Why is the car idling a bit high?

[LaLondon]

Reasons for high car idle speed: 1. Intake air temperature sensor malfunction; 2. Water temperature sensor malfunction; 3. Throttle position sensor malfunction; 4. Air flow meter or intake manifold pressure sensor malfunction; 5. Switch signal malfunction; 6. Idle speed control valve malfunction; 7. Throttle body malfunction; 8. Fuel injector malfunction; 9. Engine control unit malfunction; 10. Matching setting malfunction. Solutions: 1. Replace with SJ grade or higher engine oil; 2. Check if the radiator coolant is sufficient, replenish if lacking; 3. Inspect throttle blockage, clean if clogged.

[Duane]

I encountered the same situation a few days ago—the tachometer shot up during a cold start. Actually, there are just a few common reasons: the throttle body is caked with thick carbon deposits like a layer of sludge, forcing the ECU to compensate by injecting more fuel; an air leak in the intake system, such as a cracked vacuum hose, means the engine is secretly sucking in extra air; or a glitchy electronic throttle or a faulty coolant temperature sensor can also cause high idle. The most annoying part is when the RPM dances on its own in neutral, and turning on the AC sends it soaring to 1500 RPM—watching the fuel consumption hurts. My advice? Don’t rush to fix it. First, try disconnecting the battery’s negative terminal for ten minutes to reset the ECU. If that doesn’t work, clean the throttle body—older cars are especially prone to this issue.

[DelAthena]

My decade-old car has been acting up lately, with the engine rumbling like it's about to take off when idling at a red light. After some research, I found several possible causes: unstable voltage from a dying battery can interfere with the ECU; a contaminated oxygen sensor sends erratic signals; the system automatically enriches the mixture when spark plugs with excessive gaps struggle to ignite; and the most overlooked culprit—a PCV valve stuck open. That little valve is cheap, but if it leaks, the idle speed can shoot past 1,000 RPM. I recommend checking the trouble codes first before diving in. Changing spark plugs yourself is easier than you think—just remember to get ones with the OEM-specified heat range.

[SanIsabella]

High RPM issues generally stem from three areas: intake, fuel system, and electrical circuits. Abnormal throttle valve opening is most common - modern cars use electronic throttles, and carbon buildup blocking the idle bypass port can be disastrous. If the fuel pressure regulator's internal diaphragm ruptures, excess gasoline leaks into the intake manifold. Engine mount collapse is particularly noticeable, making the entire driver's seat vibrate. Observe whether RPM fluctuates severely when the fan operates - if so, it usually indicates a grounding issue in the AC compressor's electromagnetic clutch circuit, which tricks the ECU into increasing fuel injection.