The Causes of an Idle Surge

by Chris Weis
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Fluctuations in engine idle speed can be annoying and may even cause stalling at idle. Engine stalling can be dangerous, because power assistance is lost to the steering and brakes. Idle speeds below those specified by the manufacturer will affect the performance of the power steering pump, alternator and AC compressor. Emission control devices also require a minimum engine speed for effective operation. Surges that accelerate engine speed could cause unexpected vehicle movement.

Air Supply

A clogged or obstructed air filter can cause surging at idle, and foreign objects can enter the filter housing at any time. Any debris in the filter or housing might restrict air flow to the engine enough to affect idle speeds. Clean the throttle body or carburetor venturi(s) after replacing a severely dirty air filter. Aerosol products designed for this purpose are available at any auto parts store. Instructions for use are on printed on the can.

Fuel Supply

Fuel filters can become restricted and deny proper delivery at idle speeds. Mechanical fuel pumps may overcome restrictions and performance deficiencies at higher speeds. Incorrect idle speed settings or fuel mixture adjustments, and malfunctions in the fuel vapor recovery system can upset engine idle capabilities of carbureted engines. The appropriate fuel system cleaner added to the fuel tank can open clogged passages in injectors and carburetors that may be causing the surge.


Any extra air entering the intake manifold will lean the mixture at idle and surging will result. A visual inspection of vacuum hoses may reveal cracked or leaking hoses. A short length of tubing can serve as a make-shift stethoscope to pinpoint any hissing noises heard on a running engine. Cracked or clogged positive crankcase ventilation valves can adversely affect engine idle quality. Idle solenoids or controls should be adjusted to specifications after vacuum leaks are repaired.

Engine Issues

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Ignition system problems can contribute to idle surge. Ignition timing and the condition of spark plugs and related components should be be checked before seeking professional assistance. Lack of compression, and valve timing and condition issues are complex to diagnose and usually affect overall performance, but can be more pronounced at idle speeds. Contributing emission system flaws may require advanced testing techniques or equipment. A visit to a qualified repair facility might be necessary.

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