Why does the idle speed increase when hot but decrease when cold?
3 Answers
An increase in idle speed when hot but a decrease when cold may be due to a malfunctioning intake pressure sensor. Here are some possible explanations for this phenomenon: 1. Intake pressure sensor failure: The sensor can no longer accurately determine the intake air volume, providing incorrect information to the ECU for idle speed regulation. 2. Throttle position sensor malfunction: The throttle primarily controls the intake airflow, and sensor errors can similarly cause the ECU to misjudge the intake volume. 3. Incorrect valve timing: Valve timing controls the opening and closing moments and duration of intake and exhaust valves. Disordered valve timing can lead to improper fuel injection behavior and unstable idle speed.
I have personal experience with this issue - idle speed increases in hot weather but decreases in cold weather. When air temperatures are high during hot days, the intake air density of the engine decreases, leading to insufficient oxygen content. The engine control unit actively increases fuel supply to maintain stable air-fuel ratio, which naturally pushes up the idle speed. Additionally, the operation of cooling fans or air conditioning adds extra load. The lower idle speed in cold weather might be due to problems with the idle speed control system, such as a cold-stuck idle motor or restricted airflow caused by accumulated dirt in the throttle body. If not addressed promptly, this abnormal behavior may lead to engine stalling while driving or decreased fuel economy. It's recommended to clean the throttle body and idle control valve as soon as possible, while also checking vacuum lines for aging or leaks, which can effectively solve the problem. Paying attention to the engine compartment ventilation system during regular maintenance is crucial.
It's quite common in my driving experience for the idle speed to be higher in hot weather and lower in cold weather. On hot days, the fan runs at full speed to dissipate heat, adding load to the engine and causing the RPM to rise; the heat might also distort the readings from the air flow sensor, prompting the control unit to inject more fuel to maintain stability. Abnormally low idle speed in cold weather is mostly due to issues with the idle control device, possibly because the motor gets stuck due to temperature changes and fails to reset. Don't underestimate it—unstable idle speed can be quite dangerous during low-speed driving, especially when going up or down hills. If you want to tackle it yourself, start by checking for any fault codes. If that doesn't work, take it to a professional shop for a comprehensive check of the sensors and wiring harness connections. Keeping the intake system clean can help prevent issues before they arise.
