Why does the Audi A6L shake during cold start?
3 Answers
Audi A6L cold idle shaking has the following three reasons: 1. The temperature inside the engine is insufficient, and the temperature of both fuel and lubricating oil is too low. 2. Uneven cylinder pressure, excessive carbon deposits on intake valves and intake pipes. 3. Fuel injection issues caused by excessive internal carbon deposits in the nozzles. 4. When the water temperature is low, it is considered a cold start, and the engine noise is very loud, especially in winter, sounding like a tractor with significant vibrations, which improves when the engine warms up. This is a normal phenomenon because combustion and lubrication are poor during cold starts. The engine runs at higher RPM when cold and lower RPM when warm, leading to incomplete combustion which can cause carbon buildup, blackened exhaust pipes, and in some cases, blue smoke for the first few seconds, indicating oil burning, which is also normal.
I've driven quite a few German cars, and cold start shaking is quite common. When the engine is first started, the temperature is low, the oil flow is poor, and insufficient lubrication causes friction between components, leading to shaking. Direct injection cars like the Audi A6L are most prone to carbon buildup. If the fuel injectors get clogged and the fuel spray becomes uneven, the cylinders won't work in balance, causing shuddering. Aging engine mounts are also a common issue - when their damping effect weakens, engine vibrations get transmitted to the cabin. I'd suggest first checking if the idle stabilizes. If it keeps shaking after several dozen seconds, have the spark plugs and ignition coils inspected. Last time I had this issue, it was due to cracked ignition coils - replacing them fixed the problem.
From a mechanical principle perspective, cold start shaking involves air-fuel mixture concentration and combustion efficiency. At low temperatures, the ECU enriches the fuel injection, and slight deviations in air-fuel ratio control can cause shaking. If Audi's electronic throttle accumulates sludge, imprecise opening and closing affects the intake volume, leading to instantaneous fluctuations in intake manifold vacuum. Before the oxygen sensor reaches operating temperature, it operates in open-loop control, potentially causing abnormal short-term fuel trim values. Based on my study of repair manuals, priority checks for such issues include: whether the crankcase ventilation valve is stuck (abnormal exhaust gas recirculation), intake pressure sensor data drift, and fuel pump pressure retention capability.
