I have a 1992 Grand Prix that is having rough idle problems. I will try
the best I know how to describe what is happening and what we have had
The car runs what seems to be fine when traveling down the road. The problem appears when it is idling or traveling at low speeds when coming to a stop. It occurs when in either Park, Neutral or drive.
The car was taken to a local shop and they informed us that the #4 injector was bad. I will try and explain what I was told that he did to determine that the injector was bad. He took a hose off the engine and sprayed some type of flammable liquid in it and the engine ran smooth. He then proceeded to pull each indivudal plug wire off. There was a noticeable difference in the way the engine ran with each wire he unplugged until he came to plug #4. When he came to wire #4 then there was no difference.
He suggested an injector flush. As the car is getting quite old and has quite a few miles on it I didn't want to spend a lot of money on it. So taking their word we had the injector flush done and it did not help at all. The mechanic then informed us that replacing the injector would cost between $300.00 and $400.00 dollars. I did some further checking and the going rate at the other shops was right around $188.00 dollars to replace one injector.
We took the car to a different shop and had the #4 injector replaced and the problem is absolutely no better. The car still vibrates (pulses, shakes) when it is idling and it sounds like it is missing.
The mechanic said that the compression was checked and is OK
The ignition system was checked and was ok
But it still is very rough when idling
At road speeds it runs fine
The service engine soon lite has been coming on and going off intermittently after the injector flush was done
Neither mechanic referred to any codes
The plug wires have never been changed but the mechanic said that they were OK and didn't need to be replaced. I personally don't know much about vehicles, but with over 100K miles on them, should they have been changed. He also said the plugs have been changed and that they look fine.
I hope I have given you enough information to possibly enable you to give me some suggestions for repair for this problem
I greatly appreciate your help.
I don't believe the part about the ignition system checking out OK. I believe thats where your problem is. Over 100K and the wires SHOULD be changed.