What Causes Engine Idle Shaking and Jerking?

[SanEmerson]

Idle shaking of the vehicle body may be caused by the following four major reasons: 1. Severe engine carbon deposits: The most common cause of car shaking is excessive dirt in the throttle body or carbon buildup in the fuel injectors. When there's too much carbon inside the engine, the gasoline sprayed by the cold-start injector gets largely absorbed by the carbon deposits, resulting in an overly lean air-fuel mixture during cold starts and making ignition difficult. In this situation, the engine only starts easily after the carbon deposits become saturated with gasoline. After starting, the gasoline adsorbed on the carbon deposits gets drawn into the cylinders by the engine's vacuum suction for combustion, making the mixture too rich. This alternation between lean and rich air-fuel mixtures causes idle shaking after cold starts. 2. Ignition system issues: Check the condition of spark plugs, high-voltage wires, and ignition coils. Poor performance in the ignition system or spark plugs can also lead to such symptoms. 3. Unstable fuel pressure: If you've already cleaned engine carbon deposits, throttle body, and replaced spark plugs but still notice idle shaking, it's recommended to check the fuel supply pressure and intake pressure sensors. Abnormal fuel pump pressure or faulty intake pressure sensor readings can both cause body shaking. 4. Aging engine components: Car shaking may also be related to worn engine mounts. Engine mounts are responsible for absorbing minor vibrations during engine operation. If they malfunction, these vibrations will transmit to the steering wheel and cabin, causing shaking at idle.

[LaAdrian]

Last month, I worked on five or six cars with this kind of idle vibration issue, and most of them were related to carbon buildup. When the engine's throttle body or fuel injectors get clogged with carbon deposits, it becomes difficult to intake air or spray fuel properly, causing the RPM to fluctuate unpredictably. First, check the misfire data with a diagnostic tool—if a particular cylinder has fewer ignition counts, it’s likely due to worn-out spark plugs. The weirdest case I encountered was an old Camry that shook like a massage chair; it turned out the engine mount rubber had cracked—this part is supposed to absorb vibrations, so when it fails, the shaking goes straight to the steering wheel. Fuel quality also plays a role; long-term use of cheap fuel can clog the injectors, but this usually requires disassembly and cleaning to diagnose properly.

[NolanAnn]

In my 20 years of driving, I've experienced three severe vibrations: The first time was due to excessive spark plug gap, which was fixed with an 80-yuan replacement. The second incident involved a chugging exhaust pipe, and the mechanic diagnosed it as a leaking ignition coil. The most recent repair was the most expensive – all four fuel injectors were clogged! I recommend feeling the exhaust pipe when the engine is cold; if you notice a distinct pulsing sensation, it usually indicates a cylinder misfire. You can start by checking these three areas yourself: 1. Remove the spark plugs to see if the electrodes are blackened. 2. Check if the ignition coil connectors are loose. 3. Inspect the air filter box's intake pipe for cracks. By the way, pay special attention to vacuum hoses in older cars – my Passat had a vacuum leak that caused an overly lean fuel mixture.

[VonJourney]

During idle, the engine completes hundreds of combustion explosions per minute, and any hiccup in the process can cause shaking. For instance, wear in the valve train leading to insufficient cylinder pressure, or a malfunctioning oxygen sensor sending erratic data, can cause the ECU to incorrectly adjust fuel injection. Last week, my neighbor's Sylphy shook so badly that a tea cup couldn't stay put—turned out the EGR valve was stuck, excessively recycling exhaust gas and diluting the air-fuel mixture. Actually, a faulty coolant temperature sensor can also trigger shaking; if it falsely reports low temperature, the ECU will aggressively enrich the fuel mixture. It's advisable to focus on checking these electronic control components rather than just mechanical parts.