What is the normal idle speed for a car?
2 Answers
Car idle speed between 700 to 900 RPM is normal. Idle speed refers to the engine running without any load, only overcoming the friction resistance of its internal components, without delivering power output. The speed that maintains stable engine operation is called idle speed. The reasons for unstable idle speed are: 1. Improper air-fuel mixture ratio in the engine compartment; 2. Prolonged use of spark plugs, aging of ignition coils, aging of spark plug high-voltage wires, and leakage issues can reduce ignition capability; 3. Carbon deposits on the engine's valves and intake passages, which absorb fuel and reduce the car's power; 4. Malfunction of the water temperature sensor. Solutions for unstable idle speed include: 1. Replacing spark plugs; 2. Cleaning valves and intake passages; 3. Checking the water temperature sensor.
For most household cars in a warmed-up state, an idle speed of 650-850 RPM is normal. I make it a habit to glance at the tachometer during my daily commute. During a cold start, the needle jumps above 1000 RPM, then gradually drops to around 700 RPM once the coolant temperature gauge reaches the midpoint. Cars with larger displacements may idle lower—like my neighbor's old 3.0L Accord, which consistently stays at 600 RPM. Last time, I noticed my car's RPM increased by about 100 when the AC was on. The mechanic said this was due to the compressor's workload and completely normal. If the needle fluctuates erratically like it's dancing, or the rearview mirror shakes noticeably, it's likely due to excessive carbon buildup or worn-out spark plugs.
