1986 Other GMC Models heater

Heater problem
1986 Other GMC Models 4 cyl Two Wheel Drive 128, 00 miles

I have a gmc diesel 40 box truk (ISUZU) and the heater will not get hot, gets luke warm. I had checked the temp of the radiator fluid with a thermometer and after 7 miles of driving and then idling for 20 minutes the water was only 140 degrees. I also have over half the radiator covered with cardboard I opend the thermostat housing and tested that water and it was only 140 degrees also. I then replaced the thermostat and still the same problem.