Why does the engine speed fluctuate after starting the car?
3 Answers
Reasons for unstable engine speed: Oil accumulation in valves and carbon buildup in throttle: Blockage in the intake tract can cause idle fluctuations, while carbon deposits or oil in the throttle body are the most common causes of unstable idle. Carbon buildup in the throttle affects its opening degree and air intake volume, resulting in erratic engine speed fluctuations after cold starts, accompanied by noticeable vibrations. Spark plug malfunction: Spark plugs can also accumulate carbon deposits, which can be cleaned to restore their activity. However, as spark plugs near the end of their lifespan, their performance may degrade. Fuel line blockage or intake pipe failure: When the fuel line, particularly the fuel injectors, becomes clogged, it leads to insufficient fuel supply pressure, poor fuel atomization, and an imbalanced air-fuel mixture, causing incomplete combustion and related issues. Sensor malfunctions: Faulty sensors can prevent the ECU from accurately controlling the air-fuel ratio. Common examples include malfunctions in the coolant temperature sensor, oxygen sensor, and throttle position sensor.
Recently, my car's RPM keeps fluctuating up and down after starting, which is really annoying. I checked and found that the spark plugs need replacement—they've been used for over three years, and the electrodes are worn out, causing weak ignition, so the engine can't draw stable fuel and shakes badly. Also, the fuel injectors are clogged with dirt, causing uneven fuel spray and making the RPM go crazy. If the fuel pump is aging and can't keep up with the fuel supply, the engine will growl like it's starving. I also checked the air filter—it's clogged with dust, blocking airflow in the intake system. I replace parts during every maintenance session and recommend using high-quality engine oil and cleaning the fuel system regularly. If the problem persists, take the car to a shop immediately to read the fault codes with a computer—don't wait until the engine gets damaged. Leaving these small issues unresolved poses a high risk of sudden engine shutdown on the highway. Safety first.
After starting, the engine speed is unstable. What I fear most is the shaking when waiting at a red light at an intersection, which makes my heart race. Common causes include slight leaks in the vacuum tube, where aging rubber tubes crack and allow impurities to enter, disrupting the air-fuel mixture ratio. If sensors like the oxygen sensor or throttle position sensor fail, the electronic system miscalculates data, causing the ECU to incorrectly adjust the idle speed. I’ve dealt with this myself—I removed the cover to check for oxidation or loose connections in the battery and fuses, but ultimately, replacing the sensor stabilized it. Another issue could be carbon buildup or dirt in the idle control valve, which can be resolved by cleaning it. When driving, avoid slamming the throttle during cold starts, regularly inspect hoses and connectors, and maintain smooth acceleration habits.
