1996 Chevy Lumina 96 Lumina Rough Idle

SMARTI01

MEMBER
1996 CHEVROLET LUMINA
6 CYL
FWD
AUTOMATIC
143,000 MILES

I’m stumped. I have a "˜96 Lumina 3.1 with a rough idle problem. The car runs and idles fine in park and runs fine when moving, but once I stop with the brake pedal depressed the engine starts to run as if it’s missing. When the brake pedal is released the car goes back to normal almost immediately.

I think it’s a vacuum problem and I have checked all of the vacuum lines and replaced a few, but the problem has not gone away. Could it be the EVAP Vacuum Switch or the EVAP Canister PURGE Valve? Since the brakes are involved could it be a bad Vacuum Brake Booster?

I also noticed that there is a vacuum line coming off the bottom of the EVAP Vacuum Switch (at a right angle) that is not connected to anything.

I appreciate your help.

3 Replies

MERLIN2021

MECHANIC
17,250 POSTS

Since it only happens when the brake is applied, test the check valve at the booster, remove it and blow thru it both ways, air should go only one way! Does the brake require more effort now to stop the car?
You may have a vacuum leak, get a can of spray gumout, spray the vac lines and maniflod area, when the idle changes, youve found the leak! You want the engine running and spray on the outside but do direct the stream onto the hoses, if it is a vacuum leak, the engine idle will change speed, then you have detected a leak, repair the vacuum leak and see how it runs!