What are the causes of unstable idle speed?

[ElenaDella]

The causes of unstable idle speed are: 1. Severe engine carbon deposits; 2. Excessive carbon deposits in the throttle body and intake manifold; 3. Faulty fuel pressure regulator; 4. Clogged fuel injectors; 5. Clogged fuel filter; 6. Spark plug carbon deposits; 7. Incorrect spark plug gap; 8. Excessive resistance in high-voltage wires. When there is excessive carbon buildup inside the engine, the gasoline sprayed by the cold-start injector will be largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture during cold starts, making it difficult to start. Excessive carbon deposits and dirt in the throttle body and surrounding intake manifold alter the cross-sectional area of the air passage, preventing the control unit from precisely regulating the idle air intake. This leads to an overly rich or lean air-fuel mixture, causing abnormal combustion.

[GraceDella]

I'm all too familiar with unstable idling issues – there are plenty of common causes. Severe engine carbon buildup can clog fuel injectors, disrupting the air-fuel mixture ratio; a dirty throttle body causes erratic air intake volume; worn spark plugs lead to ignition misfires; or a contaminated mass airflow sensor provides inaccurate readings. The most troublesome is vacuum hose leaks – when aging rubber hoses crack, massive air leakage directly destroys idle stability. Insufficient fuel pump pressure can also make the engine gasp for air, while faulty oxygen sensors cause the ECU to miscalculate the air-fuel ratio. When these problems occur, the car shakes like an asthma patient, with the steering wheel dancing – immediate inspection is needed. Don't forget to check for poor electrical connections, as loose plugs alone can make idle speed go crazy.

[DaniellaLynn]

Don't panic when encountering idle vibration—it's likely a minor engine issue. I'm particularly concerned about carbon buildup problems, especially when intake valves and cylinder heads are severely carbon-fouled, as incomplete fuel combustion directly disrupts idle. A clogged EGR valve is another troublemaker, or when the throttle position sensor sends erroneous signals to the ECU. In the fuel system, a blocked fuel filter or weakened fuel pump can cause intermittent fuel delivery. For the ignition system, check spark plug gaps and high-tension leads for leakage, as these directly affect ignition quality. When the cooling system malfunctions, the coolant temperature sensor may give false readings, causing the ECU to incorrectly adjust idle speed. Worn engine mounts transmit vibrations directly to the cabin, making them more noticeable.

[VonFinley]

Idling shakes like a pile driver? Dirty throttle body is the first culprit to clean. Clogged fuel injectors cause uneven fuel spray, leading to imbalanced cylinder operation. Aging ignition coils leaking electricity can result in a cylinder misfire, making a three-cylinder work naturally shaky. Cracked vacuum lines leaking air disrupt pressure balance, and a clogged air filter restricting intake adds to the trouble. When the ECU receives incorrect data, it continuously adjusts the fuel injection, only to make things worse. Fuel quality issues are something I've experienced firsthand—using the wrong gasoline grade or low-quality fuel can cause knocking. Frequent short-distance driving accelerates carbon buildup, making the engine throw a tantrum when parked.