What Causes Engine Shaking During Cold Starts?
3 Answers
The reason for engine shaking in cold weather is that the engine coolant temperature is too low. To quickly raise the engine to its ideal operating temperature, the fuel supply system automatically enriches the air-fuel mixture, causing the engine to run more intensely, which results in unstable operation and significant idle shaking. The following are the reasons for abnormal engine shaking during cold starts: 1. Low temperature: The temperature inside the engine is insufficient, and both fuel and lubricant temperatures are low. Therefore, during cold starts, more fuel should be injected to meet power requirements. The larger the spark plug gap, the smaller the ignition energy. Poor fuel atomization at low temperatures requires higher ignition energy. Over time, the ignition gap of spark plugs tends to widen, reducing ignition energy and affecting power output, leading to engine shaking. Additionally, aging ignition coils, high-voltage wires, or electrical leaks can also reduce ignition energy. 2. Incorrect air-fuel ratio: Inaccurate air-fuel mixture ratios occur in both open-loop and closed-loop control systems. In vehicles with closed-loop control, the oxygen sensor's minimum operating temperature is 370 degrees Celsius. If the exhaust pipe temperature does not reach 370 degrees Celsius upon startup, the oxygen sensor will not function. This causes the ECU to make incorrect judgments, leading to errors in controlling the air-fuel mixture and ignition timing, thereby reducing power output and causing shaking. 3. Carbon deposits on valves and intake ports: If carbon builds up on the engine valves and intake ports, the carbon can absorb some of the fuel (similar to soil absorbing water from a riverbank), causing the ECU to misjudge. For example, if the ECU assumes it is injecting 100 units of fuel-air mixture, but only 90 units actually enter the cylinder (with 10 units absorbed by carbon deposits), even complete combustion of the remaining 90 units will not achieve the required power, inevitably resulting in shaking.
I've been repairing cars for over 20 years and have seen countless cases of engine shaking during cold starts. The most common cause is aged spark plugs that can't provide sufficient ignition energy in winter, especially for vehicles with over 80,000 kilometers. Another major culprit is excessive carbon buildup in the cylinders - particularly in direct injection engines where it alters the compression ratio. Throttle body carbon deposits also cause trouble by disrupting accurate air intake. Engine oil viscosity matters too; using 10W-40 in older cars makes the oil thick like paste in winter. One customer's car was shaking violently, and upon disassembly I found the ignition coil was leaking current. My recommendation is to first replace the spark plugs, clean the throttle body, and switch to 5W-30 oil. For older vehicles, always remember to check cylinder compression - worn valve stem seals are another frequent offender.
Last week my car had the same issue - it shook like a massage chair during cold morning starts. The maintenance check revealed multiple problems: the aging battery was the most critical, with voltage dropping to 10V at -10°C, causing fuel injector malfunctions. Severe carbon buildup on injectors resulted in poor atomization, making the gasoline combust with crackling sounds. The technician showed me cylinder carbon deposits via borescope - thick enough to write on. The solution was straightforward: replaced the battery and spent 200+ yuan on fuel system cleaning. Switching to 0W-20 full synthetic oil made morning starts noticeably smoother. One practical discovery: turning the key to ON position for 5 seconds before cranking allows the fuel pump to build pressure, resulting in much more stable ignition. Regular use of fuel additives also helps.
