Bought truck with no heat now trying to fix, flushed core flow was good very little dirt came out, full flow from 5/8 clear hose, was going to use some clr but i think core is good. how can i check blend door. hot to cold switch is good a/c works good. so the problem must be whatever changes from a/c to heat. Also temperature gauge goes up to 200 then drops, then back to 200 then drops whats the deal. does this driving or sitting

 

i've heard of some people cutting into their heater box and moving the blend doors by hand, then taping or epoxy the plastic back to the box.

dunno, haven't had that experience... yet.