
Audi A4 shakes when idling in park due to severe carbon buildup in the engine. Excessive engine carbon deposits: When there is too much carbon buildup inside the engine, it can cause the engine to shake during idle after a cold start. This happens because excessive carbon deposits absorb a large amount of the injected fuel-air mixture, making the mixture too lean and causing difficulty in starting. The engine can only start once the mixture reaches a certain level. However, due to the excessive carbon deposits, the mixture in the cylinders then becomes too rich, leading to engine shaking. Solutions for engine idle shaking: Clean the fuel system; check if the idle speed control valve has carbon deposits and clean it if necessary; inspect the spark plugs for excessive carbon buildup and replace them if needed; check the fuel pressure and replace components if necessary.


