What Causes Low Vehicle Idle Speed?
2 Answers
Vehicle low idle speed can be caused by: oil accumulation in the throttle body, carbon buildup in the throttle body, dirty intake passages, or weak spark due to faulty spark plugs. It is important to clean or replace spark plugs promptly. Vehicle idle is not a speed but rather an operating condition where the engine runs without load. When the accelerator pedal is fully released during engine operation, the engine enters idle state. The engine speed during idle is referred to as idle speed. Idle speed can be adjusted by modifying the throttle opening size, and generally, the optimal idle speed is the lowest rpm at which the engine runs smoothly without shaking.
Recently worked on an old car with idle speed dropping to 500 RPM, causing jerky driving. The most common causes are intake leaks - like cracked intake pipes or aging gasket leaks. Another frequent issue is a severely dirty throttle body getting stuck; cleaning it often restores normal operation. Once encountered a bizarre case where injectors were half-clogged with deposits - cleaning them eliminated the problem. For vehicles with PCV valves, that component requires special attention as diaphragm leaks can cause idle drops. The trickiest scenario is corrupted ECU data - last time had to reflash the program on a Focus to fix it. With older cars showing these symptoms, don't push your luck - you might get startled by sudden stalling at traffic lights.
