Why does the engine shake at idle but not when accelerating?
2 Answers
Reasons for engine shaking at idle but not while driving: 1. Leakage in intake pipes and valves: Common issues include cracked intake manifolds or poor intake manifold sealing. When the air supply system leaks, the engine control unit receives inaccurate intake air volume signals, leading to misjudgment and causing unstable engine idle. 2. Excessive carbon buildup on valves and intake pipes: Since carbon deposits can absorb a certain amount of fuel, the engine control unit makes incorrect judgments. For example, if the computer controls the injection of 100 units of air-fuel mixture, but only 90 units actually enter the cylinder (10 units absorbed by carbon deposits), even if the remaining 90 units burn completely, the required power output won't be achieved, resulting in shaking. 3. Malfunction of idle control sensors and circuits: For instance, when the idle switch fails to close, the engine control unit mistakenly assumes the engine is under partial load, leading to incorrect air intake control. Issues like delayed response or sticking of the idle air control valve due to oil contamination or carbon buildup, or incomplete closing of the throttle valve, prevent the engine control unit from making accurate idle adjustments. Additionally, malfunctions in sensors such as the intake air temperature sensor, air flow sensor, coolant temperature sensor, or short circuits/open circuits in their circuits can all cause unstable engine idle. 4. Ignition system: Generally, spark plugs are a major component contributing to unstable engine idle. If a spark plug fails to ignite, it directly affects cylinder combustion, resulting in uneven power output among cylinders and consequently unstable engine idle.
I remember my old car used to shake a lot at idle, but it would stabilize when I stepped on the gas, which really gave me a headache for a while. I learned that this is usually an issue with the idle control system, such as excessive carbon buildup in the throttle body, which restricts air intake and causes the engine to shake at low speeds. Once you accelerate and the RPM increases, the system naturally adjusts and the problem disappears. Other possible causes include a stuck idle control valve or worn-out spark plugs, leading to unstable ignition at idle. The last time I encountered this issue, cleaning the throttle body fixed it. It’s important to develop a habit of regular maintenance—checking the air filter and throttle body every 20,000 kilometers or so to prevent carbon buildup. If the problem persists, taking the car to a shop for a diagnostic scan is more reliable to avoid minor neglect turning into major repairs.
