Why does the engine have high idle speed when cold?

[Barrett]

If the idle speed is around 1500 rpm when the engine is cold and gradually decreases as the engine coolant temperature rises, eventually stabilizing between 600-800 rpm (idle speed varies by vehicle), this is a normal phenomenon. The reasons for high idle speed when cold are as follows: 1. A cold engine refers to when the engine has cooled down. When cold, due to the low temperature inside the cylinders, gasoline atomization is poor. To meet the necessary combustion conditions, the electronic fuel injection system will increase the concentration of the air-fuel mixture. Therefore, the engine raises the idle speed to increase fuel injection and enhance the mixture concentration. 2. As the cylinder temperature increases, an overly rich air-fuel mixture is no longer needed, so the idle speed gradually decreases, and the engine's fuel injection also reduces accordingly. Once the engine temperature fully normalizes, the ECU maintains an optimal air-fuel ratio of 14.7, which is the perfect proportion for complete combustion of air and fuel. At this point, the engine enters closed-loop operation, where fuel injection is no longer based on the coolant temperature sensor but adjusted according to the oxygen sensor levels in the catalytic converter to achieve complete combustion and optimal emissions.

[Todd]

I've often encountered high idle speed during cold starts over the years of driving. Actually, this is mostly normal because when the engine is cold, the temperature is low and friction between parts is high. The ECU (the car's computer system) automatically controls the throttle to open wider, raising the RPM slightly, usually around 1200-1500 RPM. The purpose is to quickly heat up the coolant and engine oil, allowing the engine to reach its operating temperature faster. This process typically lasts just a minute or two before the RPM drops. However, you should pay attention to details—for example, it's more noticeable in winter due to lower oil temperatures. If the high idle persists for too long, say not dropping even after four or five minutes, then there might be an issue. Possible causes include a faulty coolant temperature sensor or a dirty mass airflow sensor sending no signal, leading to system misjudgment. It's advisable to observe after each start. Under normal circumstances, warming up the car should resolve it, but if abnormal, head to the repair shop promptly to check the sensors or intake system to avoid affecting fuel consumption or emissions. In short, don't stress too much—it's just the car's design for self-protection.

[OAbigail]

As someone who frequently tinkers with cars, I can share some insights on the principle of high idle speed when the engine is cold. Right after startup, the engine temperature is low, and the engine oil is thick with poor lubrication. The ECU automatically increases the RPM to around 1500 based on sensor readings (such as coolant or oil temperature sensors). This boosts fuel injection to accelerate engine warm-up. The normal range typically returns to standard idle speed within 30 seconds to a minute after startup. However, if you notice the RPM remains high for too long, accompanied by shaking or unusual noises, there could be multiple issues at play—such as an air intake leak causing the ECU to compensate with higher RPM, or a dirty throttle body sticking open too wide. A quick check of coolant levels and air filter cleanliness can help diagnose the problem. Regular maintenance, including throttle body cleaning and oil changes, can minimize such issues. Remember, always observe idle speed changes before driving off—don’t rush to hit the road.

[LeBradley]

I see the high idle speed when the engine is cold every day when driving my kids to school. It's just the car warming itself up to prevent parts from sticking in cold weather. Normally, it takes a minute or two, and as long as the RPM drops to around 700, there's nothing to worry about. If it stays high, it might be a faulty sensor not signaling the ECU to reduce speed, which would need some time to fix. In short, it doesn't affect daily driving—just wait a few minutes before driving off.