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62 Posts
Discussion Starter · #1 · (Edited)
I just bought a 1990 standard Vette with the L98 engine about a month ago. It ran great except for a bit of hesitation and idle a little rough. I recently have been getting an intermittent check engine light.
Saturday I took the car to the service center to change the oil and filter, differential and transmission(auto)service and had them change the fuel filter while I was there. Drove it home and there seemed to be no big change to the performance.
I let it set for couple of hours and got a chance to use the code readers I had recently purchased on line. The code 45 came up indicating rich exhaust. I had already bought a o2 sensor and went to the local service center that afternoon to have it installed.
On the way the car stalled out on the road after reaching road speed. I shifted into N and it started up and I proceeded to my destination. I parked the car and handed over the keys.
When the mechanic tried to start my car it would turn over fine but would run for only a second before shutting down. I have read other blogs in situations when the EMC would get hot and would cause a no start. I let it set for a while trying now and then. I began tapping this and wiggling that esp. the HEI coil. After about 15-20 the car finally started and they brought it into the bay to do the service.
After completing their work they went to start it to get it off the rack and of course would turn over run for a second and shut down. Frustrated I went back to the same procedures as before but no progress. NO-ONE at the station had any ideas. I then asked if anyone had a distributor wrench and retarded the timing a bit. Amazingly it started (coincidence?).
I left the center and a few miles down the road it began to stall while driving. Once(restarted), Twice(restated), Three times(restarted), Fourth time---nothing just cranked-ran for a second-then shut down. I tried several things within my scope of knowledge at that time. I ran the scan tool But came up with NO codes. Finally had to call for a tow.
At home I began looking through my Factory and Haynes Manual for help. The first procedure was to make sure the pump was working. I installed a fuel pressure gauge at the rail, turned on the key and could hear the fuel pump working and saw a pressure of about 43psi. I let the pressure set there for a while to see if it falls. After about 15-20 it fell maybe 3-4 psi. I then turned the ignition off and then back on to check again but the same result. Trying to start the car and running for the second before it shuts down it falls a bit and begins to rise like the pump is getting ready to give another shot . Will start and run for a second then shut off, no codes.

Any help on this issue would be appreciated.

56 Posts
Remove the dust cap off the HEI coil and check the wires going from the plug to the coil. I had an 88 with the same symptoms, come to find out one of the little copper wires was broken and while cranking was making intermittent contact. Start run for a sec then die. also check the temp sensor for the computer. They can fail, telling the computer it is extrememly cold out when it is actually hot and over fuel on the start up creating a no start condition. Check with a scanner to see what the sensor is telling the computer and compart to the actual coolant temp. If all that checks out get a noid light and test for injector pulse.

62 Posts
Discussion Starter · #3 ·
Hey, Thanks for the reply. I was working on it last night and I think that I completely eliminated the possibility of a fuel problem. I was going to start on the electrical tonight. I will take your advice and begin with the distributor HEI coil and check the connectors. I did a scan a couple of days ago and as I remember it indicated the temp being mid 80s but I will recheck. Thanks for giving me a direction to follow. I will post my results as they occur.

Super Moderator
8,550 Posts
info that might help (before you ask, yeah the LT1 is very similar)

L-98 Engine Start Sequence

knowing whats going on and WHY can help

When you start an L-98 engine Corvette, a series of events take place that causes the engine to run. Knowing the sequence will help you troubleshoot no start conditions.

Fuel Rail Pressurization:

When you first turn the key to the “on” position, the fuel pump will run for 2 seconds pressurizing the fuel rails. There is a Shraeder valve on the passenger side fuel rail near the rear of the engine and if you measure the pressure there after the pump runs, you should see between 40-42 pounds of pressure. The reading will go to 38-40 pounds nominal once the engine is running.test by attaching a fuel pressure gauge to the fuel rail at the shrader valve, on TPI and LT1 engines its located on the pass side fuel rail

Initial Crank Action:

If you then rotate the key to the start position (assuming the anti-theft system has not disabled the starter), the engine will rotate.

Once the oil pressure has reached 4 PSI, the oil pressure switch will close allowing the fuel pump to run. (Note that you should have a black oil pressure switch/sender. It is mounted behind the distributor on the driver’s side and if it is not black, it is suspect due to a run of bad units that stayed in the GM parts pipeline for some time).

The distributor will send a string of pulses to the ECM (Engine Control Module) in response to the engine being rotated by the starter. These pulses continue as long as the engine turns (both starting and running) and if they are not present, the engine will not run.

ECM Reaction:

If the ECM sees oil pressure greater than 4 PSI and the reference pulses from the distributor, it will energize the injector drivers which will begin pulsing the injectors on for 4 ms (milliseconds) periods. (In the L98, all injectors on one side of the engine fire at the same time followed by all injectors on the other side firing at the same time. On the LT-1, the injectors are fired individually at the appropriate time).

The ECM will also pull in the fuel pump relay in effect paralleling it electrically with the oil pressure switch. (If the fuel pump relay fails, you can still normally get the car to start and run unless you can’t make at least 4 PSI oil pressure. This is a “limp home mode” feature put in place to allow for a fuel pump relay failure).

The ECM also monitors the TPS (Throttle Position Sensor mounted on the throttle body assembly) and wants to see .54 volts at this time. If it sees appreciably more than 0.54 volts, it will assume the engine is flooded and the driver has pressed the accelerator to the floor to clear the flooded condition and restrict the fuel flow as a result. (.54 volts during start and at idle from the TPS is very important to both starting and run performance.)

Assuming the ignition module is good (meaning there is a spark of sufficient intensity to ignite the fuel), the engine will “catch”.

Engine "Catches":

When the engine catches, the MAF (Mass Air Flow sensor mounted just ahead of the throttle body) sends a signal to the ECM advising that air is flowing and also just how much air is being pulled through to the intake manifold. The ECM takes note of the amount of air being consumed and adjusts the injector pulse width to around 2.2 ms nominally so as to attain a proper air/fuel mixture to insure combustion. (This is how the 1985 through 1989 L-98 works. For information on the 1990 and 1991 L-98 variant, see the Note below).

The engine should show an initial idle speed of around 900-1100 RPM and then slowly diminish to 600-700 RPM unless the air conditioner is on in which case it will run at around 800 RPM.

If this does not happen, the Idle Air Mixture valve (located on the throttle body) may be misadjusted. Alternatively, there may be a leak in the intake manifold or another vacuum leak may be present. Listen for hissing sounds---there should be none.

ECM Mode:

The engine will now be in Open Loop mode meaning that the ECM is controlling the air/fuel mixture by referencing values stored in memory.

Once the Oxygen sensor (mounted on the exhaust pipe) reaches operating temperature of several hundred degrees, the Manifold Air Temperature (MAT) sensor shows an intake air temperature of more than 140 degrees and the Engine Coolant Temperature (ECT) has reached 160 degrees, the computer will switch to closed loop mode meaning the Oxygen sensor’s output is examined along with the MAT and ECT outputs and the ECM adjusts the injector pulse widths (more “on time” or less “on time”) to constantly strive for a 14.7:1 air/fuel mixture which is the best mixture to hold down pollution.

Note that prolonged idling can force the computer back into open loop mode.

Note: In 1990, the MAF was eliminated from the engine in favor of a speed/density system. This system uses a sensor called the MAP sensor which measures the Manifold Absolute Pressure (hence the name MAP) and compares it with the atmospheric pressure outside the intake manifold. This information, coupled with the Manifold Air Temperature, Engine Coolant Temperature and Engine RPM is used by the ECM to determine the amount of air entering the cylinders. It is a different way of reaching the desired 14.7:1 air-fuel mixture ratio but functionally is like the MAF system in that the ECM uses the feedback to control the "on time" for the injectors.

Corvette used this approach in the 1990 and 1991 L-98 engines and in the 1992 and 1993 LT-1 engines. With the 1994 model C4, they went back to the MAF system. Note that MAF based systems are far more accurate since they measure air flow directly whereas the MAP system infers air flow indirectly. A multitude of things can throw the calculation off and Corvette returned to the MAF system beginning with the 1994 C4 (with a MAP backup). From a troubleshooting standpoint, the MAP operation comes into the sequence the same place that the MAF does.


If you have a no start condition or if the L-98 starts and then dies, check the above items in sequence to see if all the events are occurring as required.

A Scan Tool makes this job much easier and is a highly recommended troubleshooting aid for these sorts of problems.

Most of the C4 Corvettes used a MAF (Mass Air Flow) sensor to determine how much air is being pulled into the intake manifold. The exceptions are the 1984 Corvette that used a speed density system--a sort of predictive method of measurement---and the 1990 through 1993 C4 models which were also speed density based. In 1994, Corvette went back to the MAF based system but used the speed density approach as a back up. (1989 Bosch MAF installation shown at right).

A Mass Air Flow sensor has an extremely fine wire inside its bore. The 1985 through 1989 C4 engines used a Bosch MAF sensor that heated the wire to 100 C. The 1994 and later C4 models used a AC/Delco MAF that heated the wire to 200 C. The amount of current required to reach the temperature is measured in each case. (Note: the LT-5 engine used in the ZR-1 used a speed density system and continued to use that system in 1994 and 1995 since the engines had already been made prior to the last two years of production. The ZR-1 therefore has no MAF even after Corvette went back to the MAF based system).

Theory of Operation

As the air travels past the heated wire enroute to the intake manifold, it will cool the wire and additional current is added to again heat the wire to the design temperature. Since the amount of air moving past the sensor is directly related to the amount of cooling experienced by the heated wire, a feedback condition is established whereby the exact amount of moving air is directly related to the amount of current passing through the wire and the intake air is therefore precisely measured.

Once the amount of air is known, the computer controlling the engine can add or subtract fuel as required to maintain the magic 14.7:1 air-fuel mixture resulting in the cleanest burn possible from an emissions (pollution) standpoint.

It does this by varying the "on time" of the fuel injectors. The injectors are pulsed on and off and the width of the pulse is lengthened or shortened as required. When you first start a typical engine, the pulse width is around 4 milliseconds but as soon as the engine "catches" the pulse width is shortened to about 2.2 milliseconds for idle. During operation, the measured air flow through the MAF will cause the computer to increase or decrease the pulse width as explained above.

MAF Operating Conditions

The Bosch MAF is more complex than the AC/Delco version. Both measure the air flow but the Bosch MAF has a circuit called the 'burn-off circuit' that cycles on for about 2 seconds when you shut the engine down. This circuit heats the wire to a high enough temperature to burn off any residue that may have collected on the wire during operation. If you are in a quiet area, you can hear the relays click on and then off on a 1985-1989 C4 as the burn-off cycle occurs.

There are two relays involved with the Bosch MAF: A power relay that passes current to the MAF wire during normal operation and the burn-off relay that provides the current for the cleaning cycle. Both are located on the firewall in the engine compartment, just behind the battery on the drivers side. Bad MAF power and burn-off relays can cause hard starting problems and should be changed periodically as preventative measure and any time you experience hard starting conditions.

The AC/Delco MAF has a power relay but no burn-off relay. For this reason, you should pay even closer attention to the condition of your air filter on a later model C4 than normal since a contaminated wire in a AC/Delco MAF is going to stay contaminated for the most part and cause false signals to be passed to the computer.

Also, the Bosch MAF outputs its information as a analog signal to the computer but the AC/Delco sends its signal as a digital component of varying frequency. For this reason, you cannot measure it's operation directly.

A scan tool is generally the best way to troubleshoot engine problems and with the 1994 and later Corvette, it is virtually mandatory. (An oscilloscope will also work on the AC/Delco MAF but a regular test meter will not).

MAF Problems

Faulty MAF sensors will normally light the check engine light on the drivers information center if the problem is constant and store a trouble code. If intermittent, a trouble code will still be stored as long as the battery is not disconnected.

Normally, the problem is a poor connection at the sensor and wiggling the wires, unplugging and reinserting the connector will often cure the problem.

A faulty MAF will normally cause a no start or difficult start condition and although you can eventually get the car into the "limp-home" mode in most cases, you need to attend to the problem ASAP.

this flow chart might help

AC/Delco sensors can become intermittent or give false readings if the wires become contaminated as explained above.

The MAF is a critical part of the emission control system and as such will cause the computer to react to problems very quickly, setting trouble codes and reducing performance in ways that cannot be ignored for long.

MAF Mods

The Bosch MAF is often modified by removing the two screens that are present in the front and rear of the cylinder. Removing these screens significantly increases the air flow through them and this results in more horsepower. Removing the screens is an old trick from the Corvette Challenge days in 1988 and 1989. It does work but is illegal in many states so be advised not to do anything that will get you arrested for a pollution violation.

The AC/Delco MAF is not readily modified. It is what it is but since it is a larger diameter than the Bosch, it responds well to changing the air filter to a free flowing type such as the K&N filter.
Welcome to C4 vette codes it is very ....repeat very
important that if you are not savvy of working on your
vette would be better off - taking your car to a
dealership for repairs on your trouble codes.
However if you feel that you want to dive right in ..than you
have come to the right place.First locate your car's alcl
this component is located just below the instrument panel and
to the left of the center console. Remove the plastic cover
the first two slots to your right are the A & B slots for a drawing of
the alcl module's picture is added below.
The A slot is the diagnostic slot and the B slot is the ground
slot. insert the computer key into these slots (with the engine
off) this is very only put the ignition key
to on ( not start !!!) the check engine light will display a
code 12 which is one flash followed by two flashes.
this code will be flashed three times ..followed by the
trouble code stored in your car's computer.
what ever the code is it will be flashed three times.
have a paper and pencil ready and write down the
code .

code 13 =1 flash followed by 3 flashes =>oxygen sensor
code 14 =1 flash followed by 4 flashes =>coolant sensor
code 15 =1 flash followed by 5 flashes =>coolant sensor
code 21 = 2 flashes followed by 1 flash =>throttle position sensor
code 22 = 2 flashes followed by 2 flashes=> throttle position sensor
code 23 = 2 flashes followed by 3 flashes=> manifold air temp sensor
code 24 = 2 flashes followed by 4 flashes=> vehicle speed sensor
code 25 = 2 flashes followed by 5 flashes=> manifold air temp sensor
code 32 =>egr system
code 33 =>map sensor
code 34 =>maf sensor
code 35 => idle air control
code 41 => cylinder select error
code 42 => electronic spark control
code 43 => electronic spark control
code 44 => lean exhaust
code 45 => rich exhaust
code 51 => PROM
code 52 => fuel calpak
code 53 => system over voltage
code 54 => fuel pump circuit
code 55 => ecm
code 62 => oil temp
please remember that if you have the computer key installed
in the alcl and you start the engine ( you will ruin the engine's computer
only put the ignition to on (not to start)
If you should get a check engine soon display.. you can use
the above procedure and codes to buy the right part
or at the very least to keep from getting taken for a ride
and be made to pay hight prices for some inexpensive
module that you could have installed yourself.
You never ask a barber if you need a haircut ..
so you have to be on guard they will see you comming
a mile away.
If your engine displays a trouble code ... your engine will
go into limp mode will still run but very poorly.
you might be able to reset the computer if it will not start
( just to get home ) by disconnecting both battery cables
and re-installing them ...this is not recommended ..but if
you are stranded it might help unitl you get your car home
or to a repair shop..good luck

1985 TO 1991:

Code #12: Normal No Codes.
Code #13: Open Oxygen Sensor Circuit.
Code #14: Coolant Sensor Circuit Low.
Code #15: Coolant Sensor Circuit High.
Code #21: Throttle Position Sensor High.
Code #22: Throttle Position Sensor Low.
Code #23: Manifold Air Temperature Circuit High.
Code #24: Vehicle Speed Sensor.
Code #25: Manifold Air Temperature Circuit Low.
Code #32: EGR System Failure.
Code #33: Mass Air Flow Sensor High.
Code #34: Mass Air Flow Sensor Low.
Code #36: Mas Air Flow Sensor Burn-Off Function Fault.
Code #41: Cylinder Select Error.
Code #42: Electronic Spark Timing.
Code #43: Electronic Spark Control.
Code #44: Lean Exhaust indication.
Code #45: Rich Exhaust Indication.
Code #46: Vehicle Anti Theft Fault.
Code #51: Faulty Mem-Cal.
Code #52: Fuel Calpak Missing.
Code #52(1990-91 Corvette Only): Engine Oil Temperature Sensor Low.
Code #53: System Over Voltage.
Code #54: Fuel Pump Circuit Low Voltage.
Code #55: Defective ECM.
Code #62: Engine Oil Temperature Sensor Circuit High.

ECM CODES 1992 TO 1993:

Code #12: Normal No Codes.
Code #13: Left Oxygen Sensor Circuit.
Code #14: Coolant Temperature Sensor Circuit High.
Code #15: Coolant Temperature Sensor Circuit Low.
Code #16: Opti-Spark Ignition Timing System.( Low Pulse)
Code #21: Throttle Position Sensor Circuit High.
Code #22: Throttle Position Sensor Circuit Low.
Code #23: Intake Air Temperature Sensor Circuit Low.
Code #24: Vehicle Speed Sensor Circuit.
Code #25: Intake Temperature Sensor Circuit High.
Code #26: Quad-Driver Module #1 Circuit.
Code #27: Quad-Driver Module #2 Circuit.
Code #28: Quad-Driver Module #3 Circuit.
Code #32: Exhaust Gas Recirclation Circuit.
Code #33: Manifold Absolute Pressure Sensor Circuit Low.
Code #34: Manifold Absolute Pressure Sensor Circuit High.
Code #36: Opti-Spark Ignition Timing System. (High Resolution Pulse.)
Code #41: Electronic Spark Timing Circuit Open.
Code #42: Electronic Spark Timing Circuit Grounded.
Code# 43: Electronic Spark Control Circuit.
Code #44: Left Oxygen Sensor Circuit Lean.
Code #45: Left Oxygen Sensor Circuit Rich.
Code #51: Mem-Cal Error.
Code #52: Engine Oil Temperature Sensor Circuit Low.
Code #53: System Voltage.
Code #55: Fuel Lean Monitor.
Code #56: Vacuum Sensor Circuit.
Code #61: Secondary Port Throttle Valve System.
Code #62: Engine Oil Temperature Sensor Circuit High.
Code #63: Right Oxygen Sensor Circuit Open.
Code #64: Right Oxygen Sensor Circuit Lean.
Code #65: Right Oxygen Sensor Circuit Rich.
Code #66: A/C Pressure Sensor Circuit Open.
Code #67: A/C Pressure Sensor Circuit. (Sensor or A/C Clutch Circuit Problem)
Code #68: A/C Relay Circuit Shorted.
Code #69: A/C Clutch Circuit.
Code #72: Gear Selector Switch Circuit.

CODES 1994 TO 1996:

DTC #11: Malfunction Indicator Lamp Circuit.
DTC #13: Bank #1 Heated Oxygen Sensor #1 Circuit:
DTC #14: Engine Coolant Temperature Sensor Circuit Voltage Low.
DTC #15: Engine Coolant Temperature Sensor Circuit Voltage High.
DTC #16: Distributor Ignition System Low Pulse.
DTC #18: Injector Circuit.
DTC #21: Throttle Position Sensor Circuit Voltage High.
DTC #22: Throttle Position Sensor Circuit Voltage Low.
DTC #23: Intake Temperature Sensor Circuit Voltage High.
DTC #24: Vehicle Speed Sensor Circuit.
DTC #25: Intake Air Temperature Sensor Circuit Voltage Low.
DTC #26: Evaporative Emission Canister Purge Solenoid Valve Circuit.
DTC #27: EGR Vacuum Control Signal Solenoid Valve Circuit.
DTC #28: Transmission Range Pressure Switch Assembly Fault.
DTC #29: Secondary Air Injection Pump Circuit.
DTC #32: Exhaust Gas Recalculation.
DTC #33: Manifold Absolute Pressure Sensor Circuit High.
DTC #34: Manifold Absolute Pressure Sensor Circuit Low.
DTC #36: Distributor Ignition System High Pulse.
DTC #37: Brake Switch Stuck On.
DTC #38: Brake Switch Stuck Off.
DTC #41: Ignition Control Circuit Open.
DTC #42: Ignition Control Circuit Shorted.
DTC #43: Knock Sensor Circuit.
DTC #44: Bank 1 LF Heated Oxygen Sensor #1 Circuit Lean.
DTC #45: Bank 1 LF Heated Oxygen Sensor #1 Circuit Rich.
DTC #47: Knock Sensor Circuit Or Module Missing.
DTC #48: Mass Air Flow Sensor Circuit.
DTC #50: System Voltage Low.
DTC #51: EEPROM Programming Error.
DTC #52: Engine Oil Temperature Sensor Circuit Voltage Low.
DTC #53: System Voltage Low.
DTC #55: Fuel Lean Monitor.
DTC #58: Transmission Fluid Temperature Sensor Circuit Low.
DTC #59: Transmission Fluid Temperature Sensor Circuit High.
DTC #62: Engine Oil Temperature Sensor Circuit Voltage Low.
DTC #63: Bank 2 RF Heated Oxygen Sensor #1 Circuit Open.
DTC #64: Bank 2 RF Heated Oxygen Sensor #1 Circuit Lean.
DTC #65: Bank 2 RF Heated Oxygen Sensor #1 Circuit Rich.
DTC #66: A/C Refrigerant Pressure Sensor Circuit Open.
DTC #67: A/C Pressure Sensor Circuit Sensor or A/C Clutch.
DTC #68: A/C Relay Circuit.
DTC #69: A /C Clutch Circuit.
DTC #70: A/C Clutch Relay Driver Circuit.
DTC #72: Vehicle Speed Sensor Loss.
DTC #73: Pressure Control Solenoid Circuit Current Error.
DTC #74: Traction Control System Circuit Low.
DTC #75: Transmission System Voltage Low
DTC #77: Primary Cooling Fan Relay Control Circuit.
DTC #78: Secondary Cooling Fan Relay Control Circuit.
DTC #79: Transmission Fluid Overtemp.
DTC #80: Transmission Component Slipping.
DTC #81: Transmission 2-3 Shift Solenoid Circuit.
DTC #82: Transmission 1-2 Shift Solenoid Circuit.
DTC #83: Torque Converter Solenoid Voltage High.
DTC #84: 3-2 Control Solenoid Circuit.(Auto Only).
DTC #84: 2nd And 3rd Gear Blockout Relay Control Circuit.
DTC #85: Transmission TCC Stock On.
DTC #90: Transmission TCC Solenoid Circuit.
DTC #91: One To Four Upshift Lamp(Manual Only).
DTC #97: VSS Output Circuit.
DTC #98: Tachometer Output Signal Voltage Wrong.
_________________you really can,t be effectively at playing mr-fix-it with out the correct tools

especially on the more modern cars that are computer controlled, the days of effectively tuning by ear and vacuum gauge and engine sound went out with carbs
you need a few basic tools, now the list will vary, but you can,t get by by guessing, you neet to know and test now that sensors and CPUs control engine function
heres some basic tools

be sure to get the specific manuals your car and EFI system and ignition system,require FIRST

while it appears to be expensive, it saves you a good deal of money in the long run compared to dealing with the local chevy dealers mechanics, and makes diagnostics far faster, I bought this for the shop and it seems to be a good investment, since between a dealers diagnostics and swapping parts that don,t need changing you could easily spend close to that on just a few problems getting sorted out
youll also want a few basic diagnostic tools

and a book or two

in no time youll be the area wizz kid on chevy injection diagnostics

62 Posts
Discussion Starter · #5 ·
Hey Grumpy, Thanks for the info. Actually I have seen your posting of the starting sequence before and it has been very helpful to get me to this point. I had verified the fuel pressure was at 43 psi with the key on-- until last night. I checked it and it seems that the fuel pump died. I turned on the key and no pressure. I listened for the pump to run for the 2 seconds and no pump noise, no pressure. I swapped out the relay and still no pressure no noise.
After I replaced the pump I was back to having 43 psi when the key was on but it would STILL only run for a second. I was referencing your starting sequence that I had printed out and the oil sensor note had me thinking. You mention that after the initial squirt of fuel, the fuel pump waits for a signal from the oil sensor before it cuts back on. I swapped out the oil sensor BUT still would only run for a second. It was worth a shot and $10
As I mentioned the fuel gauge would hold at 43 psi with the key on. During start I watched the gauge and it would go down maybe 4-5 psi then when it catches for that breif second it climbed back up. Does this mean the pump is getting the signal and is continuing to do it's job?
My neighbor was with me lastnight helping me with the diagnosis and he observed the exhaust being very rich. Would the fuel regulator have something to do with this problem (flooding). Remembering another posting from a member I looked inside of the valve covers at the oil fill to see and smell if gas was present. I did not smell gas and put my finger into the filler hole and rubbed against the parts inside and still no smell of fuel.
I will probably move on to the distributor this evening to see what I find there as recommended by copperman.
Thanks for the valuable info. And I will post my progress.:buhbye:

Super Moderator
8,550 Posts
"During start I watched the gauge and it would go down maybe 4-5 psi then when it catches for that breif second it climbed back up. Does this mean the pump is getting the signal and is continuing to do it's job? ..............I installed a fuel pressure gauge at the rail, turned on the key and could hear the fuel pump working and saw a pressure of about 43psi. I let the pressure set there for a while to see if it falls. After about 15-20 it fell maybe 3-4 psi. I then turned the ignition off and then back on to check again but the same result. Trying to start the car and running for the second before it shuts down it falls a bit and begins to rise like the pump is getting ready to give another shot . Will start and run for a second then shut off, no codes.


ok the pumps obviously kicking back in as its designed to and that would eliminate the pump as a more likely source of problems, the fuel pressure regulator is (PROBABLY) ok, the reason I say probably is most pumps max out at about 43-45 psi even if the regulators not regulating correctly but your system pressures designed to be in the 38-42 psi range and 43 psi is just fine. when most regulators fail, the diaphram leaks and you fail to get near or maintain anywere near 40psi
if you have an adjustable regulator you might try turning it down 2-3 psi just to see if abnything changes, (unlikely)but it would prove the regulators working. but Im not clear as to the fuel rail pressure as its running after its starts.... does it stay near 40psi durring the time the engines running briefly?

just some quick questions

have you opened a fitting on the fuel rail return line and allowed a pint or two of fuel to run into a container, to verify that whats pumping thru the lines is pure fuel, and theres no water in the lines.?
and while you have it off shoot some compressed air into the return line and have a buddy listen at the tank that your getting airflow back to the tank, a plugged return line or a fuel sock screen on the intake line in the tank is unlikely but possiable

did you verify your getting constant ignition spark while the engines spinning over?

once it fires up, does it quit suddenly or start missfiring and slowly die?

what do the plugs look like?

does the oil pressure stay well over 10psi while your cranking it up?

its looking like IGNITION or CPU, control not fuel or injectors (mechanically) you can get a light that plugs into the injector harness that flashes to indicate a pulse from the harness to check that.

these a sensor under the plenum (MAT) that might be unplugged and (MAF) sensors can do strange things but a defective engine heat or manifold temp sensor can also cause the CPU to run the fuel overly rich


DC Crew
20,694 Posts
Another idea...

There's a big following for the Southern Maryland Corvette Club down in Callaway, MD. Great folks and there may be someone willing to give you an extra hand.

Last I heard, they meet Friday evening at the A&W.

62 Posts
Discussion Starter · #8 ·
Thanks again Grumpy for the wisdom.
To answer your questions: With the key on, the fuel pressure is 43 psi. During cranking, the pressure falls 4-5 psi. When the engine catches for that breif second it starts to climb back up to 43+.
My fuel pressure gauge has a bleed off valve with a tube attached that I stuck inside of a small container. I recall a strong gas odor comming from it but I will check futher.
Ignition spark will be my next project. What would be the best way to tell if the ECM is getting the pulse from the distributor?
According to the oil gauge in the car's dash, the oil pressure seems to be at about 60 psi at cranking and fall slightly after started breifly. Should I perform another type of oil pressure test?
I plan to pick up a Noid Light this afternoon and check the injector pulse.
I have already replaced the MAP sensor a couple of days ago and I will look at the MAT sensor.
Thanks again Grumpy for the valuable information !! :thumbsup:

62 Posts
Discussion Starter · #9 ·
Thanks DAN85VETTE,
I will look into joining them this friday. Hopefully I'll be able to drive my vette there:thumbsup:

62 Posts
Discussion Starter · #10 ·
Just an update: tested the spark at the wires and the spark seemed to be intermittent. I removed the coil, cap, rotor, and coil module. Tested the magneto as described in Haynes and seems to be fine. Replaced the module, rotor, cap and coil with all new parts and...the same. Start for a second and quit :thud: I did remove a couple or spark plugs and they were black, fouled and smelled like pure gas. Tomorrow I will proceed to change the fuel regulator and check for a clogged return line. Then maybe new plugs :nuts:

62 Posts
Discussion Starter · #11 ·
I mentioned to the guy at the auto parts store “what’s more American than working on your car on a holiday”
Started off yesterday re-checking the fuel pressure and it still looked fine with no change. 43 psi with the key on, falls a bit during cranking.
Armed with an Actron CP9087 Scan Tool we began with checking the operation of the TPS. The scanner revealed operation to be OK and with a voltage tester I found voltage to be right in the middle of required voltage range.
We then moved to removing the plenium to get to the fuel pressure regulator. We found that in order to remove the plenium easier the intakes had to be loosened. A bit of coolant will have to be drained from the system because coolant lines are connected to the port intake. When we loosed the fuel lines the supply line needed no special attention but when the return line was removed at the fitting, a lot of fuel was pouring out. We found a piece of rubber tubing and attached it. When I raised the tubing the flow stopped. The car was at a slight incline with the front end lower than the rear. We figured the fuel was in the line running by gravity then creating a siphon.
Remembering that I had purchased a new set of 8 injectors to fix an hesitation issue mentioned earlier, we decided as long as we are THIS far we may as well replace them as well.
Removed Plenium, intake tubes then the rail itself with the injectors intact. On the bench we removed the old injectors (numbering them according to cylinder ,I may get them reconditioned for the next time). The manual says the fuel regulator was not serviceable and a new one must be purchased and not to open the case. When I went to the parts store they only had the “kit” in stock (price $35) they would have to order the complete Regulator ($185). I then opted to take my chances and proceeded with the kit. Examining the fuel regulator case I noticed the screws were a tamperproof #10 torx type. As luck would have it I had one on hand from another project. Removing the cap, it revealed what looks somewhat like a thermostat with a double gasket. The old regulator was removed and new one installed. Before installing I blew thru the tubing to make sure of no blockage. Included with the kit was a bunch of rubber o rings to be used in the fuel rail joints.
Got out the box of injector and began installing them into the rail. As recommended a light coating of motor oil was applied to the o rings. The old clips were reused as I did not anticipate getting new ones, but they seemed to be OK.
Before we reinstalled the rail I got out the shop-vac and with a parts cleaning brush loosened up some of the debris on top of the head and vacuumed it up. Then cleaned off the old gaskets being careful not to scratch the aluminum. Then I wanted to check the return line for blockage. I pumped up the air compresser and turned it off. Attached a blow gun and let off some of the pressure (I wasn’t sure if a lot of pressure would do damage. I placed the blow gun at the end of the return pipe that was connected to the plenium. When I pulled the trigger a bubbling then an air sound could be heard at the tank (all clear). I wished I had done this procedure before removing the plenium it would have saved the time finding a piece of hose.
Installed the rail being careful with the o rings at the injectors and making sure the electric and vacuum connectors were on the correct side of the mounting bolts. Reinstalled the intake tubes and plenium. Reinstalled the linkage and refilled the cooling system.
NOW for the moment of truth (I couldn’t come up with something original). The first crank you could hear the rails filling. On the second try IT STARTED and purred like a kitten. :cheers:
What do you think was the culprit? Could the fuel regulator have been faulty as suspected and flooding the entire rail and not allowing fuel to return or would it have created a vacuum situation not allowing fuel to flow either way.?
I then proceeded to check the timing and or course the test drive. It seems to have more power and the slight hesitation seems to have disappeared. I will probably do many more test drives before venturing too far from home.
At this point I am very happy of coarse but I still have a few questions:
During teardown, I noticed the MAT sensor was plugged in under the plenium and was found to be operational using the sensor tester, however when we were reinstalling it was noticed the other end of the cable was not plugged in. We did a search and could not find a harness in which to plug it into. Where should this be plugged? I am not getting any codes from it.
The timing was off slightly so I followed the procedure as prescribed: unplugged the timing connecter, connected a light at #1. Started the engine, made the adjustments and retightened the dist. Reconnected the timing connector and restarted the car. I noticed the check engine light came on as suggested it would in the manual. I ran the codes and came up a 42. I cleared it (or thought I did) restarted and in a few seconds the light came back on wit another code 42. I did this a couple of times with the same result. What would this be?
When I changed out all of the contents of the distributor I installed a stock module and coil (replacing the Accell Hei coil and some kind of special module). Can I Install another HEI coil using the stock Module?

Super Moderator
8,550 Posts
first, congrats on making progress!:D
since youve replaced the injectors , cleared the retuirn line,and rebuilt the regulator and things go markedly better, obviously it sounds like the problem was injectors or the pressure regulator, or return line with clogged injectors being most likely

Code #42: Electronic Spark Timing.
Id bet your ignition timings not set where the cpu thinks it should be, or youve got a sensor not working correctly

62 Posts
Discussion Starter · #13 ·
Thanks Grumpy.
When I checked the timing just after I got the car it was set at 13 btdc. According to the tag on the cowling the correct timing is 6 btdc and I moved it to the correct location. One of the tests we did yesterday before tearing into the intake was to verify the location of the button at cyl #1 in relationship to the timing mark at compression. That test confirmed the dist.was in the correct location. Would there be any reason it would have been set incorrectly? Maybe to make the light turn off.? :huh:

62 Posts
Discussion Starter · #14 ·
Another update::::
Last night I rechecked the timing and all still seems to be fine. And when I re-connected the timing bypass I realized I may not have pushed the connection together all the way :rolleyes: . This time I took special care to do so.
This time when I started it up there was NO engine light:thumbsup: I took it out for the victory run and she runs terrific :cheers: . Plenty of power and pep. No hesitation when you stomp on it. Won't burn rubber, But it will dig in ang go.
Thanks to all you guys for all of your help and support!

Questions::: I still have the OEM coil on the cap. Should I change it out for A high performance coil?(Accell, etc.) If so will I have to change out the module? If I keep the OEM coil I need to find that plastic dust cap that covers it up like stock. Where can I find one? :huh:

22 Posts
were you able to identify the specific part that was at fault? I am having the same issues with my 91 vert. I have only had it for 4 days!

62 Posts
Discussion Starter · #16 ·
were you able to identify the specific part that was at fault? I am having the same issues with my 91 vert. I have only had it for 4 days!
I have seen other posts where the symptoms are simular but I can only speak in behalf of my experience.
First and foremost please carefully read and follow Grumpyvette's post on this string. It has been very helpful in understanding the starting sequence of my vette.
I was going to take my vette to the local mechanic....but my confidence in most mechanics is pretty low and I figured I could replace a lot of parts for the what they were going to charge me for diagnostics. Not to say I started replacing parts willy-nilly, but sometimes the symtoms are the same for different parts. Dont get me wrong I have respect for mechanics but my experience in the past they just do the replace parts until you get it right approach also.
Anyhow getting back to the story. When I got the vette, It ran pretty well except for some hesitation in acceration and idle was a little rough. I attributed this to the injectors and purchased a set from Mid-America to be changed at my leasure. Before my car got to the point where it would run for a second then die it stalled a few times then would not start, then stalled again. Were you showing a Service engine light at any time?
Before taking it to the local mechanic for him to diagnose the problem I would get hold of a service manual or a Haynes and begin familiarizing yourself with your car. There are several to be had on Ebay. I dont know your situation with your vette(If its your main mode of transportation). But I consider the time and money I spent doing my own diagnosis well spent. I figure I learned A lot about my vette while making the repairs and probably spent less than a repair shop. There are a lot of guys here on DC that are willing to help you through the process. And you will have the satisfaction of fixing it yourself. You may have to buy some special tools or equipment but most auto parts suppiers will loan tools. You can get a simple code tool for about $20 on line. There is instructions in the manual telling you how to pull codes with a paperclip(I have not been that brave yet). Just follow all common sense safety precautions when working with possibility of spraying fuel. And ALWAYS disconnect battery when working on electrical. An accidental short could be catastrophic.
Before the major issue with my vette I had a service engine light, when I pulled the codes it had something to do with the O2 Sensor. This is where one symptom and several possibilities of problems. The simplist solution to mind was to replace the sensor. However in hind sight the O2 sensor was working properly telling me the car was running rich. In my case something was obstruction the return flow of fuel back to the tank.
I was getting plenty of fuel pressure at the rail schrader valve ($35 investment in a fuel pressure gauge). I was not sure of the return line. The haynes manual says to disconnect the rubber hose at the return line and check for blockage. My return line and maybe yours as well is hard pipe all the way back to the tank. In order to check for blockage I had to remove the throttle body, Plenium, etc. If I was going that far I was going to service all I could at that point to insure suscess.
After Removing the plenium and the intake manifolds the fuel rail is right in plain view. You will need a torx bit to remove these and make sure the torx has full contact inside the bolt head to prevent messing up the heads. Some of these bolts are hidden and a little tricky. With a little patience you will do just fine. Disconnect the harness from the injectors and mark then accordingly. A few more bolts and the rail is free. Just remember when you disconnect the supply and return fuel lines fuel will most likely flow out of the return. I had a blow gun attached to a conpressor. When the fuel started to flow from the return I applied a LOW amount of pressure and the fuel returned to the tank. I had someone open the fuel cap and listen to fuel and and air sounds at the tank. The fuel comming into the engine compartment where you are working is no longer a problem and you have just insured the return line is not clogged.
I bought a rebuild kit for the fuel regulator. You will need a tamperproof torx bit in order to remove the cover. Rebuild kit is $13 a new fuel regulater is $120, your choice.....New seals for the rail came with my kit. at this point replacing the Injectors is just a twist and a pull. The injectors will be locked in place with a clip that will swing away from the injectors. Make sure to lubricate the new injector seals with motor oil for ease of installation.
It is highly recommended to use new gaskets when reassembling the manifold and plenium. Be careful noe to strip the heads so make sure the torx biu makes good contact into the bolt heads. Follow the recommended tork schedule.
Turn the key and let the fuel pump charge the lines for a few seconds before trying to start.
I tried to go through my proceedure to the best of my memory. Some of the details have been omitted for fear of boredom. Please get a manual and follow it carefully. If you are not sure when you take something apart as to the reinstall, take digital pictures to use as reference. Especially the fuel linkage.
If anyone can think of anything I have missed please chime in.
If I can help please ask. DG has been valuable to me in my time of need and I am willing to offer my education to others in return.

22 Posts
c4 no startr

thanks so much for the quick reply.
I agree 100% whith your comment regarding taking it in for service. I am dure I can replace parts as well as someone I am paying $100/hr. I really want to work on it and I am not really afraid of doing this, I just wanted to drive the darn thing for a while asap!

I am going to do it. I just found a local Chiltons for a 91 vette and I am on my way out today to get it and a fuel pressure guage. I have a voltmeter and compressor. I put 200 miles on the car including when I drove it home and just in a split second it died? I just find it hard to believe that all of the injectors are bad at the same time. Shouldn't the car at least start an run rough if it were 1 or 2 bad injectors.

here is my plan of attack:

Check the pressure first, maybe just a fuel pump?
Then the electrical readings on the FIs
If all this is good, my next step is to do the FIs
If I am doing the FIs I may as well do the regulator as you described - right

A few other ?, does this 91 have an adjustable fuel regulator?
Does it matter what FIs I buy? (remans vs new)
I seem to hear that the std is a #22 (22 lb?)

Thanks again to all, I will have to get my profile set up cause I think I will be on this site a lot!

62 Posts
Discussion Starter · #18 ·
Glad to help....Again I am not a mechanic by any means, But given enough time a the right direction I can usually get pretty good results.
As it is with me,when I feel frustrated during the proceedure I would just cover it up and walk away for a while. It is not worth stripped nuts and broken screws. Getting the bolts out of the intake may be challenging but as I said Take time to make sure the torx is seated inside the bolt. some are hidden so a universal on your ratchet may be useful. A little bit of PB Blaster may go a long way. Give them a squirt a few min before starting.
As far as the fuel pump. If the pressure is low, this will be the easiest fix ever. Get a new pump and you can be back in business in about an hour.I have heard of situations where the pump had good pressure but not enough voltage. Have you checked the fuel pump fuse? If your fuel pump seems not to operating don't forget there is a relay under your dash on the passenger side. There is a test to confirm a good relay but I just swapped around like relays in that same cluster. If you have fuel pressure your pump is working.
I don't believe the injectors are your culprit or mine, as you mentioned it would take all of them to fail all at once..not likely. I replaced mine because of the lack of performance (hesitation, idle, etc). Mine are 24lb Accel's that I orderd from Mid-America. There are other sites and you may be able to shop around for the best price(ZIP, Jegs, etc). I have purchased a lot of stuff from Mid-America, they have a good customer service and i like the warm fuzz feeling.
If you choose to check your injectors a noid light is the best choice for a diagnostic tool. You clip it to the injector wires and will light up during pulse. Great for a one person job.
Are all of your plugs firing? There are cheap testers you just put against the wires for testing during cranking. A clip on timing light works the best.
Usually a stock fuel regulator is not adjustable. If yours is it is most likely installed by the last owner. If you do not have any mods and you engine is more or less stock you are better off with a std regulator. Takes away the guess work. If you plan to upgrade you may consider it. Mine is stock and it has plenty of power. More fuel does not mean more power. As a matter of fact it could lead to floodiing. Just remember you will need a special tamperproof torx to open the regulator case.
Again I am just passing on information I have aquired thru thes site and my own experience. There others on this site more qualified than me to advise. But I am happy to lend a hand when I can. Good luck to ya.:thumbsup: Remember.....take your time and think it through. Half the fun of vette ownership is the satisfaction of working on it...yourself.

22 Posts
no start

Thanks, I am going to wait on the injectors till I get the rest of the diags done. I di want to replace the FIs and do an overal tune up just becasue they are probably original and the car did have a rough idle and slight hesitation. It only hs 49.7k on it and in the past 3-4 years only had about 4k driven, (till I get it running)

I am looking for a fule presure tester and a noid light, I need to find one that will handle 40+ lbs right?

I have the firing seq doc and that is basically what I am going to work from. I think the first thing to do is verify I have fuel pressure at the engine. I can hear the pump run for a few sec when I turn the key to on, so I will pressure the system and test the pressure. I understand I can check the pressure at the schrader valve on the TB right. I assume I am looking for a bicycle tire type fitting I can attach the guage to?

Also, I did check the FP fuse it is good. You mentioned a relay for the FP, what am I looking for. I have dropped the panel under the pass side but I do not know what the relay would look like?

I have not checked the plugs yet, at first, I guess I could remove the boot and just use a spare plug right? I know this does not validate the actual plug, but I would know that the ignition system is delivering to the plugs.

When I read your article, I was thinking that the regulator might have been the main cause of your problems, that is why I was asking if they were adjustake or not.

The only other fuel issue I can think of is the filter, I hear these are a real pain in the a** and I do not have any safe way to lift the car. If all else fails I may have to pay for a fuel filter swap.

I am sort of thinking that after having set for such long periods of time, when I started driving it, maybe some debris or gunk (water) got into the fuel system and is causing the problems. I am also going to make sure I have a clean return line as you suggested. I do not have a lot of time to work on it so this will probably be a wekk ro more project for me. No problem since this is just a "pleasure" car for me. - well it will be if I can ever drive it ;o)

Also, how is your's running now that you have worked out your issues? Anything else you would recommend for me to test/replace while I am under the hood?

Thanks again.

62 Posts
Discussion Starter · #20 ·
When you get a fuel pressure gauge, get one that has a releif tube attached. It is not only handy when draining down the pressure but you can use it to put some fuel into a clear container. Let the container set for a while a see if you have a fuel/water separation. This will tell if you have contamination. I bought mine from Auto Zone.
A fuel filter change is always a good idea anytime. I have read on this site some guys replace their filter yearly. Grumpyvette has come up with an ingenious use of 4 stepped 2x12's to lift you vette that extra 6". Ill try to look for his string. He just started out with a 2x12 approx 4 feet then stacked more on top making each one 10 or so inches shorter than the one below it. Creating a stepped ramp. Just make sure you releve the pressure with your pressure gauge before proceeding. some will recommend to remove the fuel pump fuse.
After My episode with my vette I had a couple of weeke of driving pleasure until the check engine soon light came on again. The code indicated something with the EGR system. It is just a emissions system but left as is will turn into a problem. I intend to replace the silenoid to see if thats the problem. If not I will Have to remove the plenium again and replace the EGR valve. That being said It may be wise for you to do so at that time.
The fuel relay looks like a 1"x1" silver box. In mine when you take out the panel under the passenger dash you will have to stand on your head or get a mirror because they are actually up further behind the dash. There should be a row of 4 or 5 of them. Your manual should direct you to which one.
Let me know how it goes:thumbsup:
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