Heater blows cold

fly boy · Dec 25, 2015

What's happening here? Vacuum? Heater will be blowing warm air, I'll kick it down to say pass and after I pull back in from passing and let off throttle at all the heater will blow cold (winter) air for couple minutes and then eventually blow warm again. Coolant level is good and new thermostats.

78f100 · Dec 25, 2015

Vacuum pump is electric and has no relation to engine speed. If I had to guess the coolant level is low or has a restriction in the line to the heater core. Low coolant will cause the same issue as the level is not high enough to be circulated into the heater core, check that first. If that's good check for a restriction in the heater hoses. Blend door issues are common on these trucks also but shouldn't change with engine temp or speed.