Hello, I am the owner of a 1992 Trans Am with an L98. The car ran absolutely flawlessly for several years until my injectors began to fail. I understand this was a common problem, so I decided to replace the injectors. This was at 90,000 miles on the original engine.

The engine was a virgin engine that had not been apart at all until then. While replacing the injectors, I decided to replace most everything else that would be a problem if it went bad. I replaced the EGR valve, solenoid for it, thermostat, and replaced the air temp sensor, both coolant temp sensors, fan switch sensor, and knock sensor. I decided to go even further and replaced the Idle Air Control valve and clean out the intake of any deposits, replaced the TPS, and even the MAP sensor. Because I wanted no problems, and because I had a few good connections at a GM/ACDelco dealer, I used all OEM parts.

I'm no stranger to mechanics, and found most of this fairly easy. I went ahead and also replaced the plugs, wires, coil, and got a brand new distributor. Basically everything electronic on the engine is brand new.

However, when I began to set the timing and start the engine up, a serious problem made itself very clear. I had the engine perfectly timed at 6 degrees, and everything is hooked up correctly, I have checked it dozens and dozens of times at this point. The engine runs perfectly with the EST bypass connector unhooked, but the moment it is hooked up, the engine runs extremely rough, pinging and will neither idle, nor will it go above 2000 rpms, and dies within 20-30 seconds no matter what.

I put the old pickup and ignition control module back in, and it made basically no difference. Stumped, I folded and drove it to a dealership, who told me to replace the whole distributor. They didn't really know what was wrong. I have taken it to two other places, but they are all quite stumped on this problem, as am I.

Could my ECM have gone bad while sitting for a couple months? It was in a nice, heated and air conditioned shop/garage, and it did run perfectly before...I am at the limits of my knowledge here, and am not sure exactly how to troubleshoot this problem anymore. Does anyone have any advice on how to solve this problem?

 

check the distributor shaft they had problems with them.

 

put your old distributor back in

 

I know this isn't really helpful, but shotgunning parts is never a good idea. Especially when most of them are aftermarket and of unknown quality.

That said, try plugging the EGR vacuum line and see if it helps.

 

Does your timing stay at 6 degrees when you recheck it? Have you tried other settings?

You know the old adage, "If it ain't broke, don't fix it?" I wonder if coincidentally while you were changing all these other parts the distributor itself suffered some failure.

No check engine lights? Does the check engine light flash 12 when you start the car?

 

Alright, I did forget to mention some things. I also resealed the intake manifold with new gaskets, and the valve covers, the car has no vacuum leaks.

antoine. What exactly do you mean they had problems with the distributor shafts in these cars? A new distributor can easily be defective? The pickup coil can not work correctly because of the mechanical portion of the distributor?

Chevy, I sort of did this, I put the old ignition control module back on fearing the current one was defective and there was no change. I will try putting the old distributor as a complete assembly back in. The only difference will be the coil as far as I can see, can the mechanical part of a new distributor affect it in this way?

jv These parts were not aftermarket parts of questionable quality, these were all new OEM parts, and as far as I can see, all of them are working well, especially the injectors. Most, if not all, of the parts appear to be the exact same as the ones I removed from the car, right down to casting/part numbers. I did not want to have to tear apart my engine again anytime soon, so I replaced many of the parts at once to keep anything else from hopefully happening in the near future. I will try plugging the EGR vacuum as well.

rezin Timing stays at 6 when EST is disconnected, but it appears the computer is trying to advance the timing when it is connected and not recieving all of the information it needs, so something is defective. With the EST connected, I did get a check engine light once, but now it just dies with no engine light. I will have to check more into it.

Thank you everyone for the suggestions, I will try what I can tomorrow, and post the results.

 

Alright, status report.

I put the entire old distributor in, which worked well before it seems. This part has been sitting in the garage under a few rags, and hasn't been touched for a month or so. I find it hard to believe anything has gone bad in that time in those conditions. Anyway, I put it in, and there was some improvement. When the car is timed correctly, or very close to correctly, I can now accelerate the engine through the rpms with no sputtering, no misfiring, etc. It accelerates smoothly, but at idle there is still a major problem. The engine runs rather rough at times, and at first seems to idle fine, with some slight play, and then starts surging between 500-1500 rpm, getting progressively worse until it eventually dies. This happens every time. Unplugging the EST bypass again solves the surging idle completely.

Checked the EGR vacuum, no vacuum at idle like it should, EGR ports on the manifold are all cleaned out, the valve is new and was working before I put it on the car.

No check engine lights, I'm getting a clear and continuous code 12.

It appears the problem is still isolated to whatever components the computer's spark timing is drawing information from. I'm not totally sure the distributor can be totally ruled out...but putting the old one in to see some improvement seems to really suggest there is something else at work here. Are there any other components the computer will use the information from for the electronic timing? Knock sensor? Or will this problem be isolated to the distributor?

 

Anyone have any ideas?

 

Have you put a vacuum gauge on it? Statements like "I have no vacuum leaks" often come back to bite you (or me anyway). :-)

With the EST connected, the ECM is trying to advance the timing at idle. When it does, the RPM falters so it retards it. Then repeat. The question is why does it falter? Vacuum leaks (including a leaking EGR) are good usually good guesses.

A bad distributor is another common cause. Put a timing light on it and watch the flash with the EST disconnected. Then connect it and see if it starts to miss a beat now and then. You can have two bad parts.

Good luck!

 

Hello all, it turns out that my main problem was a grounding wire for the ECM not doing its job. I've been chasing this problem for weeks, and of course, it is something small and hard to find, but easy to fix.

Thanks to everyone who had their own suggestions!

 

Can you share more detail as to what you did? I'm having a similar problem

 

I tracked down someone with much better diagnostic and scanning equipment than I have access to at home, who was very familiar with TPI motors. The ECM was recieving a bad signal because it was not grounding, as there was corrosion on a grounding wire attached to the body. Cleaning that solved my problem, but I am afraid that in this sort of situation it could be a lot of other things as well. But if the car runs perfectly with the EST disconnected when timed correctly, then it has to be isolated to either the ignition control module, the pickup coil, the ECM itself, or any of the wires associated with them.

I hope that at least helps you in searching for the culprit of your problems, some of these little gremlins can be a huge pain to track down.

 

I have the same problem right now, it runs great with unplugged but when i reconnect it the rpms flucuate then stalls. Ive already tried the ecm, dist module, knock module, and coil. So in the morn im gonna change the distributor and the knock sensor. But if this dont fix it id like toknow where the ground wire that was corroded on yours is located.