My 92 stock lt1 had developed a pretty bad miss. I figured it was the optispark, so i replaced it myself this week. At first the car wouldnt start. I let it sit overnight and in the morning it fired right up, but died after a minute or two. Now the car will idle but has the same miss it did before, stumbles horribly while goin down the road and gets worse with more throttle. I put it on the code reader at the parts store and got these 3 codes:

26 QDM fault

55 Lean Fuel Monitor

64 Right Oxygen sensor

What do these mean and what is most likely causing the miss? The only thing i can think of thats left is injectors or fuel pump? Ive already wasted money and i dont wanna waste more chasing the problem. Any help appreciated.

 

Code 26 is a quad driver code. The quad driver is part of the ECM and is what cycles the injectors off and on. I'd start by checking all the connections that you disconnected during the opti swap. Also check engine grounds.

 

so that means the problem could possibly all be in the computer?
If all my connections are good i mean. I dont think it would be connections or grounds, because the engine was missing before i tampered with anything or took the opti out.

 

Sounds like a bad ECM. You can listen to the injectors with a mechanics stethoscope. The one(s) that aren't clicking aren't working. If the wiring from the ECM to the injecstor is good the ECM is bad.

 

im dreading the answer to this but what kind of price am i looking at for an ecm? and where can i get one? just from General Motors?

 

Not for a '92 ;Rare as the proverbial.
Only exchange rebuilt on yours most likely

http://www.autopart.com/ecc/ECC1992C...ETCORVETTE.htm

 

Another source: www.autoecms.com

 

Definitely not the optispark with those codes.

 

Check your fuel pressure, that could explain the lean fuel code.

 

I cannot imagine a true ECM failure. These things are tough as hell. A friend services hundreds of Corvette (C4s included) and has only replaced two. One car was hit by lightning and the second was in a wreck and the ECM was crushed. I don't have my 93 manual front of me but I would hazard a guess that you might have a vacuum leak.

As an engineer, cause and effect are critical thought processes cause one to not troubleshoot with ones wallet. You removed the water pump and the OptiSpark and all the associated stuff around it. Start with what you know you did and don't buy trouble by expanding your quest to include the car from bumper to bumper.

For diagnostics, I use TTS Datamaster software to see what the O2 sensors are seeing in relative real time. It also IDs codes that occur. This software is free to download and can be used for I think 20 times before they require you pay for a license. Great POWERFUL application. I have only used probably 30% of it's capability so far.You do need an ALDL cable to connect to the data port under the dash and the the USB port on your computer. The cable will cost you about $60 and takes a few days to the there.

A gross vacuum leak (or a series of small one) would certainly explain the lean O2 codes and the inability of the ECM to compensate by driving the injectors "on time" sufficiently long to satisfy the EMCs need for a certain DC voltage value supplied to it by the O2 sensors.

In the interim I would use a vacuum gauge to see what baseline vacuum you are at and start looking at hoses and fittings and improvements as you go. I would insert a T into a vacuum line and leave the gauge hooked up and mess with the vacuum hoses and look for changes. It should probably read in the neighborhood of 18-20 inches of vacuum when running good with adding to those values driven by age of engine, cam type etc. But that ought to get you close.

 

Suggest you follow the diagnoses detailed in the FSM for the codes you got. As you have found, it's not a good idea to replace parts at random. IMHO, the FSM is worth owning, whether you use it yourself or show it to the person working on your car who may not have the specific info that's in there.

Diagnosis and repair is not voodoo or a roll of the dice, but logic plus some understanding of how the various systems work.

 

Vacuum leaks or fuel pressure problems will not cause a quad driver code. ECM problems are rare, but quad driver failure does happen.

 

Read the OPs post.

He had the miss first, changed the distributor, and still has the same thing.

Not firing one bank of injectors will definitely cause a lean condition on that side.

Pull the computer out and have it tested.

 

We reman 2-3 92-93 ECM's per week and , AFAIK, are the only company that has the parts to fix them and the proper test equipment to test them. GMAutocomputers.com770-777-1031 EST

 

took it to the shop. fuel pressure is fine and no vacuum leaks. they think i got a couple injectors that arent spraying well. they are gonna do a fuel injection cleaning. whatever that is.

 

Have they checked the injector connectors with a noid light?

 

I'm not a technician, but had a similar set of symptoms. Car would start OK, but after warm up to operating temp (a good heat sink but not overheated a all) it would start to miss, then buck and misfire badly. I disconnected the oxygen sensors as a crude test, and the miss went away. It idled a little rough, but at full throttle on an autocross it ran fine. A quick way to cheaply / crudely test it, and if that is the problem, a cheap way out. I also had replaced the opti beforehand.

 

The 02 sensors are coming up a lot as opti issues, I would say they are in a dead head with damaged plug wires. Things to keep in mind for those searching through these threads with issues.

 

Is the car "missing" &/or misfiring at idle?If it is disconnect each injector while car is running.Engine idle should change the same with each injector being unplugged.If no change is noted on that specific cylinder then that is the problematic cylinder.Next step is to check for spark,injector pulse,ohm injector & do a compression test on that cylinder.Also check to verify fuel pressure is within spec.You can also put a screwdriver on that body of injectors & put ear to end of handle & listen for the injector clicking/Keep us posted on what you find