What Causes the Engine to Shake When Idle Speed Drops and Then Recovers?
2 Answers
Relevant introduction about the engine shaking when idle speed drops and then recovers is as follows: 1. It is generally caused by engine misfire or severe carbon and oil deposits. 2. Changes in the gas force of one or several cylinders in the engine lead to an imbalance in the power of each cylinder, resulting in shaking. Carbon deposits and oil contamination in the throttle valve and idle intake passage prevent the control unit from accurately controlling the intake air volume, causing the air-fuel mixture to be too rich or too lean, leading to abnormal combustion. 3. Air intake manifold leakage, damaged or detached vacuum tubes, causing abnormal combustion, can all result in idle shaking. There are many reasons, and specific ones need to be checked one by one.
I've driven for over a decade and encountered the issue of idle speed dropping, shaking, and then recovering. The most common cause is severe carbon buildup in the throttle body—sludge obstructs the air intake, leading to unstable airflow and causing engine RPM fluctuations. This results in sudden drops and shaking, followed by automatic adjustment and recovery. Additionally, aging spark plugs can cause poor ignition, especially noticeable during cold starts. Low fuel pressure or a misfiring cylinder may also trigger shaking, and clogged fuel injectors can lead to uneven fuel supply. These issues can make the car shake noticeably at traffic lights. It's advisable to regularly clean the throttle body and replace spark plugs, as prolonged neglect can damage the engine and shorten its lifespan. For older cars, also check for vacuum hose leaks, as incorrect engine computer readings can cause similar symptoms. In short, don’t ignore it—get it checked at a repair shop early. Regular maintenance with high-quality engine oil and fuel additives can help prevent carbon buildup.
