What is the optimal idle speed for a car?

[DelDaniel]

The normal idle speed for a car is 700-800 RPM. If the speed is too low, the engine may vibrate; if it's too high, it will increase fuel consumption. Below are the hazards of unstable idle speed: 1. Excessively high idle speed will significantly increase fuel costs and lead to more carbon deposits in the engine. Check for any looseness or air leaks in the intake manifold. 2. Excessively low idle speed can cause the engine to stall unexpectedly while driving, posing a safety risk. 3. Fluctuating idle speed may cause the spark plugs or ignition coils to malfunction, leading to engine piston failure and severe car vibration.

[DeAutumn]

Normal idle speed is generally most suitable around 750 RPM. Don't panic if cold starts spike above 1000 RPM—it will drop back as the engine warms up. In my daily driving of German cars, the RPM stays steady at 750 when in D-gear with the brake pressed, but oddly shifts up to 900 RPM in neutral—the transmission is indeed slacking off here. For turbocharged cars, note that RPM may slightly rise when running the AC or multiple electronic devices, but as long as the needle isn't dancing, it's normal. If an older car shakes like a sieve, check the spark plugs and carbon buildup. For newer cars, RPM consistently below 600 likely indicates throttle body sticking. Remember, a rock-steady tachometer is king—if it fluctuates wildly, head to the shop immediately.

[LydiaFitz]

As a long-distance driver, maintaining idle speed between 700-800 RPM is most worry-free. My experience shows: after the engine reaches normal operating temperature, shifting to D gear while holding the brake with the needle steady at 750 RPM on a 3/4 thousand RPM dial is ideal. Manual transmission cars may show slight vibration at 700 RPM in neutral, but it's normal for RPM to rise to 850 when engaged with AC on. Previously driving an older Japanese car, I noticed dirty throttle bodies could drop idle to 500 RPM, causing spine-massaging vibrations. Modified exhaust systems are a different story - their deep growls can mislead RPM judgment. The key is observing whether the tachometer holds steady; fluctuations exceeding 100 RPM warrant inspection of intake or fuel systems.

[Tabitha]

Tested over a dozen cars, 800 RPM is the most balanced for family vehicles. Recently drove an American SUV with 750 RPM in D gear, shifting to N gear raised it to 900 RPM due to torque converter resistance. Don't overthink it with auto start-stop systems—when the engine's off, RPM is irrelevant. Be wary if AC usage drops RPM instead of increasing it; likely a load compensation system failure. ECU-tuned owners note: reprogramming may set idle to 1000 RPM, reducing vibration at the cost of fuel efficiency. The simplest check? Feel the steering wheel—consistent smoothness means stable idle.

[SanWyatt]

A decade of car repair experience shows that idle speed mainly depends on operating conditions: 1200 RPM when cold and 750 RPM when warm are the baseline. Last week, a Corolla idled at 650 RPM with severe shaking—cleaning the throttle body immediately restored it to 800 RPM. For turbocharged cars, be cautious as the idle speed may increase by 50 RPM when the turbo is active. Manual transmission drivers should avoid revving in neutral for extended periods, as it damages the release bearing. During diagnostics, check data streams in P mode—as long as the oxygen sensor voltage fluctuates steadily around 0.45V and RPM variation stays within 20, it's considered healthy.

[DePeyton]

As a female car owner, the most dreaded sight is an abnormal dashboard: when the engine is warm, an RPM between 700-900 is reassuring, but the key is to watch if it stabilizes when the coolant temperature hits 90°C. Once, with the AC on, the RPM got stuck at 600, and the car shook so much the rearview mirrors vibrated—the mechanic said it was a faulty ignition coil. Now, driving an electric car eliminates such worries, but I still glance at the tachometer in my husband's fuel-powered car. For cars with AUTO HOLD function, note that when you press deeply to engage parking, the RPM will be about 50 lower than in Drive (D) mode. Remember, if the idle RPM needle shakes like a phone vibration, it's a warning sign; smooth as water surface is normal.