Why does the car shake at idle when hot but not when cold?
4 Answers
Vehicle idle shaking may be caused by the following reasons: 1. Intake system malfunction. Excessive dirt on the throttle valve and intake passages or a clogged air filter element reduces the cross-sectional area of the idle channel, causing less air to enter the cylinders than normal, resulting in a richer air-fuel mixture and incomplete combustion in the cylinders. 2. Intake pipe or EGR valve leakage. Leakage in the intake pipe or EGR valve allows more air to enter the cylinders than normal, leading to a leaner air-fuel mixture and reduced cylinder performance. Here is some additional information: 1. External combustion engine. When a large amount of coal is burned to generate heat energy that turns water into a large amount of steam, high pressure is created, which then drives mechanical work, completing the conversion of thermal energy into kinetic energy. 2. Internal combustion engine. The main difference between this type of engine and an external combustion engine is that its fuel burns inside the engine itself.
Over the years of repairing cars, I've encountered many similar cases where the vehicle starts smoothly without shaking when cold, but exhibits severe body vibration during hot idle. This is often related to aging engine mounts. When the rubber in the mounts hardens or cracks over time, it fails to stabilize the engine as it expands with rising temperature, causing the entire frame to shake. Simultaneously, high temperatures can degrade spark plug or ignition coil performance – for instance, poor coil heat dissipation leads to unstable ignition, while carbon buildup on spark plugs increases misfire rates. In the fuel system, clogged injectors from carbon deposits worsen during hot operation, causing uneven fuel supply. Similarly, a dirty air filter in the intake system disrupts airflow, resulting in erratic idle. A faulty thermostat causing engine overheating exacerbates these issues – don't forget to inspect it. Overall recommendation: prioritize inspecting and replacing mounts and ignition components, as safety comes first.
Last time my old car had this issue - it started up super smooth when cold, but after driving for a while when hot, the idle shook so badly I could barely hold the steering wheel. The mechanic explained it was due to worn-out engine mount rubber that became loose and unstable when thermally expanded, plus spark plugs failing at high temperatures causing ignition interruptions. I later replaced the mounts and installed new spark plugs which fixed it, and also cleaned the fuel injector carbon deposits - fuel consumption dropped too. From experience, excessive dust in the air intake system causing poor airflow can also lead to this, especially in hot environments which tend to expose minor issues more easily. Timely maintenance avoids excessive engine component wear, saves money, and prevents breakdown risks.
The common reasons for a car running smoothly when cold but shaking when hot are usually simple: engine mounts aging and loosening when hot, or spark plug coils failing at high temperatures. It could also be due to a clogged fuel filter causing unstable fuel supply, or an air leak in the intake system. The issue should be identified and resolved by replacing the necessary parts promptly.
