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Rough Idle with intermittent idle miss

9

99GTPMLA

New member
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.
 


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.
 
Vacuum leak perhaps. Most popular spot is the dry-rotted rubber T off the LIM on the left side, under the supercharger snout.
 
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.
 
Yeah, browser killed my other response. If you're using Bosch, E3, or Champion spark plugs, stop and do something else.
 


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.
 
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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