I just replaced the heater core on my 1988 F150 and now it doesn't seem to be blowing warm air at all. Both hoses are hot running into the firewall. Novice mechanic, so any ideas would be great. Thanks!

 

I just did this same thing today.
Here was my problem:
When i put the cover back over the heater core, i put the cable back in the little metal clip with too much black part towards the drivers side. What was happening is even though i had it to max warm the door that stops cold air from coming in was still partially open, mixing the hot air with the cold.

hope this helps

 

Thanks, I'll check that out tommorrow. Let you know if it helps.

 

Not blowing at all? Or cold? If it's not blowing at all then check you fuses and switches. But other than that I agree that your hot/cold gate is stuck or linkage is bad since your lines are hot. Good luck!

Rick Z.
____________________________

1990 4x4 300 6L Go 4.9ers!!!

 

I had a " friend " one time who forgot to remove the little plastic plug on the new core--
coolant couldn't pass through it---but---1 hose never got warm either----

Gary

 

Toyed with the linkage and didn't seem to change a thing. It blows air, just not warm air. I noticed today that the upper radiator hose didn't get warm, but the heater core hoses did. Anyone think it could be a thermostat issue?

 

No.
The heater core and associated plumbing bypass the thermostat entirely.
The fact that the hoses are hot assures that heat is getting through the core.

If the under dash diverter is selecting various outputs (defrost, panel, floor, mix) then I would look at the fresh air "blend" door.
The vacuum operator for this damper is located beneath the passenger side hood hinge (or close to it) on top of the plenum, in the engine compartment.
If this does not seal or move to a fully closed position then the air going through your new heater core has only 1 pass through to get warm.
**EDIT**
You can see if the actuating arm moves with vacuum applied.
If the diaphram is blown or otherwise jammed you may be able to disconnect the retainer and fix the arm in place with a zip tie or something.

Here's a photo of the actuator:

 

Thanks alot Jim, that was the issue. It wasn't sealing and was rusted. Worked it with some wd-40 and got it to seal. The air coming out got noticeably warmer. I hope the fix isn't temp, but good for now. I'm glad their are people willing to share their knowledge! Thanks to all who responded also.