What Causes Engine Shaking at Cold Idle but Normal When Warm?

[BrookeDella]

Reasons for engine shaking at cold idle but normal when warm: 1. Excessive carbon buildup in the throttle body/intake manifold or clogged air filter: Excessive dirt in the throttle body/intake manifold or a clogged air filter reduces the cross-sectional area of the idle passage, causing less air to enter the cylinders than normal. This results in a richer air-fuel mixture, incomplete combustion, and engine shaking. 2. Intake pipe or EGR valve leakage: Leaks in the intake pipe or EGR valve allow more air to enter the cylinders than normal, creating a leaner mixture and reduced cylinder performance. If the EGR valve doesn't close properly, some exhaust gas enters the cylinders, increasing residual exhaust gas volume and worsening combustion, leading to reduced engine performance and shaking. 3. Faulty mass airflow sensor or intake pressure sensor: If the mass airflow sensor or intake pressure sensor malfunctions, the intake air volume cannot be accurately measured, causing inaccurate fuel injection. This results in abnormal air intake and uncertain air-fuel mixture concentration, leading to engine shaking.