could be a clogged heater core.
I don't know the difference and I wouldn't think it would make a negative difference if they used the Hi output one over a standard.

 

Disconnect the heater core and back flush it. It likely is clogged. If that does not fix the problem then replace the heater core. It should put out enough heat to drive you out of the truck even at -20...

 

Amen, brother! Preach it! On my '84, in the middle of winter you could use the cab as a pottery kiln. The heater seems to have only two settings: Really hot and really, really hot.

A few more things to check:

Make sure the airflow is good and strong. If not, the face of the heater core may be clogged.

If you also have factory air, check the face of the evaporator, too. All airflow passes through the evaporator, even when the AC is not in use.

Don't just guess and say the airflow seems okay. On my truck, with the fan on high and the heat to full hot, I could practically use the dash vents to dry my hair, ala Ron Burgundy.

Beg, borrow, or steal an infrared thermometer. Make sure the radiator inlet is fully warm and close to the thermostat rating. Don't assume (Danger! Danger!) the thermostat is good just because it is fairly new.

With strong airflow and hot coolant, check the temperature of the two lines to the heater core. When all is well, and you've got good heat transfer from the heater core into the cab, you should see at least a 30 degree drop at the heater return line. Any less indicates limited heat transfer from the heater core. If you've got strong airflow on the outside of the heater core, then internal clogging or scale buildup is the culprit.

 

I can't recall if you are running a 351M in that truck. If so, they take a special thermostat that most parts houses want to sell you a Windsor variety instead. I don't know if the Windsor variety will fit or not, but something to keep in mind if all the good advice given above doesn't help, and indeed if you are running an M variant.

 

In order to install a high output heater core, the heater plenum chamber (heater box) would have to be changed, because it's different. I doubt that anyone would go thru the trouble.

There are actually 3 different heater cores: (1) Standard heater // (2) high output heater // (3) with factory installed integral A/C or with standard heater and Hi/Lo Comfort Vent System.

 

You can tell if the heater core is clogged. Drive it for a long drive and let it get fully warmed up. While it's idling lift the hood and touch the heater hoses going to the core. If it's right, you can't hardly hold onto either hose, they are that hot. If they are just warm you are not getting enough flow through the heater core. The upper radiator hose should also be almost impossible to touch for very long.

 

Dave, I'd like to add one thing to your excellent advice. Unless one has had their hand recently calibrated, use an infrared thermometer instead. With the two heater hoses, ideally the return line should be about 30F cooler than the inlet. If both are about the same, that means not enough heat transfer into the cab is taking place. That could be due to poor external airflow, or scale on the inside acting like an insulator. (Scale obviously not a problem with a new heater core, only mentioned for troubleshooting)

To determine the actual temperature drop, it's a little tough to confirm with your hand:

160F on the return line = "Ow, that's hot!"

190F on the inlet = "Ow, ow, that's hot!"

One could also compare the size of the patches of skin left adhered to the two hot pipes, but that has a lot of potentially misleading variables. Or if you have a decibel meter, you could compare how loudly you scream with each scalding pipe. But for the most accurate and repeatable results, I vote for the infrared thermometer.

 

Yeah, I had a similar problem in '77 Corvette......When the heater is working (and it was raining....you know the T-tops always leaked) you could rent the car out as a 2 person sauna! When it stopped working, the culprit was a stopped up core. My mechanic at the time just took the hoses off, and used his high pressure air hose to 'blow it" out. Whole fix took less than 10 minutes and the sauna was back in business.....now the leaking T-tops is a subject for another thread!

 

When my 86 wasn't putting out enough heat, I opened up the heater core box and literally removed a squirrel nest. Blew much hotter then!

Also, maybe the blower resistor has gone south?