My 06 f350 is haveing an issue with the heater. Any suggestions?
Symptom: blows cold air with heater on, air heats up at high rpm like on the freeway the goes cold when rpm drops.
Fan is working, good flow through vents, just cant get hot air unless rpms are high...
Please help its mid winter and kids are freezing....

 

I bet your thermostat is stuck open. Depending on your outside temps, you may need a winter front or. Large cardboard in front of rad. Just leave a 8*8 inch hole in the middle. To much cooling for winter on these trucks. If working it u will have remove, watch water temps closely, may have to tune the size of hole for how you work it.

 

If it's a stuck T-stat you should really fix that. It's not a hard job and the
part is about $30 at Ford. Make sure that truck is cool and drain off about
2 gals into a clean bucket. Long extension on the socket makes it easy to
reach the bolts. remove old one and pop the new one in and but bolts back.
Dump in coolant and your off.
Also by going to Ford for the T-stat you get the gasket and a new housing.
Save to old one so that when you need to do a flush you don't have to take
the T-stat apart. Just put the empty housing back in.

Sean

NOTE : A diesel does not like to run well when cold

 

Checked the coolant level was low.....added 2 gallons and heat is working well...now where did i loose 2 gallons, oil is clean and no leaks found.hmmmmmm

 

Sounds like it could be head gaskets or MAYBE (less likely IMO) a bad EGR cooler. Put a pressure gauge on your degas bottle.