no heat

[Kerin OBrien]

The heat has crapped out on my 2007 Sport Trac, but not completely... Oddly enough, if I run down the highway, after a while it will blow hot air. But then if I let it sit and idle, it blows cold again. Ideas? It was 27 degrees here today, some heat would be nice :sad:

k~

 

[OLE442]

Could it be the thermostat is stuck open?

 

[Kerin OBrien]

That's what I was thinking. BF says no, but doesn't have a better answer... I guess there's no way to 'test' a thermostat? The temp gauge goes to normal and stays there...

 

[swshawaii]

Hopefully a simple fix like a heater control valve. Check if the plunger moves when the heat is turned on.

V8 picture. Worst case would be heater core or blend door. Don't think I've used my heater living here. LOL

[Broken External Image]:

 

[Vic Sorlie]

Do you have the electronic HVAC control (i.e., buttons)? If so, have you lost control of the fan speeds and modes? I ran into this problem on my 07 Limited, and the culprit was the fan controller mounted in ducting behind the glove box. It was an easy fix after I got the right part (depends on whether you've got analog system or the electronic). You can see my write-up w/pics in the Maintenance section of Projects.

 

[Scott Tanner]

check the coolant level before anything...

 

[Aaron Yarbough]

If you recently changed the coolant I bet there is a large air bubble in the heater core. What happens is that when driving down the highway enough coolant is forced through the line and the heater cools. When the engine slows down there is not enough pressure to push the coolant through the line so the air goes cold. The exact thing you described happened on my Infiniti G35 and sure enough once I bled the air out the problem was solved.

To bleed the heater core line disconnect the hose that goes through the firewall to the heater core (while cold). Use a small funnel and slowly pour antifreeze into the line so it allows the air to escape while you slowly fill the line with coolant. Reattach the line and I bet it solves the problem.