Strange heater issue 95 F150
 

My dad's owned a 95 f150 supercab Eddie Bauer for the last few years, never done much to it before it has 175k miles on it now. Last winter everything worked fine on the heat, I replaced the fan clutch this summer no other changes.

Fast forward to last weekend, it is cooling down and he kicked the heat on during his drive to work and nothing but cold air.

I have checked the cable for the blend door and you can hear it closing, flushed the entire coolant system ( unhooked the heater core first) and back flushed the core itself. It was hot for a minute and then back to cold, today I put a new thermostat, T-stat housing, bypass hose, water pump and radiator cap on it. The cap is a factory replacement 13lb model and the thermostat is a 192 degree fail safe.

I'm stumped on what to do next, it's his only vehicle so I need to get it back together for him ASAP.

Ii dislike 'failsafe' and would not install one in my truck, but 'whatever'...

With the engine warm are the hoses at the heater core both the same temperature?

Have you checked for brittle/broken vacuum lines?

Your dad's truck is a newer generation than mine, but my understanding is that vehicles with AC all have blend and diverter doors controlled by vacuum.
If the truck defaults to Max AC the heater core will be bypassed.

He specifically asked for a fail safe, one hose is slightly warmer than the other going to the core. I've checked for vacuum leaks and have only found the fresh air/recirculation door to have issues and repaired it.

From what I understand the cable off of the temp dial controls the blend door for hot/cold air?

But the dial moves fully, from one stop to the other?
Can you hear the door close at each end of travel?

Max AC would open the recirc door and block the heater core entirely.

Yes the dial moves all the way, about halfway on the warm setting it has a spot that gets stiff but then continues to move and the lever moves all the way in the top of the box. It makes noise as you move it but I can't verify 100% it closes completely as I haven't pulled the box apart.

I went back out this morning and unhooked the core then back flushed it a couple more times, now it has the best heat it has ever had and works great! I'm not sure if it had a blockage or air pocket trapped but either way it's fixed now. Thanks for the input with troubleshooting.

Well that's good news. I back flushed mine last year and it puts out better heat but still not scorching hot like I'd like it. May have a cool stat too I really don't know. Where is your temp on the dash gauge with the 195 deg thermostat? Mine is not even to the N in normal most of the time