Originally Posted by drivebymashing
i recently installed a new heater core in my 1997 ford f150 new heater hoses
flushed radiator coolant was very rusty and nasty
ran for 45 mins with cap off and heat on high
well the heater works fine if your at 3k plus rpms idling or in drive
once below that it blows cold air
so i installed a new thermostat thinking that might be stuck open or something
no better than before
will blow hot if driving hard
has 153k miles btw
the coolant will not boil over unless revved up with cap off
even after 50 miles of driving
is it the water pump?
please help thanks
I had this same problem, changed everything except water pump, nothing worked. Took water pump off, and there were no impellers left to circulate the water, changed with a new one and everything is nice and toasty, would consider trying this!!