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Discussion Starter · #1 ·
Have several questions and have included them in a single post. If I should break them up into separate posts I will.
Have a 1994 coupe / basic engine. When scan for fault codes none or IDed.
  1. Mass Air Flow (MAF) sensor has the following readings: KOEO = 0, at idle = 8, at 2500 RPM = 28. Are these acceptable readings? Based on web searches have not IDed the proper values for my 94. Note: The MAF sensor passed the three basic test: voltage, ground and signal (HZ), and the MAF sensor and throttle body have been cleaned.
  2. Knock sensor has the following readings: volts – 6V, Knock retard degrees at idle = 0 and at 2500 RPM still 0. As I understand the degrees should change to approx. 30 to 55. Are the two zero readings acceptable or is the knock sensor faulty or ???.
  3. EVAP duty %. Receive the following reading: when running = 0. Again, as I understand the readings should be 15 to 85 percent. FYI, have replaced the Vapor Canister Purge Valve Solenoid and 2 vacuum hoses. Do I have a problem and, if yes, any potential areas I should be looking, e.g. a faulty canister?
  4. EGR duty %. Received the following reading: when running = 0. Assume should be getting a reading other than 0. FYI, have replaced the EGR Valve. Again, any potential areas I should be looking?

Thanks. Again, if I have posted too many questions in a single post please let me know. Any and all input appreciated. Bob
 

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Hello. As a long time driveability tech and successful diagnostician I appreciate your atttention to specific PIDs. It is how I get by. However, staring at these things requires a mess of knowledge most simply are not in possession of. And so for the many to stare at these individual PIDs with the hope of actually diagnosing a problem is just a futile effort because it ignores the amount of skill/knowledge required to filter that information into logical diagnostic information. Without specific engine repair, engine performance, or advanced level engine performance training we really should rely on trouble codes for diagnostic purposes. And when we have problems without codes we should seek professional help. You know so we don't butcher our wire harness or spend thousands of dollars on parts we don't need.

With that said let me try to help with the list you provided..

1. The mass air flow sensor will report back different values at different RPM but also at the same RPM and different coolant temps and also with or without the load of the A/C and in and out of park or drive/rev. It can report back different values at different elevations or different ambient temps. It will even report back slightly different values with all of those conditions the same and just the purge valve open or close. This sensor is simply a small wire in the middle of the airstream that uses electric current in pulses to heat it to a certain resistance and the controller looks at how many pulses of power it takes to hold that resistance. More power means there is more air flowing across the wire and less power to maintain resistance means there is less airflow. The amount of airflow an engine will need or use can change in a moment and for many different reasons. That is why we need a sensor to measure this airflow.

2. The knock sensor listens to engine noise. It is a piezo electric sensor and is very sensitive. We can test these sensors by looking at their value in the datastream and gently rapping on the block with a mallot or hammer. There is a specific calibration for a given sensor and engine that determines what level of noise becomes knock. Sadly outside of the factory there is no good way to calibrate a knock sensor just because by the time you hear the change rattling you are already many degrees into knock. This value should stay at zero always. You should never see knock retard and if you do there is a problem you need to chase.

3. EVAP is your evaporative emissions. It is vapor control for your fuel system/tank. When this valve opens it takes fuel vapor from the tank and feeds it into the intake manifold to be burned by the engine and passed through the catalyst before being allowed to enter our atmosphere. For this to work effectively we need a few conditions to be right first. Like the catalyst temp needs to be at a place where it can be effective. The engine coolant temp needs to be at a certain point, also. And the engine needs to be at a low load situation so there is plenty of vaccum in the intake and the controller will start to work the purge solenoid but based off of tank pressure. If the tank is at a negative pressure the PCM will open the vent solenoid and let fresh air into the tank and if the tank goes into a positive pressure it will open the purge solenoid to remove the pressure.

4. EGR is your Exhaust Gas Recirculation. You can think of this as a precursor to the displacement on demand. When the motor is at decel or low load driving conditions where we might need a smaller engine the controller will use exhaust gas to rob air out of a cylinder so less fuel and air are needed. This also helps lower cylinder head temps and helps to prevent the NOx emissions. But again for this to be working we need low load or high vacuum in the intake manifold and a fully warmed engine. If we opened the EGR valve at idle the engine stalling would be the verification of a properly working EGR valve.

Are you having a specific driveability issue with your C4?
 

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Discussion Starter · #3 ·
Hello. As a long time driveability tech and successful diagnostician I appreciate your atttention to specific PIDs. It is how I get by. However, staring at these things requires a mess of knowledge most simply are not in possession of. And so for the many to stare at these individual PIDs with the hope of actually diagnosing a problem is just a futile effort because it ignores the amount of skill/knowledge required to filter that information into logical diagnostic information. Without specific engine repair, engine performance, or advanced level engine performance training we really should rely on trouble codes for diagnostic purposes. And when we have problems without codes we should seek professional help. You know so we don't butcher our wire harness or spend thousands of dollars on parts we don't need.

With that said let me try to help with the list you provided..

1. The mass air flow sensor will report back different values at different RPM but also at the same RPM and different coolant temps and also with or without the load of the A/C and in and out of park or drive/rev. It can report back different values at different elevations or different ambient temps. It will even report back slightly different values with all of those conditions the same and just the purge valve open or close. This sensor is simply a small wire in the middle of the airstream that uses electric current in pulses to heat it to a certain resistance and the controller looks at how many pulses of power it takes to hold that resistance. More power means there is more air flowing across the wire and less power to maintain resistance means there is less airflow. The amount of airflow an engine will need or use can change in a moment and for many different reasons. That is why we need a sensor to measure this airflow.

2. The knock sensor listens to engine noise. It is a piezo electric sensor and is very sensitive. We can test these sensors by looking at their value in the datastream and gently rapping on the block with a mallot or hammer. There is a specific calibration for a given sensor and engine that determines what level of noise becomes knock. Sadly outside of the factory there is no good way to calibrate a knock sensor just because by the time you hear the change rattling you are already many degrees into knock. This value should stay at zero always. You should never see knock retard and if you do there is a problem you need to chase.

3. EVAP is your evaporative emissions. It is vapor control for your fuel system/tank. When this valve opens it takes fuel vapor from the tank and feeds it into the intake manifold to be burned by the engine and passed through the catalyst before being allowed to enter our atmosphere. For this to work effectively we need a few conditions to be right first. Like the catalyst temp needs to be at a place where it can be effective. The engine coolant temp needs to be at a certain point, also. And the engine needs to be at a low load situation so there is plenty of vaccum in the intake and the controller will start to work the purge solenoid but based off of tank pressure. If the tank is at a negative pressure the PCM will open the vent solenoid and let fresh air into the tank and if the tank goes into a positive pressure it will open the purge solenoid to remove the pressure.

4. EGR is your Exhaust Gas Recirculation. You can think of this as a precursor to the displacement on demand. When the motor is at decel or low load driving conditions where we might need a smaller engine the controller will use exhaust gas to rob air out of a cylinder so less fuel and air are needed. This also helps lower cylinder head temps and helps to prevent the NOx emissions. But again for this to be working we need low load or high vacuum in the intake manifold and a fully warmed engine. If we opened the EGR valve at idle the engine stalling would be the verification of a properly working EGR valve.

Are you having a specific driveability issue with your C4?
Thanks for taking the time to provide such a detailed response. I find it very informative and helpful. Obvious I have a lot learn but info such as you have provided is definitely educational. Re: your question "Are you having a specific driveability issue with your C4?" Two issues: As stated, but not a driveability issue, my ASR Service light comes and goes on and off. Just an aggravation and does not seem to impact performance or the shifting of the transmission. And the other issue is there is a slight rough idle. But other than that performance is find. Put the pedal to the metal and away we go. Once again, thanks for your time and the provided info. Bob
 

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So ASR is basically the precursor or very early version of traction control. ASR is a modifier to your mechanical cable operated throttle body. You will always identify the ASR engagement as the gas pedal fighting you. ASR is a motor attached to cams on the throttle body cables. This ASR unit can be found on your 1994 just in front of the brake master cylinder or if you follow the cable back from the throttle body. There is a procedure for setting the adjustment on the ASR that we need to verify before changing parts that are no longer available. And that last sentence usually will stop most shops from even spending time tracking down your problem. Just FWIW.

Whether it is the ABS module or a wheel speed sensor or anything else they are very difficult to find and with these early pre OBD 2 systems on board diagnostics are very limited to start with. Most will remove a bulb or use a piece of the black electrical tape to cover the ASR and ABS lights on a C4. Because the cost of repair vs. the value of the car is so lopsided.

The ruff idle can be from a few things. First the optispark absolutely sucks. But the controller will generally code for the low or high resolution pulse failures. The ignition wires on these cars are always problematic. There were specific thin plastic keepers to hold the wires where they need to be but they all break real early on. If your ignition wires are melted, or touching each other, or touching ground anywhere then you might want to replace the ignition secondary circuit and just be careful about routing of your wires.

And also the fuel injectors and regulator are known sources of problems. You could use a DVOM and ohm out each of your 8 fuel injectors making sure they all report back the same resistance value. So long as this is good you could remove all 8 injecotrs and clean them in an ultrasonic bath. If any are out of spec or if you just want the better design, Accel makes the replacement injectors for your car that are more tolerant to the ethanol used in our fuels today.

Another thought is your MAF sensor. This sensor uses a small wire to measure airflow. The simple way this works is the controller sends a given amount of the electrical current to this wire to heat it up to the point of achieving a certain resistance. Then the controller just measures how much current is required to keep this wire at this temp/resistance. If it takes more current the controller knows there is more airflow and if less current is needed to hold the resistance value then the controller knows there is less airflow across the sensor. When this wire gets dirty or contaminated it gets insulated from the airflow around it a little and starts reporting back slightly skewed data that can bring about the rough idle conditions. IF you have a Mass Air Flow sensor in your intake duct between the throttle body and the air filter you might want to consider removing this MAF and cleaning it with some carb cleaner or brake parts cleaner and some compressed air. Be careful with this sensor.
 

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Discussion Starter · #5 ·
So ASR is basically the precursor or very early version of traction control. ASR is a modifier to your mechanical cable operated throttle body. You will always identify the ASR engagement as the gas pedal fighting you. ASR is a motor attached to cams on the throttle body cables. This ASR unit can be found on your 1994 just in front of the brake master cylinder or if you follow the cable back from the throttle body. There is a procedure for setting the adjustment on the ASR that we need to verify before changing parts that are no longer available. And that last sentence usually will stop most shops from even spending time tracking down your problem. Just FWIW.

Whether it is the ABS module or a wheel speed sensor or anything else they are very difficult to find and with these early pre OBD 2 systems on board diagnostics are very limited to start with. Most will remove a bulb or use a piece of the black electrical tape to cover the ASR and ABS lights on a C4. Because the cost of repair vs. the value of the car is so lopsided.

The ruff idle can be from a few things. First the optispark absolutely sucks. But the controller will generally code for the low or high resolution pulse failures. The ignition wires on these cars are always problematic. There were specific thin plastic keepers to hold the wires where they need to be but they all break real early on. If your ignition wires are melted, or touching each other, or touching ground anywhere then you might want to replace the ignition secondary circuit and just be careful about routing of your wires.

And also the fuel injectors and regulator are known sources of problems. You could use a DVOM and ohm out each of your 8 fuel injectors making sure they all report back the same resistance value. So long as this is good you could remove all 8 injecotrs and clean them in an ultrasonic bath. If any are out of spec or if you just want the better design, Accel makes the replacement injectors for your car that are more tolerant to the ethanol used in our fuels today.

Another thought is your MAF sensor. This sensor uses a small wire to measure airflow. The simple way this works is the controller sends a given amount of the electrical current to this wire to heat it up to the point of achieving a certain resistance. Then the controller just measures how much current is required to keep this wire at this temp/resistance. If it takes more current the controller knows there is more airflow and if less current is needed to hold the resistance value then the controller knows there is less airflow across the sensor. When this wire gets dirty or contaminated it gets insulated from the airflow around it a little and starts reporting back slightly skewed data that can bring about the rough idle conditions. IF you have a Mass Air Flow sensor in your intake duct between the throttle body and the air filter you might want to consider removing this MAF and cleaning it with some carb cleaner or brake parts cleaner and some compressed air. Be careful with this sensor.
Once again, thanks for spending the time to provide such detailed info.
Just as a little background on my 94 should it help.

Re: optispark – agree re: your comments. Based on cost and work associated with its replacement I have just ignored the fact it could be associated with my rough idle. Other than the idle the 94 is running great. Unless it becomes associated with a performance issue I have just put the optispark on the back burner.

Re: fuel injectors. Replaced the injectors just as a maintenance issue re: the injectors were original as best as I could determine. Conducted a Noid light test - passed on all cylinders. Injector resistance / Ohm test within spec each injector. Injector voltage 11.70 V per injector. Fuel pressure at fuel rail within spec. Just FYI: I use 100% gas – no ethanol. But that does not mean the prior owner also used 100% gas.

Re: MAF Sensor: Have cleaned the MAF sensor and throttle body (removed to clean). MAF sensor passed the signal (Hz), ground and 12 volt tests.

Appreciate all feedback and educational information. Thanks Bob
 

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Just FYI: I use 100% gas – no ethanol. But that does not mean the prior owner also used 100% gas.
There is at least 10% ethanol in 98% of the fuel sold in the US. If you truly are in touch with a source of the pure gas, you need to share with the rest of us. We will come from far and wide for your guzoline. unleaded88 is 15%, E-85 is 85% or at least 70%, and the 87, 89, and 93 you get at the pump contains up to 10% ethanol by volume. And that can cause problems for the early fuel injectors but also might shorten the life of your 1994 fuel pump or regulator. I think there is rec90 that is ethanol free if you can find it.

Another thing you could explore that might bring you to a rough idle without setting codes is a failed idle air control valve. These early IAC's have small plastic toothed gears inside that can break a single tooth off and create a dead zone in the IAC that manifests itself as a degraded idle condition.
 

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Discussion Starter · #7 ·
There is at least 10% ethanol in 98% of the fuel sold in the US. If you truly are in touch with a source of the pure gas, you need to share with the rest of us. We will come from far and wide for your guzoline. unleaded88 is 15%, E-85 is 85% or at least 70%, and the 87, 89, and 93 you get at the pump contains up to 10% ethanol by volume. And that can cause problems for the early fuel injectors but also might shorten the life of your 1994 fuel pump or regulator. I think there is rec90 that is ethanol free if you can find it.

Another thing you could explore that might bring you to a rough idle without setting codes is a failed idle air control valve. These early IAC's have small plastic toothed gears inside that can break a single tooth off and create a dead zone in the IAC that manifests itself as a degraded idle condition.
First re: IAC valve, have replaced. Thanks for the suggestion. Re: gas. There are several stations in Pensacola that say they sell 100% gas. Either 91 or 93 octane. The included pic is from one of the stations I buy gas. Maybe it is false advertising.
Gas Fuel Machine Font Gas pump
 

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I would pay money to see some of that gas measured for content. Just saying. To think you'd buy some coke without any cut or it being stomped on is a crazy idea. I mean I am sure there are a lot of drug dealers that are gonna tell you how pure their powder is but really, why would someone do that and not make money? The gov't said they can cut the fuel by up to 10%. I challenge you to test that fuel. The kits for this are cheap and simple to use. Here is one: Fuel-It! Ethanol Content Tester
 
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