Is an Idle Speed of 500 RPM After Warming Up Indicative of a Misfire?
3 Answers
An idle speed of 500 RPM after the car has warmed up does not indicate a misfire. Typically, the engine's idle speed after warming up should be around 800 RPM. If it's only 500 RPM, this indicates an excessively low idle speed, which is one of the common faults in vehicles. Symptoms of engine misfire: Unstable vehicle idle, abnormal engine vibration, noticeable power loss, difficulty starting the vehicle, engine prone to stalling, black smoke from the exhaust pipe. Causes of engine misfire: Severe burning of the contacts in the ignition system's distributor cap, causing cross-firing; failure of the ignition coil; wear of the cylinder wires; carbon buildup in the fuel injector or clogging of the injector filter screen, leading to imprecise injection and atomization; excessive carbon deposits on the spark plugs.
As an experienced driver who has been behind the wheel for many years, I've encountered situations where the idle speed dropped to 500 RPM after the engine warmed up, and this isn't necessarily a misfire. A misfire refers to the failure of one or more engine cylinders to ignite, causing the car to shake violently and the exhaust pipe to make a popping sound. However, low idle speed can also be caused by other factors. For instance, carbon buildup jamming the idle control valve, inaccuracies in the mass airflow sensor, or a vacuum leak can all lower the RPM. When my own car exhibited this behavior, an inspection revealed that the throttle body was excessively dirty, and cleaning it restored normal operation. Misfires typically result in significant RPM fluctuations rather than a consistently low idle. I recommend starting with simpler checks, such as examining whether the spark plugs are worn out or if there are issues with the ignition coils. If the problem persists, it's best to visit a repair shop to read the diagnostic trouble codes for confirmation.
Having worked in an auto repair shop for a while, a hot idle speed of 500 RPM is indeed on the low side, and a misfiring cylinder could be one of the possible causes, as cylinder misfires reduce engine output and lower RPM. However, this isn't exclusively due to a misfiring cylinder; issues with the engine management system are more common, such as crankshaft position sensor signal drift, insufficient fuel pressure, or oxygen sensor faults, all of which can interfere with idle speed settings. A misfiring cylinder is usually accompanied by vibrations and the illumination of the check engine light, and can be confirmed by detecting P0300 series codes with a diagnostic tool. Low RPM after the engine is warm could also result from intake system leaks or ECU adjustment errors. Regular maintenance, such as replacing the air filter and cleaning the idle control valve, can help prevent such issues.
