Why does the idle speed increase in hot weather but decrease in cold weather?
3 Answers
The reasons for higher idle speed in hot weather and lower idle speed in cold weather include faults in the intake pressure sensor, throttle position sensor, and inaccurate valve timing. The specific reasons are as follows: Fault in the intake pressure sensor: The sensor can no longer accurately determine the intake air volume, providing incorrect information to the ECU for idle speed regulation. Fault in the throttle position sensor: The throttle primarily controls the intake air flow, and sensor errors can also lead to the ECU misjudging the intake air volume. Inaccurate valve timing: Valve timing mainly controls the opening and closing moments and duration of the intake and exhaust valves. Disordered valve timing can result in incorrect fuel injection behavior and unstable idle speed.
I drive an older model Buick Excelle. Last summer, I encountered an issue where the idle speed would surge above 1000 RPM in hot weather, but drop to around 600 RPM in winter, which really puzzled me. This is usually caused by problems in the idle control system—such as the coolant temperature sensor or the idle air control valve. In hot weather, when the engine temperature is high, an aging sensor might falsely report a low temperature signal, prompting the ECU to increase the RPM to 'warm up' the engine. In cold weather, with denser air, the ECU might think everything is normal and lower the RPM. Carbon buildup in the throttle body is another common culprit, as the dirt deposits expand and contract more dramatically with temperature changes. At the time, I tried cleaning the throttle body myself, and it worked for a while. Later, I found out the coolant temperature sensor connector was loose, and simply reattaching it fixed the issue. I recommend checking these areas, as unstable idle not only makes the car noisy and consumes more fuel, but fixing it early will save you a lot of hassle.
As an amateur car enthusiast who loves tinkering with vehicles, I believe the issue of high idle speed in hot weather and low idle speed in cold weather is mainly related to temperature and ECU adjustments. The ECU (the engine's brain) relies on temperature sensors to balance the idle speed. In hot weather, if the sensor misreads low temperatures, the engine may rev higher to warm up; in cold weather, if it misreads high temperatures, it may lower the RPM thinking it's saving fuel. It could also be due to the idle air control valve getting stuck from thermal deformation and failing to reset, or the mass airflow sensor providing inaccurate feedback due to dirt buildup. I once used an OBD scanner to retrieve a trouble code and found that the oxygen sensor was faulty, causing improper adjustment of the air-fuel mixture and unstable idle speed. The simplest solutions are to check for loose connections or clean the throttle body carbon deposits yourself. Don’t forget to inspect the spark plugs for aging, which can affect ignition. High idle speed leads to loud noise and overheating, while low idle speed in cold weather can cause stalling at red lights, which is dangerous. Address these issues promptly and don’t ignore them.
