I have a 91 Sedan Deville and it idles great until I turn on the headlights, then it becomes very rough. It has a knocking sound, I have replaced the ISC snd I think I have set it up OK. I have replace the Oxygen sensor, cleaned the throttle body and the egr ports. I am at a loss, no codes. Please Help !
So, the engine idles fine (and I suppose the car runs fine), unless you turn on the headlight, is this correct?
To me, it sounds an electrical problem, or to be specific, bad ground. If I were you, I would make sure the ground is fine, especially one for the computer or the ground wire from the battery to the body.
I just checked the ground on the batt. and cleaned off everything. I checked the voltage and when at idle with lights on I have 12.65 volts, with lights off I have 12.82 volts. When the engine is running rough I give it a little gas and it quites running rough, I knocks and pings when it really gets rough. I am going to check the computer ground after work tonight. Ihope this works, if anyone has any other clues please let me know. Thanks from Florida
I don't post too often, however; I've been having the same battle with my car. It is a well-maintained 1993 Seville with 71500 miles and the 4.9 engine. I too have changed the ISC motor, and performed the idle re-learn procedure and cleaned the TB. Still no joy on the rough idle under load condition. Interestingly, the car wasn't throwing any codes - - until now I am getting PO91 - PRNDL problem. I have been doing a little research and was thinking maybe the computer "brain" doesn't know what gear the car is in. Could this cause a problem with the idle when the car is in gear and a load is placed on the engine; i.e., lights, a/c? Would this cause a problem with the car going into looped and closed looped operation? I'm not a mechanic but I do like fixing cars myself, if possible. Does anyone know if I'm going down the right path here?
Mine throws the po91 every once and awhile , i have no idle problems
For you , you need to check the tightness of the swtich on the tranny int he area where the shift cable connects . the 3 bolts sometimes come loose causeing slop in that switch/sensor confuseing the car ....
on the other car , voltage looks fine to me , but check your grounds , ive also heard for a couple people running an extra ground wire helps
boath cars :when was the last time the fuel filter was changed ?
I checked the computer ground it checked out good, I then removed the power to alternator and cleaned it up, then removed both cable connections on batt. the pos. terminal was really cruddy. I reset my codes on computer and tried it out. Darn it still started running rough after all that. So when it started running bad this time I put it in Park, it smoothed out. I then put it back in drive, it ran smooth for about 1 min. the it became rough again, so I tried park again it got smooth. I then put it back in drive, it idled smooth, I let it idle for 10 min. It ran smooth all that time, I hope it is fixed. I will let you know in a few days. Still do not know what fixed it, but it is great now. Thank everyone for the help and if you know what helped let me know.
Well just got back from a drive of 150 miles, bad news. It idled great going over, but when I stopped to get something to eat when I started to idle rough, I tried putting it in park but no good it keep getting rougher. I gave it some gas and tried again, not good. After about 10 miles it started doing great again. I just do not understand what is going on. Please help, Thanks caddy john
Still having problems with rough idle when car is warmed up, when it starts to become rough it knocks and makes bad noises. I give it some gas and it will smooth out, it only seems to do it when it is driveat highway speeds and then comes to a stop and beins to idle. It takes about 15 sec. after it is at an idle then it begins. I am at a loss any help is needed. Thanks for Florida
It does not make any difference now if the lights are on or off, when it begins to idle rough I put in in park and it continues to get rougher. I give it some gas and it stays rough for about 1-2 seconds, smooths out then I put it back in drive and it usually smoothes out.