There are many possible causes of heating system malfunctions. These are but a few of the more common ones:
- Stuck thermostat.
- There is a vacuum actuated heater (hot water) cutoff valve in the input heater hose, located in the engine compartment at the end of the engine block very close to the firewall. Looks like a plastic cone with a visible small actuator attached to a valve in the heater hose. It's function is to cut the hot water flow into the passenger compartment (heater core) when the air conditioning is working and calling for cold air (i.e. climate control in "auto" position and temp setting below the cabin ambient temp). Set climate control to "econ", temp to 85, and check to see that the actuator is extended away from the cone (a little finger help is acceptable). Test by hand feel to see if the hot water is flowing past the valve (after engine is hot, of course). If it does you probably had a sticking valve. If it doesn't, make sure the climate control is in "econ" mode, you may then disconnect vacuum line from the valve actuator (the cone), and plug line with a golf tee. If you can move the actuator away from the "cone" to the open position and the valve is not stuck, you will have heat in the passenger compartment, and maybe a sick climate control programmer. All above assumes coolant levels are ok.
- The foam that covers the flaps inside the heater box tend to deteriorate, especially in humid climates.
- The heater valve control motor is located under the dash on the passenger side. If it goes bad, the climate control diagnostics in the Bentley manual should tell you. There is no electrical connector at the motor and if you want to get the whole harness out it's routed over and behind what I think is the heater core housing which you do not want to touch, and it runs to a red connector that holds an additional set of wires that are of unknown function. If you prefer, you can cut the 5 wires to the motor and crimp (or solder) in the new ones.
Faulty heater core: The upper tank, lower tank and spiral inserts of the heater cores tubes are constructed of plastic. The upper tank is divided in half by the plactic tank housing, one half directing the water fomr the inlet pipe to the down tubes and the other half directinthe water from the up tubes and outlet pipe. When the divider in the upper tank break, the hot engine water goes in the inlet and directly out the outlet, bypassing the actual heater core entirely . This will be evident if there are small yellow-ish gravel-like pieces of the plactic tank divider visible in the coolant overflow tank.
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If you do see small yellow-ish gravel like pieces of plactic in the coolant overflow tank it could be the spirals in the tubes breaking down or the tank divider itself. Either way get ready to tear out that dash and replace the heater core.
