rpm not stable at idle position

[Partha]

Hi,
I am going through a very common problem that a lot of other gran prix owners are goin thru. When I start my car the rpm bounces up and down (not always...only sometimes) with the inner lights getting dimmer and brighter.

is it the intake gasket? If so... how much do they cost me??

can somebody let me know a.s.a.p

thanks so much


here is the vehicle detail:

yr: 99
model : gt coupe
eng: 3.8 v6

[richtazz]

It sounds like a vacuum leak. Are you losing coolant but see no external leaks? how many miles on car? How long has it been since you tuned it up (plugs, wires, pcv, fuel filter, air filter)?

[Partha]

Originally Posted by richtazz

It sounds like a vacuum leak. Are you losing coolant but see no external leaks? how many miles on car? How long has it been since you tuned it up (plugs, wires, pcv, fuel filter, air filter)?

its got 90 k miles on it and was tuned up last week.

[Partha]

Originally Posted by richtazz

It sounds like a vacuum leak. Are you losing coolant but see no external leaks? how many miles on car? How long has it been since you tuned it up (plugs, wires, pcv, fuel filter, air filter)?

its got 90 k miles on it and was tuned up last week.

[Partha]

I am also getting a bad gas mileage these days... almost like 15 miles per gallon. (may be its due to city driving..i dont know).

does the vacuum leak affect the gas mileage too

plzz let me know.

one more thing, how much will it be to fix a leak if there is one?

[Partha]

Originally Posted by Partha

its got 90 k miles on it and was tuned up last week.

I dont think that the coolant level is goin down.. it seems to be fine.

[richtazz]

The reason I asked about a coolant leak, is that you could have the infamous cracked upper plenum. These cars also had a lower intake issue, that causes a vacuum leak, which in turn causes an irratic idle. Is your check engine light on? If you haven't had the car scanned, do so and post any findings, as there are many things that can cause your symptoms.

[troy1]

Hey Richtazz what the deal with the lower intake on the 99's? My 99 gets about 19 intown which i think is normal but on the hiway its only 23/25 which i think is low but its cold out now.

[richtazz]

Alldata and other repair service info systems suggest replacing the lower intake gaskets (the GM computer acutally suggest replacing the lower manifold itself) when doing an upper plenum job. What happens is with the increase in manifold pressure and the introduction of hydrocarbons caused by the cracked EGR passage and the presence of coolant (from the throttle body coolant passage tubes failing from the heat of the EGR passage problem) can degrade the material of the lower intake gasket causing vacuum leaks. I've done quite a few of these plenum jobs and never needed to deal with the lower, just for info.

[Partha]

Originally Posted by richtazz

The reason I asked about a coolant leak, is that you could have the infamous cracked upper plenum. These cars also had a lower intake issue, that causes a vacuum leak, which in turn causes an irratic idle. Is your check engine light on? If you haven't had the car scanned, do so and post any findings, as there are many things that can cause your symptoms.

no the check engine light is never on and as i said before the bouncing of the rpm occurs only sometimes (lets say once out of 6 starts)

[richtazz]

Without a check engine light on, I would look at the IAC (Idle Air Control) valve in the throttle body for carbon build up both in the bore it rides in and the pintle itself. It sounds as though it is sticking in it's bore just enough to affect the idle quality, but not so much as to trip a check engine light.