Thread: rough idle

as soon as i start my grand prix 3800 non supercharged the idle goes up and down what could it be

could me many things spark plugs vacume leak intake inconsistancy anything get it scanned at a auto store

Mine is doing the same thing. I tested my spark plug wires and found that they are on their way out. I'm changing the plugs and wires soon. I got some ngk platinums and some champion wires for it.

If you test the plug wires, they should have at least about 6000 ohms of resistance.

i have new plugs and wires, i did them 2 months ago new mass air new filter no vacuum leaks and a new intake

Coil? I'd like to know what fixes it. Might be the same thing with mine. My car actually cut out one time last week.

my car cut out once before too lol its driving me nuts i checked for a vacuum leak didnt find anything

i just checked the vacuum lines they are good its so weird the rough idle doesn't happen ever start

lol its not my gtx its my gt

Originally Posted by stanggreg90

as soon as i start my grand prix 3800 non supercharged the idle goes up and down what could it be

Did you try removing the oil filler cap with the engine running. It sounds goofy, I know. I have a similar problem on my 2000 gt. I changed the UIM recently also and the engine started making this awful squealing noise once it ran for about 10 min. At higher rpm 2500 or so noise will stop.

At first I thought it was something in the trans, but when my neighbor and I were trying to tell which pulley was squealing he happened to remove the oil cap. There was a TON of suction(vacuum) inside the engine. It would actually stall the engine if you removed it too fast.

As soon as you broke the vacuum on the cap the noise would disappear. I think something on a vacuum line is clogged, or a bad head gasket.

White:
It sounds like the PCV valve wasn't installed, or is internally failed open. The PCV is supposed to allow a *slight* vacuum on the crank case... without the valve, at idle you'd have whatever vacuum your engine creates in the crank case... that makes for increased oil consumption, besides the noise you observed.

stanggreg90:

did you hook a vacuum gage up to the dipstick tube and see if it's steady or bouncing? That's really the best (least dangerous) way to check. if you have wild fluctuations in your reading, you probably have a leak. Just because your vacuum lines are in good shape does not rule out an UIM leak, or worse a LIM leak. another method you could use to check for a leak is the propane method. if you have a propane torch, let some gas go (NOT BURNING) around the intake while the engine is doing it's fluctuating. When you get close to the leak with the nozzle, the idle should even out.

be very careful with the propane bottle! Any time raw fuel is unleashed under the hood can end with disastrous consequences. That said, propane is safer to use than starter fluid, carb cleaner, or (shudder) raw gasoline, all of which will achieve the same thing.

Good luck!

Moips,
Thanks for the info. I thought I transferred/installed everything properly, but must have missed something. Will take a look later. Thanks again.

It could be a clogged injector also... trust me I just got done replacing mine and it made a huge difference. Does it only idle roughly or do you notice anything during driving? I would also check coil packs, possibly could just need some cleaning often slight corrosion build up on it could cause issues too..

I had the same problem on my 200 GP GT. was a combination of the PCV valve and the Large vaccuum line on the rear side of the upper intake.