Rough Idle

caddy john · #1 (**permalink**) 09-07-04, 06:26 PM

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 !

100 · #2 (**permalink**) 09-07-04, 08:28 PM

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.

Good luck.

Stoneage_Caddy · #3 (**permalink**) 09-07-04, 11:42 PM

how many volts are at the alternator(or battery) with the headlamps on ?

if its not a mid 12 to low 13 volts they sometimes will run rough , not positive on why but weak volts/amps sometime cause rough running

CadiJeff · #4 (**permalink**) 09-08-04, 01:52 AM

I would recomend checking the batt and alternator sounds like a low volt situation to me.

caddy john · #5 (**permalink**) 09-08-04, 11:11 AM

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

maclugie · #6 (**permalink**) 09-08-04, 11:28 AM

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?

Stoneage_Caddy · #7 (**permalink**) 09-09-04, 12:15 AM

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 ?

maclugie · #8 (**permalink**) 09-09-04, 08:42 AM

I changed the fuel filter last year - 15000 miles ago.

caddy john · #9 (**permalink**) 09-09-04, 09:42 AM

I changed my fuel filter 1100 miles ago, I am going to run another ground today and also make sure the gound on the computer is ok. Will let you know later today, I hope it works. Thanks caddy john

caddy john · #10 (**permalink**) 09-09-04, 12:47 PM

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.

caddy john · #11 (**permalink**) 09-09-04, 08:18 PM

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

caddy john · #12 (**permalink**) 09-11-04, 10:03 AM

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

Stoneage_Caddy · #13 (**permalink**) 09-11-04, 12:41 PM

does it still only do it with the headlamps on ?

100 · #14 (**permalink**) 09-11-04, 04:09 PM

It happens when:

all these 3 conditions are met,
(1) in "D"
(2) but idle (no throttle)
(3) the headlight is on.

Am I right?

caddy john · #15 (**permalink**) 09-12-04, 09:35 AM

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.