What causes engine idle shaking and unstable idle speed?
3 Answers
The causes of engine idle shaking and unstable idle speed include: unstable operation of engine cylinders, excessive carbon deposits in the cylinders, electrical circuit issues, aging of engine mounts, and problems with the ignition system. If there is an issue with the ignition coil, it will directly cause abnormal spark plug firing, leading to continuous engine shaking. Excessive carbon deposits in the cylinders can also result in unstable idle speed and shaking. The reasons for excessive carbon deposits in the cylinders include prolonged idling in place and substandard gasoline causing incomplete combustion. Excessive carbon deposits in the cylinders can lead to poor spark plug ignition, causing abnormal engine shaking. Excessive carbon deposits in the combustion chamber can also clog the valves, resulting in poor air intake and causing abnormal engine shaking.
Just experienced this issue with my old car - the steering wheel shakes violently at idle, accompanied by the tachometer needle jumping up and down. The mechanic said it was due to carbon buildup clogging the fuel injectors, causing an imbalance in the air-fuel mixture. Upon inspection, we found the throttle body was sticky like it had been coated with caramel, and it improved immediately after cleaning. The cracked engine mount bushings also played a role - when those age, they become like rusty springs that can't suppress vibrations anymore. Try to avoid frequent short trips as cold engine operation promotes carbon buildup. Regularly driving at highway speeds helps clean out carbon deposits effectively. Also remember to replace spark plugs at 40,000 km - weak ignition energy makes the engine shake like a stove lacking firewood.
Having repaired cars for 15 years, the most common issues in such cases usually stem from three areas. Carbon deposits accumulating on the back of the throttle body affect the air intake flow, akin to a person choking while eating. Worn-out spark plug electrodes cause delayed ignition, and if one cylinder in a four-cylinder engine fails, it throws the entire machine into a spasm. Another issue is unstable fuel pressure, where poor fuel injector atomization resembles a clogged showerhead spraying water intermittently. Don't easily believe that additives can cure it—you need to disassemble and inspect the carbon deposit thickness inside the cylinders with a borescope. Also, pay attention to aging engine mounts, as rubber components harden and lose their cushioning effect after five or six years. Finally, it's advisable to check the crankcase breather valve; if it leaks, it can affect vacuum levels, leading to erratic idle speeds.
