Rough Idle with no Codes 2005 V8 STS
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Cadillac STS Forum - 2005 through 2012 Discussion, Rough Idle with no Codes 2005 V8 STS in Past Cadillac Vehicle Discussion; I get a rough idle while the car is in gear and in idle (not as bad when not in ...
  1. #1
    danscan is offline Cadillac Owners Member
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    Rough Idle with no Codes 2005 V8 STS

    I get a rough idle while the car is in gear and in idle (not as bad when not in gear). I think the idle is too low personally but there is no screw to turn out to raise it (old school).
    The car has 125K miles.
    I replaced the spark plugs at 110. they were not AcDelco.
    It has been run on regular for a long time (I just got it back from the wife).

    What could be the cause and solutions. Cheapest to most expensive. I have heard EGR,Throttle body, MAF, and Idle control unit (IAC).

    Also there are no codes being thrown according to the ODBII reader I have.

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    Carvone is offline Cadillac Owners Fanatic
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    Re: Rough Idle with no Codes 2005 V8 STS

    I would try in this order...Clean throttle body and MAF, check/new air filter, check/tighten intake bolts. If no change then look into fuel filter then spark plugs (but if these plugs have been fine for the last 15K then probably not the problem.) Also check plug wires and boots. If still no change then you have bigger problems.

  4. #3
    C&C's Avatar
    C&C
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    Re: Rough Idle with no Codes 2005 V8 STS

    Could also be a broken motor-mount.

  5. #4
    MoistCabbage's Avatar
    MoistCabbage is offline Cadillac Owners 10000+ Posts
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    Motor mounts have absolutely nothing to do with a rough idle.

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    1BadCadSTS's Avatar
    1BadCadSTS is offline Cadillac Owners Connoisseur
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    Another vote motor mounts. If the car was running rough it would throw a lean/rich/misfire code. A blown mount will give a bad vibration and seem like a misfire issue.

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    orange57's Avatar
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    Another vote for mounts. If they are bad it could just be transmitting more vibration to the rest of my car.

  8. #7
    EChas3's Avatar
    EChas3 is offline Cadillac Owners Master
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    Re: Rough Idle with no Codes 2005 V8 STS

    Intake manifold bolts or if they were over-tightened, damaged gaskets/intake. The rest would be noticed or throw codes.

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    Re: Rough Idle with no Codes 2005 V8 STS

    Quote Originally Posted by MoistCabbage View Post
    Motor mounts have absolutely nothing to do with a rough idle.
    So, what was my clue; well, 'idle feels rougher in gear' (load of transmission/drivetrain) putting more pressure on mounts. It still may not be the mounts, but with no engine codes, mounts do needs to be eliminated (as cause).

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    orange57's Avatar
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    I believe the mounts are actually considered hydraulic mounts, even though they are just big rubber blocks.

    I had bad mounts on an old car, you could feel more engine vibration. Pretty much all of it went away after replacement.

    Although if they are good, intake bolts tend to loosen themselves, really easy to check (as others have said)

  11. #10
    MoistCabbage's Avatar
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    Felling vibrations from the engine, and a "rough idle", are two completely different things. A "rough idle" IS NOT caused by motor mounts. The OP will have to clarify which it is.

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    orange57's Avatar
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    Re: Rough Idle with no Codes 2005 V8 STS

    Quote Originally Posted by MoistCabbage View Post
    Felling vibrations from the engine, and a "rough idle", are two completely different things. A "rough idle" IS NOT caused by motor mounts. The OP will have to clarify which it is.
    Good point. Didn't think of it that way. I could see how one could confuse one for the other though as well.

  13. #12
    danscan is offline Cadillac Owners Member
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    Re: Rough Idle with no Codes 2005 V8 STS

    Update. I did the following: Used Carb Cleaner on the the Throttle body. Used Electrical Cleaner on the MAF. Put in a new Air Filter (needed it no matter what). I also put Sea Foam in the almost full gas tank. The shutter seems to have calmed down but the air temp was 57 and I will have to see how it does later today when the temp goes up again. It is not perfect and I still get 25% of the vibration I had earlier. Although it is not embarrassing now.

    To clarify for everyone the car was shaking noticeably while in drive and at idle. It did not sound like it was missing or misfiring. When I would take it out of gear it would lessen. When I press the gas to get the idle to 800 rpm line it smoothed out. Tested with my foot on the brake and in gear also and had a similar result (seemed to become smoother).

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    1BadCadSTS's Avatar
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    Re: Rough Idle with no Codes 2005 V8 STS

    It's pretty easy to check for a broken motor mount (though it may not always find a bad one and you need two people); with the engine on and in gear (with the brake firmer depressed), rev the engine and check for the engine to 'torque'(twist) more than what would seem normal (appropriate). The second man, of course is doing the looking (and you don't need to rev the engine too high, just enough to put some load on the mounts).

    Again, it may not be the mount(s) (but easy to check); best of luck in your diagnosis.

  16. #15
    danscan is offline Cadillac Owners Member
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    Re: Rough Idle with no Codes 2005 V8 STS

    I just wanted to let everyone know it was a motor mount. I will be looking to replace it soon. Any tips?

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