What are the causes of unstable engine idle speed?
2 Answers
Reasons for unstable engine idle speed: oil accumulation and carbon deposits in the throttle valve, dirty intake passages, clogged fuel system represented by fuel injectors, intake pipe malfunctions, weak sparks caused by individual spark plug failures, etc. The following is a detailed explanation of unstable engine idle speed: 1. Oil accumulation and carbon deposits in the throttle valve, dirty intake passages: Blocked intake passages can cause idle speed fluctuations, and carbon deposits or oil in the throttle valve are the most common causes of unstable idle speed. 2. Clogged fuel system represented by fuel injectors, intake pipe malfunctions: When the fuel system, especially the fuel injectors, is clogged, it leads to insufficient fuel supply pressure, poor fuel atomization, an imbalanced air-fuel mixture ratio, and incomplete combustion, resulting in related malfunction symptoms.
I've been repairing cars for decades, and unstable engine idle is a common issue with various causes. The most frequent culprit is worn or aged spark plugs leading to poor ignition, causing erratic RPM fluctuations. Another common issue is a dirty or dust-clogged air flow sensor sending incorrect signals to the ECU. The idle control valve is prone to carbon buildup and sticking; once it's not smooth, the car shakes. Aging vacuum hoses with cracks or leaks can disrupt the intake system. Fuel-related problems like clogged injectors or unstable fuel pump output can also disturb idle. ECU control malfunctions are relatively rare but do happen. Long-term carbon buildup in the throttle body or intake manifold is a silent killer. I recommend paying attention to unusual noises while driving—if the idle fluctuates, don’t delay repairs. For prevention, regularly clean the throttle body, drive at high speeds to clear carbon deposits, and check spark plugs during oil changes. This can save a lot of trouble and extend engine life.
