Why Does the Car Shake When Idling with AC On?

[Christian]

Car shaking when idling with AC on is primarily due to increased engine load and severe engine carbon deposits. Here's a detailed analysis: 1. Increased engine load: When the air conditioning compressor operates for cooling, it consumes engine power, increasing the engine load and reducing RPM, which causes engine shaking. 2. Severe engine carbon deposits: When there's significant carbon buildup, the fuel injected by the cold start injector gets largely absorbed by the deposits, resulting in an overly lean air-fuel mixture during cold starts. This makes cold starts more difficult and leads to frequent shaking. Solutions include regular fuel system cleaning, inspecting the idle control valve, and cleaning the throttle body.