What Causes High Idle Speed in a Car?
2 Answers
Vehicle high idle speed reasons are as follows: Idle switch not closing: The rotation signal determines the fuel injection quantity and timing. When the engine operates at idle condition with reduced air intake, it causes an over-rich mixture and increased RPM. Faulty idle control valve: When idle speed exceeds the set value, the ECU commands the idle control valve to reduce the bypass air intake. Issues like sticky movement due to oil contamination/carbon deposits or improper throttle closing prevent proper idle speed adjustment, resulting in unstable idle. Intake manifold leakage: The idle control valve opening strictly follows a functional relationship with air intake volume. Leaks disrupt this relationship, making the air flow sensor unable to measure actual intake air. This leads to inaccurate ECU air intake control and unstable engine idle.
There are quite a few reasons for high vehicle idle speed. Based on my experience over the years, common issues include a dirty throttle body with excessive carbon buildup, which affects air intake control and causes unstable RPM; it could also be a problem with the idle control valve, such as aging or sticking, preventing proper adjustment; or sensor failures, like a coolant temperature sensor falsely reporting low temperatures, causing the ECU to inject more fuel; or air leaks in the intake system, where external air disrupts the air-fuel mixture; sometimes, a timing belt skipping teeth can also cause abnormal RPM. If you encounter this issue, don’t rush into major repairs—try cleaning the throttle body first, as excessive dirt is often the main culprit. Prolonged high idle speed is harmful to the engine, increasing fuel consumption and putting a heavy burden on the exhaust system, even leading to overheating and breakdowns. It’s recommended to regularly maintain and inspect these components. If problems arise, seek professional diagnosis at a reputable shop—never attempt to tamper with the ECU programming yourself, as it may lead to more severe malfunctions.
