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.

Take the vacuum hose off the booster, block it off and see how the engine runs.