What Causes Engine Shaking During Vehicle Startup?
4 Answers
Here are the reasons for engine shaking when starting a car: 1. Low temperature. The temperature inside the engine is insufficient, and both the fuel and lubricating oil are not warm enough. Therefore, during a cold start, more fuel should be injected to meet power requirements. The larger the spark plug gap, the smaller the ignition energy. 2. Poor fuel atomization at low temperatures. Fuel requires higher ignition energy. After prolonged use, the ignition gap of the spark plugs tends to widen, leading to reduced ignition energy, which affects performance and causes the car to shake. Additionally, aging ignition coils, aging high-voltage wires of the spark plugs, or electrical leakage can also reduce ignition energy. 3. Incorrect air-fuel mixture ratio. The air-fuel mixture ratio may be inaccurately adjusted, occurring in both open-loop and closed-loop control systems. In cars with closed-loop control, the oxygen sensor's minimum operating temperature is 370 degrees Celsius. If the car has just been started, the exhaust pipe temperature may not reach 370 degrees Celsius, preventing the oxygen sensor from functioning. In this case, the ECU makes incorrect judgments, leading to errors in controlling the air-fuel mixture and ignition timing through the actuators. This reduces the car's power output and results in shaking.
As a veteran driver with decades of experience driving various models, I've seen my fair share of engine shaking issues during startup. Usually, this happens because the spark plugs are old and have too much carbon buildup, causing the spark to fail to ignite the fuel properly, leading to uneven combustion and vibration. Another possibility is clogged fuel injectors, which result in uneven fuel spray and unstable power output. A dirty air filter can also restrict air intake, making the engine work harder and shake. I recommend first checking the spark plugs; if they look blackened, replace them with new ones—it's an easy DIY job. If the problem persists, it might be a faulty fuel pump or sensor, and it's best to take it to a professional shop for a diagnostic test to prevent stalling accidents. Don't ignore minor shaking—over time, it can wear down engine parts, making repairs more expensive.
I just bought a car not long ago and was quite nervous when I encountered startup shaking for the first time, but later found out it's quite common. Possible reasons include a faulty ignition coil or a dirty throttle body, leading to unstable ignition and poor air intake, causing engine RPM fluctuations and shaking. Poor fuel quality can also cause this, as low-quality fuel doesn't burn completely. My friend's car had a similar issue, and replacing the mass airflow sensor solved it, costing just around a hundred bucks. Also, check the engine mounts; if they're worn out, the lack of damping effect makes the shaking more noticeable. Regular maintenance like cleaning carbon deposits and using good engine oil can reduce the chances of this happening. Don't rush into major repairs—rule out simple fixes first to save money and hassle.
From a mechanical perspective, engine vibration involves coordination issues across multiple systems. Excessive spark plug gaps causing misfires, carbon buildup in combustion chambers disrupting cycles, or faulty ignition coils transmitting unstable currents can all trigger vibrations. If the ECU control unit receives erroneous signals—such as from a faulty oxygen sensor—and incorrectly adjusts the air-fuel mixture, idle roughness occurs. Aging wiring, short circuits, or poor grounding can also cause current fluctuations. I recommend using an OBD diagnostic tool to read fault codes and perform targeted checks, such as cleaning intake passages or replacing air filters. Avoiding high-load startups can minimize damage and enhance safety.
