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Discussion Starter · #1 ·
Ok so I have this 1986 C4 with the TPI L98. It was acting like it was miss firing on me so I changed the plugs, wires, cap, rotor and checked the timing and adjusted with EST unplugged then plugged back in. Threw a code for MAF so I changed that and the relays. I also put a new O2 sensor in as well. I adjusted the TPS sensor to .54v also.

I had a clogged front converter so I took both of those off and it's without front cats now. I have also put in a new Coolant temp sensor and sending unit for the gauge. I have some problems

First it still runs rough. At idle when you put it in gear RPMS drop to about 500-600 and it wants to stall. When revving or driving the motor seems to vibrate like its miss firing. Exhaust does not even sound like it's supposed to. I have checked and it has good compression.

Second the temp gauge inside just reads low. It never changes and the fan never comes on. That's why I changed both sensors and still does not do anything.

Somebody please help as this has me stumpted.
 

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The corvette model you have is actually designed to run at 500 rpm while at idle, warmed up, and in gear. It will idle at this even if the temp isnt fully at running temp, just past the initial warmup phaze, sometimes called open loop.

The rough idle may be caused by one or more many things.
*egr or exhaust gas recirculation valve
*clogged fuel filter
*if still equipped, AIR being stuck on always (often this is removed)
*improperly gapped spark plugs
*weak ignition (at ANY point) check coil

The computer in these early cars has a known weak point as far as ability to compute fuel air correctly. Often when the temperature is somewhere between warm and cold, they tend to flood themselves.Sometimes even when dead cold as you just started up. For a period of time you will notice that it bogs out on takeoff, and has less power than it should. It should drive normal after the engine is entirely at running temp. These engines also take longer to warm up than many other V8 chevys i have driven before.

A tune will rid you of this problem. many people dont experience this issue, but others do.

Also, check your knock sensor, located on passenger side just above the oil pan. It is supposed to retard timing when a knock is "heard" in the engine. If this part fails it may either fail to retard during a detonation, or it may retard the timing permanently, by as much as 15 degrees. This would not happen if the esc unplugged from the distributor. Meaning that you can properly set the timing and then when spark controll is returned to the computer, it retards it again. If you cant check the sensor, i dont remember the method or measurements, you can tell by advancing the timing and see if it runs as if it were correct. or set timing correctly, and when you plug esc up, check again to see if timing has been retarded.:crazy:

Hope you understand, and get your engine running top soon.

happy troubleshooting,
~wd40:buhbye:
 

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Discussion Starter · #5 ·
Have ohmed out injectors warm and cold and all the same. Have checked timing like 8 times. Idk. Could the valves be out of adjustment? :WTF
 

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Check your grounds and possibly add another from your battery main ground to your manifold alternator engine mount support.

Ask me how I know...

You could also have a clogged injector. These batch fire L98s need pristine injectors to run smooth. I am debating a set of FIC Gen III myself...

Speaking of injectors, you might have a leaking O-ring. At idle, drip some water on each injector hole. Watch a bit and move to the next.

You might be able to narrow this by pulling 1 plug wire at a time and note which changes the least. (Replace them before moving on.)
 

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This is probably not common but I had the same problem on my 1985 w/ cast iron heads, It turned out to be a rocker arm stud was pulling out of the head so the valves would not open far enough. It might be worth checking out.
Good luck
 

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Discussion Starter · #9 ·
The left side exhaust manifold gets hot really fast while the right side does not get hot and can still be touched. Any ideas what would cause this?:thud:
 

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Too rich? Too lean? One of those conditions could create hotter than usual exhaust.
 

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You'll just be chasing ideas until it is scanned.
 

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Discussion Starter · #12 ·
Cats are removed. Today this is what I did. Complete compression check again. All ranged from 95psi to 120psi and held the compression. Plugs are not fouled out but it looks like the complete right hand side (passenger side) is not firing. All of the plugs on that side are soaked in gas while the other side looks normal. Ignition timing is correct with the EST unplugged. IAC and TPS have been set to factory specs. Cap and rotor are good and the coil ohms out correctly to specs. Ignition module has been tested and that is also good. Cap rotor and plugs are all new as well as wires. Has new CTS installed and a new coolant temp sender. New MAF and relays. All valves are adjusted properly as I checked today when I did the compression.

I also checked the distributor to make sure it was pointing to number one at tdc and it is.

It runs as if it is not timed correctly or is out of time. Like I said it's like the complete RH side is not firing at all. O2 sensor is also new.


Also it is throwing no codes at all!!!:WTF
 

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did you pull a plug and see if it was sparking? or you may have a very leaky cold start injector.:thumbsup:
 

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86s are Batch Fire.

Have you checked to see that those injectors are firing? Check the fuse, I believe there are two. From memory: Inj1 and Inj 2; passenger side dash panel.

There are injector drivers in the ECM, could one of them have failed?

Do you have a friend with the same ECM that you can swap in just to test? I did it that way to find the ECM problem on my 86.

Keep us posted.

Jake

West Point ROCKS!
 

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86s are Batch Fire.

Have you checked to see that those injectors are firing? Check the fuse, I believe there are two. From memory: Inj1 and Inj 2; passenger side dash panel.

There are injector drivers in the ECM, could one of them have failed?

Do you have a friend with the same ECM that you can swap in just to test? I did it that way to find the ECM problem on my 86.

Keep us posted.

Jake

West Point ROCKS!
He is not having a problem with fuel starvation. It's soaking the plugs on the right side of the engine. My brothers 1987 corvette had a similar problem it turned out to be a ruptured diaphragm in the fuel pressure regulator. It was sucking gas out of the fuel pressure regulator and into the plenum via the vacuum line. This caused a very rich condition at idle and a long crank to start the engine due to low fuel pressure. Disconnect the vacuum line from the fuel pressure regulator to the plenum and check for signs of fuel......it ran out of my brothers it was easy to tell....Good Luck!:thumbsup:
 

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Discussion Starter · #16 ·
Ok so this is where i'm at.

I pulled one injector wire at a time on each side.

On the left side as i pulled an injector wire it ran rougher. Meaning injectors working.

On the right side as i pulled an injector wire it didn't change a thing. Meaning injectors not work. Remember this is the side soaked with gas.

I DO NOT UNDERSTAND WHATS GOING ON lol. :thud: :smack :huh:
 

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Ok so this is where i'm at.

I pulled one injector wire at a time on each side.

On the left side as i pulled an injector wire it ran rougher. Meaning injectors working.

On the right side as i pulled an injector wire it didn't change a thing. Meaning injectors not work. Remember this is the side soaked with gas.

I DO NOT UNDERSTAND WHATS GOING ON lol. :thud: :smack :huh:
You should test each injector with a noid light. If the cylinder was flooded pulling the wire off an injector won't change anything if that cylinder was already dead.
 

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You have a
Ok so I have this 1986 C4 with the TPI L98. It was acting like it was miss firing on me so I changed the plugs, wires, cap, rotor and checked the timing and adjusted with EST unplugged then plugged back in. Threw a code for MAF so I changed that and the relays. I also put a new O2 sensor in as well. I adjusted the TPS sensor to .54v also.

I had a clogged front converter so I took both of those off and it's without front cats now. I have also put in a new Coolant temp sensor and sending unit for the gauge. I have some problems

First it still runs rough. At idle when you put it in gear RPMS drop to about 500-600 and it wants to stall. When revving or driving the motor seems to vibrate like its miss firing. Exhaust does not even sound like it's supposed to. I have checked and it has good compression.

Second the temp gauge inside just reads low. It never changes and the fan never comes on. That's why I changed both sensors and still does not do anything.

Somebody please help as this has me stumpted.
You have a bad injector driver in the ecm. R&R ecm
 
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