What Causes Idle Shaking in Automatic Transmission Vehicles?

[DanielLee]

Automatic transmission vehicles may experience idle shaking due to the following reasons: 1. Dirty Air Filter: The air filter is designed to protect the engine and reduce wear, typically requiring replacement every 10,000 kilometers. If not replaced in time, insufficient air intake can cause low idle speed, leading to engine shaking. Simply replacing the filter can resolve this issue. 2. High-Voltage Wire Breakage: A break in the high-voltage wire's core can affect ignition and cause engine shaking. Use a multimeter to measure resistance and replace the wire with significantly higher resistance. 3. Spark Plug Lifespan: Spark plugs have a limited lifespan. After 30,000 to 40,000 kilometers, their ignition efficiency significantly decreases, which can affect ignition. This is a common issue that can be resolved by replacing all spark plugs.

[JuliaLynn]

I dealt with a similar situation just last week. When an automatic transmission idles, shaking like a massage chair, the most common cause is worn engine mounts—those act like shock absorbers, and when they fail, engine vibrations transmit directly into the cabin. A dirty throttle body can also cause shaking, especially if you frequently encounter stop-and-go city traffic. If the spark plugs are due for replacement, misfiring cylinders can make the car jerk intermittently. Once, a friend’s car had clogged fuel injectors, causing unstable RPMs and resulting in body tremors. Poor fuel quality can make the engine 'cough,' so try adding a fuel system cleaner. The worst-case scenario is hardened carbon deposits inside the engine, making it run like a tractor. I recommend first scanning for trouble codes—if you don’t want to diagnose, just replace the engine mounts, as this is a common issue. If it shakes badly at red lights, get it checked ASAP.

[HectorFitz]

If the car is over five years old, it's highly likely an engine mount issue—the rubber deteriorates and loses its cushioning effect. A clogged air filter causing poor intake can also lead to surging. When the fuel pump lacks pressure, the engine struggles to get fuel, shaking during acceleration, let alone idle. Carbon buildup sticking to valves can cause cylinder imbalance, with shaking worsening when the AC is on due to increased engine load. Once during maintenance, I found a cracked ignition coil leaking electricity—unstable spark plug firing also causes shaking. Don’t overlook a clogged fuel filter; poor fuel supply will make the engine protest. Aging high-voltage wires leaking electricity are also worth checking—replace any with visible cracks. For DIY fixes, cleaning the throttle body and replacing the air filter are the most cost-effective first steps.

[LillianLynn]

If the car shakes severely during a cold start but improves when warmed up, it's most likely due to carbon buildup causing the valves to not close properly. If the shaking lessens when the air conditioning is turned on and the RPM rises, it's a strong indication that the engine mounts are worn out. If the tachometer needle jumps up and down, focus on checking the ignition coils and spark plugs. Black smoke from the exhaust accompanied by shaking usually points to a leaking fuel injector or a faulty oxygen sensor. If the steering wheel vibrates like a phone in vibration mode, it's probably an issue with the engine mounts. Remember to use fuel additives regularly and change the three filters on time to prevent many shaking problems. If the cause remains unclear, check if the engine control unit wiring is loose.