I am wondering if there are separate heater cores and valves on the Expy's with the rear air feature ? I cannot get any heat to blow from the rear vents. If I leave them off for a few minutes and then turn them on, the first little burst of air is somewhat warm (not hot by any means), but then cold right away. Also, the forward vents (floor and dash) went from hot to cold on their own last week. I turned the system off for a minute and then back on and the air was hot again. I do not automatic climate control like my Explorer did. I am wondering if I have a vaccuum leak under the dash for the thermo controls OR a bad heater valve or two. Any ideas ?

Thanks,

Greg Blakeney
'98 5.4 4x4 Expy.
Las Vegas

 

 

The front heater and the aux heater each have their own heater core and their own blend door driven by an electric actuator. There is no vacuum involved. Heat when you first turn on the blower, followed by rapid cooldown, indicates restricted coolant flow. Check this by turning the heat on hot, the blower on high, and feeling the heater hoses into and out of the heater core (in the engine compartment). If one hose is a lot cooler than the other, you don't have enough coolant flow. Restricted flow is most likely caused by a kinked hose. Feel along each hose from the TEE to the rear unit to the engine connection and to the front heater core connection. You have to feel because it is hard to see the connections at the rear of the engine. If you find a kink, take off the hose, undo the kink, and you will be OK. There is a cylindrical restrictor in the supply hose next to the engine connection. It has a small opening which I suppose could get clogged. If this happens you should probably buy a new hose. DO NOT use a hose without a restrictor because it will erode your front heater core in a short period of time. Since you have the problem in both the aux and the front I doubt that you have an electrical problem or plugged heater cores.