What Causes the Metallic Knocking Sound During Cold Start of a Mercedes-Benz C-Class?
2 Answers
If the sound only occurs during cold starts and disappears after the engine warms up, it could be caused by the hydraulic lifters in the engine valves. Since the hydraulic lifters are at the end of the lubrication system, the engine oil cannot reach them quickly during cold starts, leading to insufficient lubrication and resulting in abnormal noises. Precautions for cold starts: First, turn the key to the first position and wait for the electronic system to complete its self-check before starting the engine. Ensure all electrical devices in the car are turned off before starting, which reduces the instantaneous load on the battery and helps extend its lifespan. Wait for the engine to transition from high idle to low idle before driving. Avoid sudden acceleration, and only allow the engine speed to exceed 2000 RPM after the coolant temperature gauge starts to rise. Warming up the engine: During the first few minutes of driving, let the engine idle for a while to allow the oil to reach optimal conditions before accelerating to higher RPMs. This is especially important in northern winters—always warm up the car before driving after a cold start.
Last time I drove a Mercedes-Benz C-Class, it also made a similar noise during cold starts, like a metallic tapping sound. I was quite puzzled at the time. Later, when I took it to the repair shop, the technician said it might be due to piston slap. When the engine is cold, the oil hasn't fully lubricated yet, causing the gap between the piston and cylinder wall to increase, which can easily produce this kind of abnormal noise. Another common cause is the lack of oil pressure support in the hydraulic lifters of the valve train, leading to dry friction and noise during the initial startup. It's recommended that you first check the oil level and top it up immediately if it's too low. If the problem persists, seek a professional as soon as possible to check the engine condition with a diagnostic tool. Don't delay until it becomes serious damage, as the repair costs can be high.
