Why Does the Idle Speed Increase When the Heater Is On?
3 Answers
An increased idle speed when the heater is on is often caused by excessive carbon buildup in components such as the throttle valve, idle control valve, intake manifold, intake valves, combustion chamber, and fuel injectors. When the heater is turned on, check for misfires in each cylinder. If there are slight misfires in all cylinders, issues with the throttle valve or spark plugs should be considered. If misfiring occurs in a single cylinder, the spark plug is likely the culprit. If no misfires are detected, the phenomenon may be normal. When the heater is activated, the blower starts operating, increasing electrical demand. This causes the alternator to output more current, thereby increasing the engine load. At this point, the slower engine speed can lead to incomplete combustion, resulting in a slight shaking sensation. This is similar to the shaking caused by the compressor starting up, though the shaking from the heater is usually milder than that from the compressor.
I noticed that the engine RPM increases when the heater is turned on, which is actually quite common, especially when using the heater immediately after a cold start. The engine requires additional power to drive the fan and heating system. If the car is still idling, it will automatically increase the RPM to compensate for the load, preventing stalling or shaking. After years of working as a mechanic, I've seen many people mistake this for a malfunction, but it's actually normal, especially in older cars where the control system reacts more slowly. However, if the RPM remains consistently high or fluctuates significantly, it's important to check if the coolant level is low, if the fan is stuck, or if the motor is aging. Regular maintenance of the heating system is recommended to ensure smoother winter driving.
I remember last winter when I turned on the heater while driving, the engine RPM suddenly jumped high. At first, I thought the car was broken. After asking a friend, I realized that the blower operation increases the engine load, causing it to automatically increase speed to maintain stability, especially when temperatures are low. This isn't a major issue, but if it happens frequently, there might be other underlying problems, such as a faulty thermostat or inaccurate signals from the AC control module. I recommend keeping an eye on the tachometer while driving. If the idle speed is high, try turning off the heater. If the instability persists, get a quick check at the shop early to avoid minor issues turning into major repairs.
