Feel the radiator hoses after driving at highway speed for a while. They should be too hot to hold onto for very long. If they are cool, or just warm, suspect the thermostat. If they are nice and hot, feel the smaller heater hoses. If they are just warm when the radiator hoses are much hotter, suspect a plugged heater core. Often those can be cleared by disconnecting the hoses and running water through from a garden hose. Be aware that sometimes debris is plugging a hole that corroded through the heater core and flushing it will dislodge that stuff and expose the leak.
If the heater hoses are hot, suspect a problem with the heater box. That can be as simple as a loose or misadjusted cable or as expensive as an automatic temperature control computer.
Oct 2, 2013 at 11:26 PM
