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What Are the Causes of Unstable Idle Speed?

2 Answers

HazelAnn

07/28/25 6:36pm

The main causes of unstable idle speed include the following aspects: 1. Intake system issues. (1) Intake manifold or various valve leaks causing unstable idle speed. (2) Excessive carbon buildup on valves and intake passages leading to unstable idle speed. (3) Malfunction of idle air control components causing unstable idle speed. (4) Inaccurate intake air volume resulting in unstable idle speed. 2. Fuel system faults causing unstable idle speed. (1) Injector issues where uneven fuel injection or poor atomization leads to power imbalance among cylinders. (2) Fuel pressure problems, such as excessively low pressure. (3) Inaccurate fuel injection due to sensor or wiring faults, causing the control unit to issue incorrect commands, resulting in excessively rich or lean mixtures—an indirect cause of unstable idle speed. 3. Ignition system faults causing unstable idle speed. (1) Spark plugs and high-voltage wires where faults reduce spark energy or cause misfires. (2) Incorrect ignition timing due to sensor or wiring issues—an indirect cause of unstable idle speed. 4. Mechanical structure faults causing unstable idle speed. Valve train malfunctions leading to excessive power loss in individual cylinders, resulting in power imbalance among cylinders.

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LeCarson

08/12/25 12:09am

I've personally experienced unstable idle several times. The first time was due to severe carbon buildup on the spark plugs, which improved immediately after replacement. Another time, I found a small crack in the vacuum line, allowing extra air into the engine and causing a lean mixture and shaking. A friend's car had a dirty or faulty air flow meter, sending erratic data to the ECU and causing control errors. A stuck or failed idle control valve is also common, especially in older cars with carbon buildup. Clogged fuel injectors can lead to uneven fuel delivery and unstable idle. Electrical issues like aging ignition coils or unstable batteries can also cause trouble. Prevention is simple: regularly replace spark plugs, clean the throttle body, and check hose seals. Repairs might cost just a few hundred dollars—don't wait until the engine is seriously damaged. If your car shakes badly at idle while driving, it's best to stop and check—safety first, don't push it.

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