Working on my '84. This started by having no heat in the cabin. Thought it was a blocked heater core, so unhooked the lines going to the heater core and flushed with a garden hose. Got lots of gunk out of it and was flowing good. Got so much gunk, I changed the coolant ran some flush through it and it all seems good. Went ahead and changed the thermostat (192 degree). Still no heat and temp gauge never got to the N on the gauge while driving. Found a leak around the thermostat housing, replaced housing (old one was far from a flat face). Also, put in a 195 degree thermostat. lifted front of car opened radiator cap and let it run for a good while to let out any air. Let it cool, made sure coolant level was full and put on radiator cap. Started it and ran at 2000 rpm for a while and watched the temp gauge raise up to the A. Radiator fan kicked on. Temperatures taken with infrared temp gauge:
Thermostat Housing 195
Upper Hose 197
Lower Hose 184
Heater Hose #1 198
Heater Hose #2 165
Outside Temp 42
I didn't put a thermometer on heater air, but felt pretty warm, about what I would expect. All was good so took off on a drive. Temperature gauge went down to the left of the N and stayed there. Heater air moved cooler as would be commensurate with the dash temp gauge.
And by the way, the temp gauge has always been low. I thought just a bad gauge and a clogged heater core.
The car will just not warm up on the road. Seems like a stuck open thermostat, but this is the 3rd one I have had on it with same results. Could be air pocket in the system, but have done all I know to remove any air.
Looking for ideas...