surprise, surprise. unless yours is different from my 1992 trooper, you don't have a heater valve. the coolant flows through the heater core all the time. you get the heat by turning the dial which actuates the flap in the heater box and the heat comes out of the vents.

(i think that's weird. it means you heat the underdash area all the time. do other vehicles use this system, i.e. without a valve?)

i thought i had a heater valve too. while trying to diagnose a coolant leak/odor i found out different.

[in fact, i use to make sure i turned the heat to high when i changed the cooland to make sure it drained, too. and i thought for some time a whisp of coolant when i 'turned on' the heater was just the normal kind of drip you often get from heater valves]

so if you have heat at speed, and not at idle, i'd say you *do* have a coolant circulation problem, and i'd guess it's your radiator, but it could be the heater core (a small radiator).

hth

//bc