Dandy. So it would appear the engine is staying at the proper temperature and just the heater is getting cold. Since you get good heat at times it is unlikely the heater core is plugged. The next time it's heating well, stop and feel the two heater hoses under the hood. You'll find they are too hot to hold onto for very long. Feel them again when the air from the heater gets cool. If they are still real hot, that suggests a problem with the actuators in the heater box or the controls. Usually those fail completely and don't start working once in a while.
It is more likely you're going to find the heater hoses are cooled off when the heater air gets cold. That is due to lack of good circulation. The dealer was right to suggest low coolant level as the first suspect. Feel the upper radiator hose. If that is much hotter than the heater hoses, the belt could be slipping on the water pump. On some engines it is driven by the timing belt and that belt could be worn and starting to become loose.
You may also want to consider a leaking cylinder head gasket. Exhaust gas can sneak into the cooling system and pool under the thermostat. Thermostats open in response to hot liquid, not hot air. If it closes a little there may be enough circulation to keep the engine cool but not enough to force hot coolant through the heater core. Typically you'll see air bubbles in the coolant reservoir. Your mechanic can do a chemical test at the radiator or reservoir to check for a leaking head gasket.
If the radiator hose gets cool too, you can still suspect the thermostat even though the temperature gauge reads normal. On some engines the temperature sensor for the gauge is not close to the thermostat and it takes its reading in a different area. Sometimes you can pinch the radiator hose closed for a minute or two. If the heater starts blowing hotter air, replace the thermostat.
Friday, March 22nd, 2013 AT 12:27 PM