maybe you got too much fan,,usually sixes kept a 3(w/clutch)or 4 blade fan even up into the 80s,,.you can even remove the fan & try it to see if it is the problem-I knew a fella here w/a 77 C10- took his fan off every fall-put the fan back on in the spring. Said it was the only way to get heat outta that truck in the winter, brian
