Heat/ No Heat / Heat

[4ways]

hello everyone.

I had no heat from the dash vents for the last week. I have some time off so I wanted to sort it out. On my way back to the house I tried turning it on and it works flawlessly.

I have an X-15 on an international with 192000.

I noticed a few months back there was no heat when I cranked up the defrost one morning. I tried getting into the dash to see if it was a stuck blend door but couldn’t really get to it. I got as far as carefully changing the in dash cabin air filter and later noticed the heat was working again.

Fast forward to a week ago. There was no heat from the heater vents. I tried the heat at least once every morning for 3-4 days there. Yesterday, on my way home, I turned the heat on just to see what would happen. Heat was back on.

Now I was suspecting the thermostat because after warming up, the guage seemed to be pinned at 175 for those few days. Even climbing the hills through Alabama and Georgia it was pinned at 175. But it was on the cooler side outside (maybe 70 degrees out going through the mountains) and I had a very light load (11,000 lbs) so Im not sure if it should have climbed to around 200 like Ive seen it on harder pulls. Now when the heater started working again the temp guage seemed to read a bit higher (maybe 180-185ish) but I was empty and on flat ground so Im not sure how high it should get on those conditions, honestly.

So I guess my question is, is it possible the thermostat is intermittently failing?

I was sure it was a blend door after it worked again after changing the filter but now Im thinking thermostat but the intermittent part has me unsure.

Thanks.