My car’s heater works fine when driving along, but when I stop at a light or the engine idles, the heater starts blowing cold air. What would cause this?
The mechanic says...
It sounds as if there may be an ‘air lock’ within the heater system. For the heater to work effectively at all engine speeds, you need to have coolant only within the system, with no air present. When the car is driving along, the higher speed of the engine is sufficient for the coolant to be circulated through the heater matrix (which then properly heats up the air around it, so that warm air is delivered into the car). However, if air is present within the cooling/heater system, at low engine speeds the coolant/air mix cannot be pumped around the system and the heater matrix will contain air as well as coolant, so the matrix will not be as warm as it should be, and at best the air entering the interior of the vehicle may be lukewarm or very cool.
On many cars, air locks can be eradicated by disconnecting the heater pipe connection at the highest point within the system, with the engine at normal operating temperature and running (CARE – MOVING COMPONENTS AND VERY HOT WATER!!). With the pipe disconnected and the engine running at moderate speeds, the coolant emerging under pressure from the pipe disconnection will initially contain air bubbles, but these should disappear after a few seconds, so that just coolant emerges. The pipe should be re-connected quickly while the coolant is still flowing, and the hose clip re-tightened. Note: The foregoing general information on clearing air locks relates to most ‘traditional’ heater types. However, note that heater/cooling system designs vary, and cars with combined heating/air conditioning systems can be very complex. In each case it is ESSENTIAL to work in conjunction with a workshop manual for the vehicle in question, and to follow the manufacturer’s guidance. If in any doubt, a reliable mechanic should be engaged to check for/eliminate air locks.
Further additional or alternative possibilities worth checking include: A partially blocked heater matrix, and a faulty engine thermostat, or one of too low a temperature value (in this case the engine may never fully warm up to the required temperature for efficient heater operation).
