What Causes High Engine Idle Speed?
2 Answers
Engine idle speed may be high due to the following reasons: throttle valve sticking or not closing tightly; malfunction of the auxiliary air valve; idle control valve failure; coolant temperature sensor failure; faults in the air conditioning switch or power steering pressure switch; malfunction of the crankcase forced ventilation valve; air leakage in the intake system; low charging voltage from the generator. Hazards of high engine idle speed: High idle speed wastes fuel; it increases vehicle noise; for manual transmission vehicles, it accelerates wear on the clutch friction plates; excessive coolant temperature causes the electric fan to operate continuously; it shortens the engine's lifespan. Methods to adjust high engine idle speed: When the engine idle speed is too high, first start the engine and manually control the throttle arm to close the throttle valve. If the idle speed returns to normal, the throttle return spring may be too weak and should be replaced. If manually closing the throttle valve has no effect, check whether the throttle shaft is loose or if the throttle valve is not sealing properly. If the throttle valve does not close tightly or the shaft is loose, repair or adjust accordingly. If the throttle valve is functioning normally, check for minor air leaks below the throttle valve. If leaks are found, eliminate them. If no leaks are detected, further adjust the idle speed until it reaches the appropriate level.
I've encountered the issue of elevated idle speed several times during long drives, with the most memorable instance being last summer. At that time, the RPM consistently stayed around 1200, whereas the normal idle speed should be between 700 to 800 RPM. This led to noticeably increased fuel consumption, costing me an extra hundred or so in gas money per month. There could be several reasons for this, such as excessive carbon buildup in the throttle body causing it to stick, preventing the intake from closing tightly and thus raising the RPM. Or perhaps a small crack in the intake pipe causing a vacuum leak, allowing unmetered air to interfere with the ECU's calculations. Faulty sensors, like the throttle position sensor near the accelerator, can also mislead the ECU into thinking more fuel is needed to increase the RPM. If the idle control motor gets dirty and sticks in the open position, it can similarly cause the RPM to spike. Sometimes, the ECU needs a reset to clear its memory values. Last time, I cleaned the throttle body and sensors myself, and it worked quite well. My advice is to check for signs of air leaks as soon as you notice high idle speed—listen for hissing sounds or unstable idle, which should raise red flags. Address it promptly to avoid premature engine wear or safety issues, and don't skimp on repair costs.
