What are all the things that can cause a rough idle?

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Thread: What are all the things that can cause a rough idle?

  1. #1
    Dr. Jeff Lange Jeff Lange's Avatar
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    What are all the things that can cause a rough idle?

    I'm just curious about this, what are the most common things to check for if you have a rough idle? I know for a fact I have a busted injector seal on my #1 injector, I'm just wondering if there's anything else I could check for while I'm at it.

    Though I didn't have a rough idle before and AFAIK the only thing that has changed since then is the busted injector seal, but whatever, lol, this could be a good informational thread anyways.

    Couple things I've checked so far, and I've heard to check:

    - ISC
    - TPS
    - Timing
    - Vac/Boost leaks (I replaced a busted accordian hose and a busted PCV hose)
    - Injector seals

    Anything else?

    Also, when I start the car cold, it idles at about 1300-1500rpm for a few minutes until it warms up then drops down to a normal idle, what could be causing this?



    Thanks for the input/help guys, hopefully it'll be a thread that can be searched for later.

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  3. #2
    desufnoC myMUSICveins's Avatar
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    Also, when I start the car cold, it idles at about 1300-1500rpm for a few minutes until it warms up then drops down to a normal idle, what could be causing this?
    I'd say this is normal to an extent. My idle has been off for quite some time now, but it IS supposed to idle high and lower as the engine warms up.

    Im not positive, but I think thats what the ISC is doing. As long as it doesnt idle higher than 1500 when cold, it should be ok.

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  4. #3
    BOOST87
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    I think most car idles like that.. Goes high then lowers when the car starts to warm up..

  5. #4
    Dr. Jeff Lange Jeff Lange's Avatar
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    Yeah I know that, but mine does it for longer than most, and a lot longer than it used to.

  6. #5
    SupraForums Member
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    try the cold start time switch that controls the cold start injector, i think it's mounted on the "christmas tree".

    also, clean your throttle body thoroughly

  7. #6
    wastegate hose is pulled chevyeater-on-sf's Avatar
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    Is the engine taking longer to warm up? I believe the idle speed is ramped inversely with the coolant temperature.

    Other rough idle causes:
    Low compression
    Worn/dirty spark plugs
    Faulty spark plug cables
    Faulty ignition coil(s) or circuitry
    Faulty main injectors or circuitry
    Faulty cold start injector or circuitry
    Faulty O2 sensor or circuitry (believe it or not!)
    Incorrect camshaft timing
    Clogged exhaust

    That is about all I can think of right now, besides the things you mentioned Jeff. You sort of mentioned false air readings with the accordian hose so I left that off my list.

  8. #7
    Dr. Jeff Lange Jeff Lange's Avatar
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    Well I'm pretty sure I already know what's causing my rough idle, just that one injector ring, but just thought I'd ask, everything else seems to be ok, and my idle isn't really that bad, just a little bit rough because it's sitting at about 500-600rpm instead of the usual 750-800rpm.

  9. #8
    londonsupra
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    check the timing belt..

    I was having a rough idle for about 2 weeks.. than suddendly my timing belt blew up.. now its fine and doesnt rough idle anymore..

  10. #9
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    kinda stupid but I've never seen any say it before... How do you check the TPS, ISC, and O2.. I believe I may have a problem with one of them. My idle is iratic from 600rpm-750rpm(watching avc-r) even when its warm.
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  11. #10
    DDDDUUUUUBBBBBZZZZZZZZ suprafly1986's Avatar
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    Water temp sensor, sticky throttle linkage, can also cause.
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  12. #11
    bruggles
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    egr is what my problem was. Also clean the throttle body it is easy and cant hurt.

  13. #12
    JalopySiR
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    My brother's car idled like crap until we pulled a few EGR specific vaccum hoses. Happened alot on old LT1 Caprices.

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