Could be low coolant, bad thermostat, clogged heater core, water impeller worn or blown head gasket from overheating
January, 5, 2013 AT 7:30 PM
The engine cooling system keeps the water in the engine until the temperature exceeds the threshold of the thermostat metal. This causes the thermostat to contract (or expand) allowing water to go to the top of the radiator where the heat is released thru the radiator and exits the radiator at the bottom cooled. Sometimes there is not enough airflow to remove the heat, so fans were added to insure air circulation thru the radiator vanes.
When this water heats up, there is another path for it to follow, and that is through your heater core. The fan behind your dash blows air across the heater cpore and the hot water from the engine heats the air thus you haveheat.
The entire movement of water through your engine is possible because of a water pump. The pump ushers water from the radiator cool side and pushes it thru the engine.
Each engine has a capacity that must be maintained for optimal cooling and heating.
If hot water is NOT reaching your heater core, then there is no heat exchange to take place and you will not get heat.
Check the coolant reservoir (engine cold) to see if there is enough coolant circulating in your engine. If not, check your manual and add the recommended amount. Run your engine and determine if you are getting any heat.
Another reason may be that your thermostat is stuck OPEN. If this is thecase, your engine will have a hard time building up heat as the water is going thru the cooling cycle before it gets hot. (Your temp guage will stay at the low point.)
If your thermostat is CLOSED and will not open, then your temp guage should indicate overheating.
If all of that is inconclusive, then you may have clogged hoses preventing the hot water from going to the heater core. A possible remdy to this would be a back flush.