Thread: Rough Idle with intermittent idle miss

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  1. #1 Rough Idle with intermittent idle miss 
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    I have a 1999 GTP I purchased about 9 days ago with 114,000 miles, and I've gone through a lot of tune-up procedures and am getting a stumbling/missing at idle both in park and in drive when idling only. So far have have replaced the fuel pump, fuel filter, MAF sensor, spark plugs, wires, cleaned the IAC and PCV with carb cleaner, replaced fuel filter and put synthetic oil with new oil filter. It runs great and gets real good gas mileage but at idle after car warms up and settles down to about 800 rpms, it starts stumbling/missing and varying the idle about 200 rpms.

    I have seen quite abit of other members with rough idles but not sure what else to try to fix it.
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  2. #2 Re: Rough Idle with intermittent idle miss 
    GTP Level Member FriboRage's Avatar
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    I have a freshly cleaned tb, fresh MAF, full seafoam treatment, new spark plugs all around, just replaced all vacuum lines including the booster cap itself and when I start the car hot, within 20 seconds of start up, my aeroforce was showing 26 misses. Didnt get a chance to check out which cylinder(s) were missing.

    I knew it was happening before I got the gauge though cause when I start it hot sometimes (not all the time) I can feel the rough idle and I can hear the intermittant coughing from the exhaust.

    Once I start driving though it all calms down.

    Im stumped, good question, lol.
    --------------------------------------------------------------------------------
    2004 Grand Prix GTP Comp G: Intake, F+R plogs + downpipe, 1.7 YT Rollers, Overkill, F-body front brakes, drilled/slotted r1 concept rotors.
    Sport Edition Kv5 18x8
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  3. #3 Re: Rough Idle with intermittent idle miss 
    I live here. SlowNA06's Avatar
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    Vacuum leak perhaps. Most popular spot is the dry-rotted rubber T off the LIM on the left side, under the supercharger snout.
    Irridium spark plugs last 100k mi and work just as well as copper. Copper is a waste on N/A and only lasts 15k mi. Don't use Platinum.
    Use 195* tstat unless you can thoroughly explain why not; 99.9% don't need a lower temp.
    Almost any oil filter, ever, is of higher quality than ACDelco. Spend $6+.
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  4. #4 Re: Rough Idle with intermittent idle miss 
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    Thanks SlowNA06. I did check the vacuum hoses and did not see anything obvious. The T under the supercharger snout looks like new and I check it and it seems tight. It still could be a vacuum. I disconnected a vacuum hose and the idle raised up a bit. Not sure how to check vacuum to know if it is the cause.
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  5. #5 Re: Rough Idle with intermittent idle miss 
    Turbo is the way to go. Fivefingerdeathpunch's Avatar
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    What plugs and gap?

    SMGPFC Member #1
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  6. #6 Re: Rough Idle with intermittent idle miss 
    I live here. SlowNA06's Avatar
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    Yeah, browser killed my other response. If you're using Bosch, E3, or Champion spark plugs, stop and do something else.
    Irridium spark plugs last 100k mi and work just as well as copper. Copper is a waste on N/A and only lasts 15k mi. Don't use Platinum.
    Use 195* tstat unless you can thoroughly explain why not; 99.9% don't need a lower temp.
    Almost any oil filter, ever, is of higher quality than ACDelco. Spend $6+.
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  7. #7 Re: Rough Idle with intermittent idle miss 
    GTP Level Member KeithGTP03's Avatar
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    Are you modded? Seems modded GP's like to do this. I know mine does, but only at idle. From what I've been told, if its not triggering a blinking SES light, it's nothing to worry about. Since you just purchased it, and you're not sure if it's been modded, post some pics of the engine bay.

    03 GTP...Its got pulleys and stuff.
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  8. #8 Re: Rough Idle with intermittent idle miss 
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    Thanks for the responses. The car is stock, right down to the paper air filter. I will take a picture of the engine after work and attach it. I have the Champion Spark Plugs - 7940's (CCGE double platinum) in it now gapped at .058". I wanted to wait until I do the seafoam engine treatment before I put the Iridium plugs in. I have seafoam in the gas tank right now. I have a scan guage attached to it and it is not throwing any codes.
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  9. #9 Re: Rough Idle with intermittent idle miss 
    Turbo is the way to go. Fivefingerdeathpunch's Avatar
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    Those plugs can surely cause the problem.

    Once you do whatever your going to do, swap the iridiums in there and see whats its like.

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  10. #10 Re: Rough Idle with intermittent idle miss 
    I live here. SlowNA06's Avatar
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    Quote Originally Posted by 99GTPMLA View Post
    I have the Champion Spark Plugs - 7940's (CCGE double platinum
    That brand causes misfires. Congrats, you found your problem (or one of them).

    Platinum plugs cause misfires eventually (durability issue).
    Irridium spark plugs last 100k mi and work just as well as copper. Copper is a waste on N/A and only lasts 15k mi. Don't use Platinum.
    Use 195* tstat unless you can thoroughly explain why not; 99.9% don't need a lower temp.
    Almost any oil filter, ever, is of higher quality than ACDelco. Spend $6+.
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  11. #11 Re: Rough Idle with intermittent idle miss 
    I live here. SlowNA06's Avatar
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    If you're playing by the wallet, seafoam it with Autolite 606 copper cores in. They're <$2 each.
    Irridium spark plugs last 100k mi and work just as well as copper. Copper is a waste on N/A and only lasts 15k mi. Don't use Platinum.
    Use 195* tstat unless you can thoroughly explain why not; 99.9% don't need a lower temp.
    Almost any oil filter, ever, is of higher quality than ACDelco. Spend $6+.
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