LannyL81

'95 LT1, 6 spd.
Problem: When HOT, engine will cut-out, recover, then cut-out again, like it turns off and back on. First time lasts only a few seconds, then will be fine for several minutes, then happens again...a bit longer each time. Putting car into netural during this and engine speed will vary all over the place, go from almost dieing to almost redline. While this is happening, engine has very little power. Letting engine cool-down over night or for at least several hours, it will be fine until HOT again.

parts replaced: Dec 2006: cap and rotor with GM parts, did not not replace the entire Opti-Spark, plugs, wires, water pump, front cover seals, oil pan gasket (don't ask).
July 2008: igntion module, igntion coil, had ECM rebuilt by Auto Computer Exchange (I hope they actually rebuilt it...cannot tell)

I am at a loss here....can the Opti-Spark optical sensor be heat sensitive and cause this problem? I did not think so, which is why I pulled the ECM out for repair. The Service Engine Soon light is not on, never has been.

Thoughts, comments...HELP

Lanny

MikeC4

I had similar symptoms as you, just not quite as severe. My engine would start "lurching" and "bucking" at idle only after it was hot. Turned out my bearings were shot in my WP and leaking coolant on my Optispark. I noticed my symptoms were getting worse, so I only drove the car 2 days before I stopped driving it and diagnosed the leaking WP.

When your engine is cool reach under your WP and feel all around for coolant leak and/or oil leaks.

Here is what I found when I removed the WP and Opti off my '94 :

http://forums.corvetteforum.com/show...903&highlight=

Redeasysport

Codes?

corvette-pilot

When my WP leaked onto my opti the same thing happened to me. Ran ok for a mile or 2 and once warmed up knocked and bucked. Replaced the opti and problem went away

steve40th

Whay was the cap and rotor replaced. Was there a problem before. The Optical sensor and bearing within the Opti can go bad.
Here is my Opti Sensor. Maybe a little extreme, but it shows that things do go bad over time. Mine was sealed with the Right Stuff, so I have no idea how corrosion got in there.

pcolt94

Your car uses a PCM not a ECM. There is a big difference although they do a lot of the same functions. A major difference is ECM are known for all kinds of weird run ability problems and PCMs are not. Of course it could be a problem, you can never completely eliminate it.

I would think if the optical sensor which provides the low resolution pulse is sensitive from heat, it would not cause a RPM problem but a complete shutdown because if the pulse becomes insufficient to drive the PCM, it would not clock the pulses for the injectors and provide the pulse for the ICM. Having said that, nothing is an absolute and its just the way I view it.

I would check for any codes because if you really lost the opti pulses, it should set a code for low resolution. Loss of high resolution pulse will not kill the engine.

I would also not eliminate the fuel system. I would use a gauge and check the pressure while I had the problem. You have already replaced coil and ICM and cap. Make sure you have really an electrical or ignition problem.

Trying to isolate a bad PCM can be tricky especially if it reacts to temperature. Some people have wrapped it, put ice bags on it and probably anything else you can think of to affect its behavior and determine if it is bad.

I would also use a nod light on the injectors to see if you are loosing the drive to it.

Also checking the output of the PCM which drives the ICM would be a great clue also. Pin “B” white wire of ICM has this pulse on it. Remove ICM connector and see if there is between 1 and 3 volts of AC voltage at that point when you crank the engine in the “no run condition”. You can use a normal DVM on the AC scale.

Collect data, what works, what doesn’t, make a picture to help solve the problem.

LannyL81

The Service Engine Soon light has never been lite, so I assume no codes have been set. I have a code reader, but not a scanner.


I replaced the cap, rotor, wire, plugs, water pump, front cover seals, oil pan gasket (don't ask) when I got the car in Dec 2006 as it was overheating. Also replaced the radiator (twice).

Will check fuel pressure, but how could low fuel pressure cause the engine RPM to increase?

Redeasysport

Never assume on codes Check them.

schrade

Some ignition circuit faults will not light up the SES light.

Jump the ALDL connector, and make sure no codes are stored.

gigs94c4

I was having the same problem, Just fixed it. It was the optisaprk. Water pump had been leaking onto it causing the drive bearing to go bad in the optispark. Replaced it the pump and all the wires. Now the car is feeling much better

LannyL81

I was planning on using my code reader....should do the same correct?

LannyL81

Oh man...if that new water pump has failed in 1.5 years!!!!


Checked ebay for OptiSpark units....wow...lots of choices and a wide price range. I assume that the lower price ones are junk, even though they say they are GM parts right?
I went ahead and ordered one from The Parts Lady....hope it is a good one.

pcolt94

Make sure your code reader is OBD1. And a scanner is not really needed because the on board diagnostics works very well and is clear as to what module you are in. Codes can definitly be set with out the SES light coming on.

OK - your RPM redline question. How does it go that high, are you stepping on the gas, or is it by itself. Of course fuel pressure will not increase the RPM if gas is not touched, but just want to know if pressure is OK so it can be eliminated as a possible factor.

If RPM goes up by itself, I would check into the TPS (throttle position sensor) to make sure it is working correctly and adjusts linear across its range (about 0 to 5 volts out on the wiper).

Redeasysport

Oh geeze lets throw money at it first A good diagnostic SHOULD be done before buying parts. Too many guys around here lately throw parts at the problem before they even know what it is and just create more problems. It is your money I guess but I like to fix cars not toss money at them.

A dying opti will not make a car go to redline in neutral.

LannyL81

[QUOTE=pcolt94;1566625946]Make sure your code reader is OBD1. And a scanner is not really needed because the on board diagnostics works very well and is clear as to what module you are in. Codes can definitly be set with out the SES light coming on.

OK - your RPM redline question. How does it go that high, are you stepping on the gas, or is it by itself. Of course fuel pressure will not increase the RPM if gas is not touched, but just want to know if pressure is OK so it can be eliminated as a possible factor.

If RPM goes up by itself, I would check into the TPS (throttle position sensor) to make sure it is working correctly and adjusts linear across its range (about 0 to 5 volts out on the wiper).


My code reader will do OBD I and II...so no problem there.

And the RPM increase is all by itself.

I had not thought of the TPS....good idea. I will pull it off and get the wife's hair dryer on it and check the resistance range. But if the TPS opens or quickly changes value, would this not set a code?

I agree that determining the problem is the only way to fix something...not throwing new parts at it. I took the ignition module and coil off my '96 to try on the '95...found out that the coil did not use the same connector though. I ordered a new OptiSpark as the 96 has not been replaced, so having a spare is not a bad idea...a bit expensive though....

corvette-pilot

I found the same thing. I think most of them are made in China junk. I got mine from The Patrs Lady. She seems to have great feedback and the parts were in OEM boxes

schrade

Yes - paper clip will give all codes for '94. 4rth pin socket from left top, and 4rth pin from left bottom.

I don't think you should have bought a new opti yet, before doing this test that colt said... [edited for FSM spec]


The schematic is in this thread somewhere, with plenty of pics
http://forums.corvetteforum.com/show....php?t=2069247

Do the code check first...

Redeasysport

http://www.corvettebuyers.com/c4vettes/codes.htm

LannyL81

Remember this is on a 95 not a 94, so there really is not an ALDC, but there is an OBDII type of connector. I know that I can still jumper the two pins and go into diagnostic mode....if I could only find that thread I had printed out about doing this...????

Redeasysport

I just gave you the link to do it with.