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Pontiac Repair/1997 pontiac grand prix gt idle issues


Hi there, I have a 1997 pontiac grand prix gt, 3800 series II engine

Car has around 170k.

Idle is acting funny at times.

Seems to idle smooth, the needle is slightly unsteady, and sometimes its an obvious surge.

The main thing is sometimes after driving when its put in park it revs really high, 2500 or so, if i tap the gas it just bumps it up.

Another time it was stuck around 1500 I...
-checked throttle plate, was not stuck
-unplugged the tps, no change
-cleaned the IAC valve, no changed
- cleaned throttle body, no change
- Cleaned MAF, no change

Now, I am not sure if it was a coincidence or not, but there is a little plug in the air intake tube, Air temp sensor i think I  unplugged that when it was stuck at 1500 rpms and it went right down. But I dont know if that can cause that, or if the car was just being sporadic and it was a coincidence, because this isnt and everyday thing, but often enough.

Do you have any ideas to shed some light on this? Thank you

Hi Kolby:

Is thsi a supercharged 3800 engine or just the basic engine? you said it's a GT so I'm thinking super charged?? If so there is a vacuum leak in one of two places. the gasket between the super charger and intake manifold is sucks in near the front of the engine or the vacuum line that runs to the map/boost sensors is leaking. that line is under supercharger nose and there is a "T" in the line where it attaches to the engine. the line splits there and cuases vacuum leak. If this is a plain 3800 non super charged check on the back side of the engine where the vacuum hose goes to the brake booster. ther is a vacuum line right there that breaks on ome applications and on some there is a vacuum tap there that had a cap on it possably the cap split and fell off and it's sucking air there. The reason for the incresed idle is the engine is ingestion un metered air into the intake. If extra air enters the engine that the MAF doesn't see the engine computer adds more fuel so the idle speed increases. esentially with a vacuum leak it's like you have your foot on the gas to allow more air in but you don'r the vacuum leak is cuaing the same issue.  good luck :-)

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Todd Puccio


General auto repair,electrical issues,driveability issues,anti-lock brakes,SIR systems,HVAC,engine performance,Gm specific. I have worked at my current dealership since the 1990 model year anything much older than that I have limited exposure to. I do not have very much transmission experience so those question you may want to ask of another expert. When you send me a question let me know the exact make/model/engine and year for the car or suv you have it makes it easier to answer the questions that way. If your happy with the answer I gave you please feel free to Donate. This is a free service to you and I am not compensated at all by this web site. I have 22 years of experience to share with you and my knowledge wasn't free to me. I had to pay for my education.


Experience in the area: I have been a technician in GM dealerships for over 22 years and I am a ASE master technician. I work at a Buick and GMC truck store Previously I worked at a Pontiac GMC store for 18 years. I Served a a lead technician when dealership was on a team system. I just recently moved to a Buick,Hummer,Saab Store when my franchise was purchased by this dealership. Organizations: ASE master technician


Education/Credentials:High school diploma,2 year vocational school degree 2 year college degree. Associates degree in applied sciences (auto technology)Awards and Honors: GMC master technician

Awards and Honors
Master GMC truck technician Master ASE technician in 9 categories

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