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Q: Engine accelerating on its own.

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5.7 liter 97 sierra. Accelerating on its own. Maintains speed at around 62 kmh.wont rev down when breaking at stops. Maintains an RPM around 12-1300 No cruise control. Changed IAC Valve and TPS valve.responds a little better, but still does same thing.

My car has 182000 miles.

In order for your engine to run this high, it has to have a stuck open throttle plate or a major vacuum leak. If you cannot find a vacuum leak, then try to pinch of the hose to the brake booster to see if it idles down. If it does then you may have a leaking brake booster diaphragm and need to replace the booster. If you have the hydraulic style brakes then you may have a problem with the intake leaking vacuum or the injection system leaking fuel from the regulator. You did not mention any computer service codes or fuel smells - these may help point towards a diagnosis. If you still need help a professional technician, such as one from YourMechanic, can come to your car’s location to inspect the acceleration issue and perform a repair.

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