What Causes an Automatic Transmission to Shake and Stall at Idle?
3 Answers
There are many reasons why an automatic transmission may shake and stall at idle. Issues with any component in the intake system, ignition system, or fuel system can cause this phenomenon. Intake System: If parts like the air filter box, intake pipe, crankcase ventilation valve tube, or crankcase air intake tube become detached or leak, the only solution is to replace the faulty components. Ignition System: Problems such as a damaged ignition coil, faulty electronic ignition module, or capacitor issues may manifest as the engine starting initially but stalling after a while, then restarting only to stall again. Fuel System: Issues with the fuel system, such as clogged fuel injectors due to debris, often stem from poor-quality fuel. In such cases, it is advisable to refuel at reputable gas stations.
I've encountered this situation where an automatic transmission car idles roughly and then stalls. It's mostly due to issues with the fuel system and intake system. If the fuel filter hasn't been replaced for too long, it can get clogged with impurities, causing the fuel pump to work inefficiently and leading to insufficient fuel pressure. Naturally, the engine will struggle and shake violently. The throttle body is also prone to carbon buildup, especially in older cars with 70,000 to 80,000 kilometers on them. Thick black sludge can clog the valve gaps, reducing air intake while the ECU incorrectly adjusts the fuel mixture. If the spark plugs are coated with carbon deposits or the electrodes are worn out, the ignition energy will be insufficient, causing misfires in the cylinders. Sitting in the car will feel like sitting on a massage chair. Additionally, worn or cracked engine mounts can amplify engine vibrations several times, transmitting them into the cabin. My suggestion is to start with cheaper fixes—replace the spark plugs and air filter first, then clean the throttle body. If the problem persists, you'll need to check the fuel pump pressure.
This issue requires checking three key areas: fuel injection, ignition, and the intake system. If carbon buildup on the injector needle valve causes poor atomization, fuel droplets clinging to the pipe walls result in an overly lean air-fuel mixture, leading to engine misfire and surging. Aging ignition coils with insufficient voltage cause severe RPM fluctuations at idle. A clogged air filter unchanged for over six months, filled with willow catkins and dust, starves the engine of oxygen—forcing the ECU to overcompensate with fuel, potentially stalling it. A stuck-open PCV valve also draws excess exhaust gases, diluting the mixture. Last time, the mechanic used a scanner to find an oxygen sensor persistently reporting lean conditions; replacing it restored smooth operation. Also, avoid gas from small stations—low-quality fuel with impurities easily clogs injectors.
