98 Ranger erratic heater

Have a wierd heater problem-=Heater gets hot when first started and warmed up (at idle) but when I get to highway speeds it gets cold again. Stop vehicle, go to idle, will warm back up. I have replaced water pump, had cooling system flushed at Ford dealership, replaced blend door, any ideas?

Suggest a couple more things to check. Fan clutch. Thermostat.

Took to radiator shop-reflushed heater core-replaced thermostat-replaced control valve-has already had new water pump and clutch fan--Is it possible that the core has something loose in it that is blocking the coolant flow after it warms up?

From cold start run your engine until it gets warm, feel the upper and lower radiator hoses for temperature differential. When you first start, feel the upper radiator hose, it will feel cold, but then should start getting hot when the stat opens. Then turn your heater on and feel the in and out hoses going into the core. You should detect difference in tempt but should equalize when the valve opens.

When it is first started it is like you said-both hoses cold-then gradually the inlet gets hot and so does the return-BUT- after driving apxy 6 miles the return hose gets cold but the inlet is still hot enough to burn ur hand. Driving me nuts.

Have a wierd heater problem-=Heater gets hot when first started and warmed up (at idle) but when I get to highway speeds it gets cold again. Stop vehicle, go to idle, will warm back up. I have replaced water pump, had cooling system flushed at Ford dealership, replaced blend door, any ideas?

still sounds like heater core half pluged unless thats got a heater control valve not opening all the way. i would also look at fan if it's a clutch fan or bad thermo you got a hold of?