Why did the cold car start shaking and knocking?

Engine cold jitter is one of the most common faults of automobiles, which greatly affects the ride comfort and driving experience, and even affects the driving safety. Engine jitter is generally not obvious at idle speed and high speed. So what factors can cause engine idling jitter? Let's analyze it below.

Engine idle jitter can be roughly divided into cold idle jitter and hot idle jitter. Generally speaking, it is normal for a cold car to shake slightly at first, because the fuel atomization is not good, the mixture is uneven, the ideal combustion state is not achieved, and the cylinder works unevenly, so the engine will shake. When the engine reaches the normal temperature, it will be normal. If the car is difficult to start when it is cold, it will shake seriously after starting, which is usually caused by too much carbon deposition in the engine.

If the engine still shakes badly after reaching the normal operating temperature, there are many reasons. Generally, it is analyzed from the following aspects.

1. Mechanical failures of the engine: for example, improper valve closing, different wear degrees of each cylinder, unbalanced centrifugal force when the engine is running, etc.

2. Engine installation failure: commonly known as "machine pad". If it breaks, its ability to absorb engine vibration will be reduced, which will lead to engine idling jitter. One of the most obvious fault phenomena is that when the engine load suddenly changes, the engine jitter is serious.

3. Air supply system failure: The most common failure in this respect is that the throttle valve is unclean or damaged; Engine jitter can also be caused by air filter blockage or air flow meter failure, but this situation is rare.

4. Oil supply system failure: If the pressure of the fuel system is too low or fluctuates greatly, and the fuel injection suddenly increases or decreases, the engine will shake, and the oil return pipe can be clamped with a clamp. If it is improved, it means that there is something wrong with the fuel pressure; The injector also has serious carbon deposition, and the fuel injection quantity of each cylinder is different, which leads to engine jitter.

5. Ignition system failure: If the spark plug has carbon deposition or electrode ablation is serious, or the high-voltage coil of a single cylinder is damaged, or the circuit is faulty, then one cylinder may fail to ignite, resulting in cylinder failure and serious engine jitter.

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