Need help with hunting rpm's

[99LuminaLS]

Hey guys I'm new to the site... Alot of info on here as I can see. I was wondering if anyone could help me out.

I have a 1999 Lumina LS with 145 000 miles on it and this car has been running great over the years and is litterally a tank.

The problem my car recently developed is that when starting the engine (wether already at operating temperature or cold startup) the rpm's will hunt. It is not drastic it only fluctuates between 800-1200 rpm. You also feel it jerking from inside the car sort of like hesitating. The second the fuel system goes into closed loop everything runs beautiful with no hesitation or lack of power. Some people have mentioned that it could be a vacuum leak? (wouldn't that be all the time and not only open loop?). It is really confusing me as there is no SES/CEL light on in the instrument cluster.

I would really appreciate some insight and also some possible causes and diagnostic procedures from someone with experience with this sort of problem.

Thanks

Kevin

[unchained01]

sounds like iac going bad , vaccuum leak or egr tube clogged

[TGP37]

A vac leak in open loop will create a lean fuel condition, lean misfires and lack of power. Once it goes into closed loop the O2 sensor detects the lean condition and adjusts the fuel trims to correct it. Not a solution cause when you floor it and enter PE Mode, those trims are not calculated and runs lean. Too lean while in PE can burn up your pistons, valves over time.

Spray small amounts of starter fluid around suspected areas while idling. If there is a leak near by the RPMs will surge momentarily.