Rough Idle
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HT4100 4.1, 4.5, 4.9 Discussion, Rough Idle in Cadillac Engine Technical Discussion; I have a 91 Sedan Deville and it idles great until I turn on the headlights, then it becomes very ...
  1. #1
    caddy john is offline Cadillac Owners Member
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    Rough Idle

    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 !

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    100
    100 is offline Cadillac Owners Enthusiast
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    Re: Rough Idle

    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.

  4. #3
    Stoneage_Caddy's Avatar
    Stoneage_Caddy is offline Cadillac Owners 10000+ Posts
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    Re: Rough Idle

    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

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    CadiJeff's Avatar
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    Re: Rough Idle

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

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    caddy john is offline Cadillac Owners Member
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    Re: Rough Idle

    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

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    Re: Rough Idle

    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?

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    Re: Rough Idle

    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 ?

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    maclugie's Avatar
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    Re: Rough Idle

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

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    caddy john is offline Cadillac Owners Member
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    Re: Rough Idle

    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

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    caddy john is offline Cadillac Owners Member
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    Re: Rough Idle

    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.

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    caddy john is offline Cadillac Owners Member
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    Re: Rough Idle

    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

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    caddy john is offline Cadillac Owners Member
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    Re: Rough Idle

    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

  14. #13
    Stoneage_Caddy's Avatar
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    Re: Rough Idle

    does it still only do it with the headlamps on ?

  15. #14
    100
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    Re: Rough Idle

    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?

  16. #15
    caddy john is offline Cadillac Owners Member
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    Re: Rough Idle

    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.

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