brown&tan93

Hello all,
I have a 93 5.0 f150 automatic trans. The heater works good when you warm it up in the morning, and for the majority of my highway commute. When i go to exit from the highway, after a 15 mile drive, the heater starts to gradually blow cooler. I have replaced the thermostat, heater core and had the system flushed. What could be causing this?

highballit

There is a door (blendng door) behind your glove box, it may not be staying open as you drive down the road due to vibration. To check it, pull your glove box off and look behind for a small cable attached to a little arm. Move your temperature selection back and forth and you will see it moving r not. It should have some resistance. If no resistance I would say that it is closing off from your heater core as you drive. In my 94 it is manual, however it could be vacuum operated in other year models and if so you would need to find the vacuum leak.

brown&tan93

Yeah its cable operated. I saw it when i changed the heater core. I will check that out.

brown&tan93

The door seems to be working properly. Anyone else have this issue. It has to be simple. Its a heater for crying out loud.....lol.

benbailey84

ive got an 89 5.0 and the door will change from dash to defrost with a higher rpm's, its on my to do list tomorrow to search for the leak.

brown&tan93

That definately sounds like a vacuum issue. I think i saw that is a common problem.

Good old Bill

When the heater is in its no heat mode, check temperature of each heater hoses, both should be same temp and hot to touch. If both are hot, go back to blend door for remedy.

drac484

I know that when I changed my heater core,I accidently unplugged a line.The only cycle I had was the heater.Had to reattach line for full cycle operation.Your problem may be there.

brown&tan93

This was the problem. When i took it in to have the system back flushed something must have got clogged in the heater core. Put in a new heater core and works like a charm. Thanks to everyone for the help.