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my engine revs high when not in gear.

topic posted Tue, September 26, 2006 - 9:20 PM by  Aargh
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I drive a '96 Geo Metro. It hasn't given me any real problems until today. It suddenly started revving up to unusually high RPM. when it is in gear, it feels like it's pulling, like it's still getting gas even when i don't have my foot on either pedal. i had to put my full force onto the brake pedal just to keep the car from creeping. It was a hell of a stressful drive home today.
when i put it in neutral or park the engine immediately revs up to 4000 RPM or higher.
it just started doing this today. no warning.
any ideas what is causing this? anything would be appreciated.

A
posted by:
Aargh
Houston
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  • Unsu...
     
    well, its sounds dangerous to me so, first off I would not reccomend driving it at all until the problem has been taken care of....even just running it in the driveway could lead to potential disaster. Might be a good one for a professional to see with their own hands after its towed in.

    But, heres where I think the most likely problems could be....

    If the vacuum hose to the brake booster came off the intake manifold, is cracked or broken or if the brake booster has a bad diaphragm it could certainly cause this condition. not only would it cause your engine to rev up uncontrolably, it would cause your breaks to function poorly and require a huge amount of effort to stop the vehicle. look for a hose about 1/2 to 3/4 of and inch in diameter going from the intake manifold to a canister on the driver side of the engine compartment at the fire wall. Check to make sure it's connected at both ends and has to leaks or cracks.

    This could also be caused by the throttle plate sticking open. Either due to a hung or stuck throttle cable, or mechanical damage. If the vehiche has cruise control there is another cable involved in actuating the throttle that could be causing the problem. The cruise control stepper motor could also be stuck in one position.

    There is also an item called an idle air control motor or solenoid (also known as an IAC), located near the throttle body which can often stick in position if it becomes dirty internally...sometimes they have even been known to come apart due to wear. If it only does the reving when its not in gear this is a likely scenario as depending on the position it may be stuck at you might not notice it in normal driving.

    Please be careful. This is a dangerous situation, even for a mechanic in a shop, that really requires a pair of mechanics who communicate well, to repair safely. an engine revving uncontrolably to 4000 rpm's suddenly with no warning is a recipe for disaster.

    Good Luck and please be careful...I had a 76 VW fastback with an automatic transmission many many years ago. throttle waws stuck, and I went back to investigate what was wrong and as I was getting out I accidentally bumped the shifter into drive. The car took off at full throttle down the street with a ghost driver at the wheel. It impacted the dead end wall at the end of the block and totaled the vehicle. It was a very lucky thing indeed that no children were in the street, and nothing but minimal property damage was done. This is like playing with dynamite.