Idles fast in Neutral; Accelerates w/out pushing gas pedal.

DENNISDEEMII

MEMBER
1999 CHEVROLET LUMINA
220,000 MILES

This problem is intermittent and I can only pinpoint that it seems to occur in the summer. I start the car and it will idle fast. Then, I will shift to reverse and it shifts hard @ an accelerated speed. When I shift to drive, it shifts hard again and will accelerate up to 40mph w/out pressing the gas pedal. I can usually stop the car and start again, and the problem goes away temporarily. I have no floor mat that would be pressing on the gas pedal. Anything you could suggest would be great. Thanks.

1 Reply

CARADIODOC

MECHANIC
33,871 POSTS

High idle is typically caused by a vacuum leak. Since you can make it change by restarting the engine, that would point to a valve not turning off or seating properly. To run as fast as you said it would have to be a pretty big leak which would be easy to find if it was a cracked hose or gasket.

Start by spraying water on the engine while it's running and still cold. If a gasket or hose is leaking you'll see and hear the water get sucked in. If nothing is obvious, pinch off various vacuum hoses. If you find one that slows the engine speed down, follow it to where it branches off, then pinch each hose until you reach the leak.