ucfchris84

My heat does not work. A/C blows cold and air will go from defroster to floor back up to driver vents. I flushed the radiator several times hoping that would help it didn't. I then took the feed and return hoses off the heater core and blew compressed air both directions and poured C.L.R (50/50 with water) in the core let it sit for 20 min then flushed completely out.. didn't help. A/C blows cold till I step down on the gas hard then it becomes warmer.. Any thoughts to why that would happen?

I looked on the forum for an answer but didn't see anything. If I over looked it please feel free to point me in the right direction.

98 F150 STX 4.6L with 173k miles

Bflokd

Been down this road with mine and fixed others like this. When you rev up engine it pulls the vacuum off the A/C damper and it opens the same damper to heater . Look on the passenger side where the heater -A/C box is and the damper vacuum motor . Those plastic lines have a hairline crack in one. If you have a vacuum pump you can chase it down . Or put your finger over vacuum side of line and feel for vacuum. If you have vacuum at idle , have someone rev engine and notice the lack of / loss of Vacuum. If you find the bad line it just going down that line to fine a good spot to tie into. Also , some suds water on the lines and watch for the bubble get pulled into the line . It's not as hard as it sounds. Good luck

ucfchris84

Thanks for the info! I would have never thought about vac lines being the problem. I'll def check and see if I can find any leaks. Anything beats pulling the heater core out....

jdinner

The blend door actuator for the heat has a square drive. It fits into a square hole that is part of the door pivot. I have seen the blend door square hole split and cause the actuator to slip causing no heat at all.
Test both heater hoses (hot engine) by holding them in your hands. They are fairly hot but you are testing to see if one is cold/slightly warm and the other is hot. They should both be around the same temperature with the heater fan off and engine running. If they are the same temperature then you have a blend door problem. Some of these are electric and some are vacuum controlled.