1995 Pontiac Grand Prix GTP 3.4 DOHC Question

[GTPWarrior88]

I own a 1995 Pontiac Grand Prix GTP 3.4 DOHC with 71,000 miles on it. I am the second owner of the car. When the car starts up in the morning, it shakes like crazy because the idling is very bad. It goes up to 3k RPMS, then down t 500 RPMS, then searches after that. My service engine light only comes on it the morning, or when I drive for a long period of time. I replaced spark plugs, wire, intake manifold gasket, plenum gasket, cleaned the IAC too. I know its not the intake gaskets because I checked them. What could be the problem to fix the idling..my friend's thinking the Throttle Position Sensor...but I'm not sure...please help. Thanks!

[kustomkid54]

Originally Posted by GTPWarrior88

I own a 1995 Pontiac Grand Prix GTP 3.4 DOHC with 71,000 miles on it. I am the second owner of the car. When the car starts up in the morning, it shakes like crazy because the idling is very bad. It goes up to 3k RPMS, then down t 500 RPMS, then searches after that. My service engine light only comes on it the morning, or when I drive for a long period of time. I replaced spark plugs, wire, intake manifold gasket, plenum gasket, cleaned the IAC too. I know its not the intake gaskets because I checked them. What could be the problem to fix the idling..my friend's thinking the Throttle Position Sensor...but I'm not sure...please help. Thanks!

Seen your listing, thought you were Jeff (GTP WARRIOR). That's too close. We need to find you a new name.

[Imperator]

you should check out the coolant temperature sensor, as coolant temperature effects the idle speed!

[jon95se]

coolant temp sensor, idle air controlller, throttle position sensor. i would check out all of those.

[GTPWarrior]

No kid 88 isn't me I should patent the name so I can get royalties off of it.

[auxbar]

I have a 1994 3.4 DOHC GTP and it does EXACTLY the same thing! I have no idea what the problem could be. I do not get a Service Engine Soon light with mine, so there are not computer codes to check. You should get the codes from your ECM and that will point you in the right direction. I changed my IAC, and that was not any help. One thing about mine, it really started doing it bad after I did an oil change and added Slick-50 to the oil. Anybody have any experience (good or bad) with that product? After I drove the car about 500 or 600 miles after the oil change, the idle problem subsided, but did not go away.

Looks like we are not the only ones with this problem. The general concensus seems to be intake gaskets. How are you sure that is not your problem? How did you check yours? I tried to check mine with the carb. cleaner trick, and mine seemed to check out ok too, but I could only do the front ones with this trick. I could not access the rear one.

Let me know what you come up with, and I'll do the same. Maybe between the two of us we can solve this one.

[GTPWarrior88]

Thanks to everybody who tried to help. I found out that the problem was a bad O2 Sensor. Thanks again!

[IH8RICE]

its prolly intake gaskets 3.4's have always had a problem with this due to the styler of heads a DOHC uses i also own a 95 gtp with 144k miles it has a new tranny and a stage 3 b&m shift kit its actually pretty quick. and people tell me the 3.4 is junk!!! its all how u maintain it. change the intak gaskets and u should be ok.