Guys, I have a 1990 F350 with the 7.3 idi. It never seems to get over the middle of the N on the temp gauge and the heater only blows hot when driving. As soon as you let off the throttle the heater starts to blow cool. Once stopped it is blowing cold. I put in a new core today, a new thermostat last week, and a new water pump about a year ago. I have checked the dampers and they are clear and functioning properly. The lines to the core get hot. Any ideas?

 

The hoses may get hot, but if they don't flow you won't get any heat out of the heater. You could have also put in the wrong temp thermostat.

 

I put a stock 195 degree thermostat in.

 

Have had this happen in a few cars/trucks and everytime it was that the coolant was low. Be sure the coolant is topped off and that there's no air in the system.