What is the appropriate idle speed for an automatic transmission sedan?
3 Answers
Automatic transmission sedans typically have an appropriate idle speed between 5 to 10 kilometers per hour. Idle state refers to an operating condition where the engine is running without load - when the accelerator pedal is fully released during engine operation, the engine enters idle state. The vehicle's idle speed may vary slightly depending on different engine types and models. During cold starts, the car's idle speed will be higher as engine wear is more severe when operating at low temperatures. When the engine control unit detects low engine temperature, it increases idle speed to help the engine reach optimal operating temperature faster and adjust to normal working conditions, thereby avoiding excessive wear. In idle state, the engine only needs to overcome internal component friction resistance without delivering output power.
Speaking of automatic transmission idling, I've seen plenty in over a decade of car repairs. After proper warm-up, maintaining 600-900 RPM is ideal - Japanese cars often idle around 700 RPM while German models may run slightly lower. During cold starts, it'll spike to 1200-1500 RPM before gradually dropping as the coolant temperature rises. A 100 RPM increase with AC on is normal since the compressor runs on engine power. Watch out for erratic tachometer fluctuations - that usually means carbon buildup clogging the throttle body or a faulty oxygen sensor. Too low idle makes the car vibrate like a massage chair, while excessive RPM wastes fuel and damages the catalytic converter over time. Simple self-check: observe if the tachometer stabilizes after restarting a warmed-up engine.
My automatic transmission car has been running for over five years, and I keep an eye on the dashboard during my daily commute. I've noticed that during a cold start, the RPM can reach 1400, but it drops to around 800 after three minutes. In summer, when the air conditioning is on and waiting at a red light, the RPM stays around 900 due to the compressor consuming power. The most comfortable idle speed is 700 RPM, where the car runs smoothly and saves fuel. If the RPM suddenly drops below 500, you can even feel the steering wheel vibrate, which is a sign to check the spark plugs and fuel injectors. Be cautious if the idle speed remains high after a long trip, as it might indicate an air intake system leak. Remember, idle speed isn't fixed; transmission fluid temperature and battery voltage can cause fluctuations.
