Okay I fixed up this old car (94 Plymouth Acclaim 2.5L) to flip it (was my grandma’s) after her head gasket went out and so she got a new car. I changed the head gasket and everything seemed to be fine and the heater worked on that day (around 50 outside) though I only drove it around the block.
I test drove it this week (well below freezing outside now) and the heat fluctuates from cool to warm to occasionally hot. I thought air might be in the system but I burped it multiple times and still doesn’t help. I couldn’t even get the car to run hot while burping even with the cooling fan unplugged! Max I can get it to run to is 3/4 temperature (on gauge).
I already put in a new T-stat when I did the head gasket, but everything seemed to point towards a bad t-stat, so I replaced it again figured I had a bad one. I tested the one I took out and it seemed to still be good but I replaced it anyway (only $5 part). Still same thing. All the hoses including the heater core hoses get hot and cold randomly while sitting too, so I am sure it is not the heater core (gauge seems to follow the strength of the heat while driving also.) Coolant is not leaking or getting burned up at all. Coolant in the radiator will get hot but not super hot. System will get pressurized when running hot but I can still take off the cap with minimal spitting of coolant. I tried a different radiator cap off my other car just to see and no difference there.
Do you think the water pump is the problem? I was thinking of replacing as it is an easy job and only $30 but I’d hate to do it and waste time if that doesn’t fix it. I would think it would overheat if water pump was not giving enough pressure, but that’s hard to tell being how cold it is outside (my garage door is open while car is running of course, so it is still cold in the garage).
Oh I also tried clamping the upper radiator hose and drove around the block and it didn’t help much, still not that hot of air coming out. Driving speed doesn’t seem to affect the coolant temp like you would expect from a stuck open t-stat…
How Are You Burping It ? Seems I Remember A Hex Head Plug (I Was Always Afraid To Unscrew) For That On The T-Stat Housing. Is There A Coolant Sender There That Will Come Out ?
Coolant is staying full in radiator and reservoir ?
I had a 2.5L Spirit.
CSA
I jacked it up and would let it warm up and then shut it off and wait a while and open the cap to release pressure. Maybe that’s not enough? I saw two hex head bolts and I tried to release pressure from those but they were really on there and I didn’t want to break the housing trying to break those loose! I might try to take the sensor off to bleed it instead. Should I do it warm while running or what?
Coolant level stays solid in both radiator and reservoir. I was worried the head gasket was still the issue! Better not be! 
I always thought that the car would run hot with air pockets, but maybe not always?