My truck isn't making heat in the cab and it has me stumped. The heater hoses are warm coming to and from the heater core and when I put my hand up under the dash I can feel heat at the heater core. The fan blows lots of air but the air is cold. I really don't want to pull the whole dash out to get a better look at things if I don't have to but at this point I don't know what it could be.

 

I had this same issue when i bought my 90. The problem was that the cable for the temp selector had come off the block off cover for the heater core. You can see this if you look below your glove box without having to disassemble anything.

If you follow the ducting from your blower motor youll see it neck down then back up to where the heater core is. On the bottom of that housing there is a white plastic post sticking down and that is your block off plate. check if your wire is connected.

 

Is the heater valve functioning properly?
The black valve/box-looking-thing on the heater hose on the firewall should move when you switch from cold to hot on the dash.
I'm guessing you have a linkage problem connecting the toggle on the dash and the valve.

 

The cable from the dash to the valve does have a bad spot on it where it sticks, but if you push a little bit it pops through. I got my friend to move the temp selector and I watched the valve and it seems to be working properly.

I am wondering if the core is plugged. I have the fan out right now and when i put my hand up under the dash i can feel heat at the core but with the fan in there was no heat at the vent. I am thinking the core is heating up but with the fan blowing through it isn't circulating enough to stay warm.

I was thinking about trying to backflush the heater core with a garden hose or air compressor but don't know if it will work. I figure I will at least try that before I tear the whole dash out.

 

I finally got around to fixing this issue now that the cold weather is coming. I didn't realize that the heat selector had 2 cables from it, 1 to the valve and 1 to the block off plate. The cable to the block off plate must be broke as i can close it with the selector but it will not open it. If I reach in and open it with my hand the cable will close it again.