Cab heat issue

U may have two separate issues,a bad thermostat and a bad heater valve.

 

Sounds like one of the two thermostats is stuck open.

 

Usually if a thermostat is stuck open, it tends to cause overheating.
It does sound like flow to the heater cores is restricted.
Sounds like a winter front could help out.

 

I had a 2013 4900SF, first two winters here in Saskatchewan cab was cold, side windows would frost up going down the highway, would only clear when stopped.
Third winter I unhooked the cab heater hose off the heater control valve to flush out the heater core.
I happened to glance at the port of the valve and saw that it was mostly closed, only open about 1/8”. I knew my cab setting was full heat. I double checked and confirmed it was selected hot.
I got looking at the valve. The arm were the control cable was attached was mounted into a hole that was too far from the body of the valve so it had lots of leverage but didn’t rotate the valve very far.
I drilled a hole into the arm about half way down and installed the cable.
It was now harder to turn the control knob, but the valve now would rotate fully open.
After that the cab was roasting hot, and the windows stayed clear.

 

Whatever the factory thermostat temp rating is what you want. Though that engine probably doesn’t have near the complexity of the latest stuff, the ECM is still going to expect to see temps in a specific range. I wouldn’t deviate from that.

It’s common for engines to lose heat, when not doing work. All that can be done is to bump the idle even more or add an auxiliary bunk heater.

I agree with the others, you definitely need to be looking for heater control valve issues and plugged heater cores. I’ve experienced both. With a truck that old, if the heater cores have never been changed, they need to be.

 

Thermostats stuck closed, restricting circulation causes overheating. Stuck open, engine will run cold in cold weather, especially at an idle.