jumping idle problem

[stock_am]

its a 1999 grand am with th 3400. when i start the car in the morning or when its cold the idle jumps up and down. i cleaned the throttle body but there is no difference. any help would be great. thanks

[rixGAphx]

'99, 8 yrs old, about 100k miles/160,000km?

How long has this been happening?
* Several months?
* Recent, with advent of very cold weather?

Other than the TB cleaning (as an attempt to rectify this condition), has any other work been done on the car recently?
* When a problem is encountered, I go back to the last thing that was done to the car (even if it's in another part of the car or totally unrelated); often a tube or electrical connection was disturbed, causing the 'new' problem.
Are there any other problems with the car, like electrical or HVAC, that you haven't mentioned?

I *assume* the SES/'Check Engine' icon isn't lighted, or you would have said so.
* * *
All major sensors/systems of the ignition/fuel injection/emissions will set a Diagnostic Trouble Code (DTC) and illuminate the SES icon, except:
* Fuel pressure,
* Vacuum leak,
* Obstruction in the intake.

Check the air filter element; when dirty, it really blocks the smooth flow of air.
* At idle, with small volume of air being drawn, the effect is very noticable.
* At higher engine speeds, the restriction is still present but it isn't as noticable to you or the engine management.
When the engine is cold (under 140*F, the bottom edge of the 'OK' range on the gage), it is on 'open-loop' mode where the computer ignores the O2 sensor.
When the engine warms, the computer switches to 'closed-loop' mode and starts responding to all the sensors. The O2 sensor then tells the 'puter about the richness/leanness of the exhaust, and the 'puter balances the air-fuel mixture to accomodate the restricted intake air.

Same with a vacuum leak: Most pronounced at idle, and when in 'open-loop' (cold) mode.
There are many vac lines, connections, and devices.
* There's a decal on the passenger front strut tower that shows the EMISSIONS vacuum tubing and devices. These are NOT your problem, since a defect in this system will ALWAYS illuminate the SES icon.
* At the back of the intake manifold is a small tube that provides vac to the HVAC controls; it runs to the vac reservoir (small black plastic grapefruit-shaped ball, under the battery).
* There's also a 5/8" black hose from the intake manifold to the brake booster.

Check the vac system visually, and spray some 'Winter Starting Fluid' at the tubes and devices and connections while the engine is idling.
If there's a leak, the fluid will be sucked-in and the engine rpm's will increase noticably.

Hope this helps,
-Rick

[stock_am]

awesome thanks alot. I bought the car about a month ago. i did a tune up. i have no HVAC problems, but a vacuum leak might be the problem. i will do what you said and look for a leak. thanks again