What causes unstable idle speed in a car?

[Sienna]

The reasons for unstable idle speed in a car include: 1. Severe carbon buildup in the engine: Carbon buildup in the engine is the main cause of car shaking. 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 startup, which can cause sudden stalling. Solution: Clean the fuel system, check the idle speed control valve, and clean the throttle body. 2. Malfunction in the ignition system: Spark plugs are wearable parts and need to be replaced regularly. Solution: Replace the spark plugs. 3. Issues with fuel supply pressure and the intake pressure sensor: If the fuel pump's supply pressure is abnormal or the intake pressure sensor's readings are incorrect or malfunctioning, it can cause the car to shake or even stall. Solution: Inspect and repair the relevant components.

[Chad]

My previous car had unstable idling, with the engine shaking like an earthquake, making the drive feel uneven and almost stalling on the road. It might be due to aging spark plugs or excessive carbon buildup, causing inaccurate ignition and making the idle speed fluctuate. A clogged air filter, leading to insufficient air intake, could also cause this. If the throttle body is covered in oil residue and stuck, it can't control the air intake properly, affecting balance. Issues in the fuel system, like dirty fuel injectors causing uneven fuel supply, can also be problematic. Faulty sensors, such as a bad oxygen sensor, leading to computer misjudgment, are common causes too. I suggest first checking the spark plugs and throttle body yourself. If there's no improvement, take it to a repair shop for a thorough inspection—don't delay, as it wastes fuel and damages the car. Regularly replacing spark plugs and cleaning the throttle body every 20,000 kilometers can prevent most issues. I've stuck to this routine, and my car runs much smoother now.

[Rhonda]

Unstable idle issues stem from the engine system, with the ignition process being critical—worn spark plug electrodes or carbon buildup interfering with discharge can cause rough operation; unstable output voltage from ignition coils is equally problematic. On the air supply side, a clogged throttle body affects the opening angle, and a dirty air filter restricts airflow. In the fuel system, clogged fuel injectors lead to uneven fuel atomization, while faulty fuel pressure regulators cause inaccurate fuel delivery. Don’t overlook vacuum leaks or loose hoses, which introduce excess air and mislead sensor data. ECU or related sensor failures can also disrupt computer control, making idle erratic. During routine maintenance, scan for trouble codes with a diagnostic tool—cleaning the throttle body and replacing the air filter often resolves the issue. Ensure high-quality fuel and inspect the ignition and intake systems every 30,000 kilometers to prevent problems proactively.

[McHazel]

Unstable idle speed in a car is quite dangerous. Severe shaking may lead to sudden stalling, increasing the risk of rear-end collisions in traffic jams or at intersections, which demands immediate attention. Common causes include faulty spark plugs causing irregular ignition, carbon buildup in the throttle body leading to unstable control, insufficient air supply, fuel issues like clogged fuel lines or stuck fuel injectors, and sensor errors misleading the engine control unit. It's advisable to first check for any warning lights on the dashboard and inspect the condition of spark plugs and filters. Don't risk driving it—seek professional diagnosis and repair promptly to prevent minor issues from escalating into accidents. Unstable idle also increases fuel consumption and emissions, causing long-term engine damage. Spending a little on inspection and repair is far better than dealing with an accident. Safe driving requires constant attention to vehicle condition.