No hot air from heater, only warm

[mp223]

Pretty straight forward symptoms. When I set the heater to 90F, it feels like its blowing, at best, 75F air. Set it to 80F and it feels like 70F.

Truck heats up normally after a few miles, coolant temperature gauge reads slightly on the cold side of the C-H scale after warming up.

I don't think it's the blend door because the AC works fine, as does selecting defrost, vent, or floor.

What could be the issue, the heater control valve? Thermostat? Blend door?

 

[scdix50]

Heater control valve, works/not works or leaks. I run a 192degree thermostat and gage is dead center of range. When was the late time thermostat was replaced. They do not last forever and would be wise to replace every few years. It would be good idea to flush the whole system the same time you do a thermostat change.

 

[mp223]

Thermostat has never been replaced to my knowledge, and its a 1999. Flushed the radiator about a month ago, the fluid was slightly off color but otherwise looked excellent.

 

[scdix50]

When you get a thermostat, make sure you are given the correct one. The early SOHC takes a longer one than the later update housing. If you running a V6 ohv or V8 you should be OK.