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DIAGNOSE – ENGINE IDLES TOO HIGH

by Kyle McFadden


SYMPTOM SUMMARY
The engine idles too high.

USUAL CAUSE
The usual cause of abnormally high idle speeds are vacuum leaks or an inoperative idle speed control motor. A vacuum leak on a fuel injected engine will cause high idle. The oxygen sensor will detect the extra oxygen that is entering the engine through the vacuum leak and will signal the computer the exhaust is lean. The computer in turn will then provide additional fuel which will result in an abnormally high idle.

DIAGNOSIS
The first step in the diagnosis is visual inspection. Inspect the throttle cable and linkage for binding. Inspect the engine for vacuum leaks. With the engine at idle, visually inspect each vacuum hose on the vehicle. Listen for a hissing sound which is a key indicator of a vacuum leak. Repair the vacuum hoses as necessary. Inspect the PCV valve and hose. Using pliers, pinch off the hose that supplies the PCV valve. The engine speed should drop slightly. If the engine returns to a normal idle speed, the PCV valve should be replaced. Consult a vehicle specific service manual for location and specific testing procedures. If the vehicle is free from vacuum leaks and the PCV valve is functioning normally, the idle speed control motor or valve may be defective.

PRECAUTIONS, TIPS, and NOTES
You should consult a vehicle specific service manual for proper diagnosis of the system used on your vehicle. Use caution when working around hot or rotating engine parts.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)

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One Response to “Diagnose Engine Idles Too High”

  1. glenn says:

    Put new carb kit in boat now it idels really high

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