First I have seen it happen before, is the spring side of the thermostat facing in to the engine, just had to ask, check your fan clutch, with engine off spin fan with your hand, should only turn about 3/4 of a turn, if it just spins freely bad fan clutch replace it.
These cooling systems can get rusty,realy bad, if this is your case then the first thing i would do is a block test,this is done with a tool and a liquid that detects exhaust entering the cooling system,if it passes this test and t-stat was replaced as you said and radiator is flowing then i would look at the waterpump to sure the impeller is not rusted away, i worked on many of these thar ended up with bad heads or gaskets for the cylinder heads.
Answers & Comments
First I have seen it happen before, is the spring side of the thermostat facing in to the engine, just had to ask, check your fan clutch, with engine off spin fan with your hand, should only turn about 3/4 of a turn, if it just spins freely bad fan clutch replace it.
These cooling systems can get rusty,realy bad, if this is your case then the first thing i would do is a block test,this is done with a tool and a liquid that detects exhaust entering the cooling system,if it passes this test and t-stat was replaced as you said and radiator is flowing then i would look at the waterpump to sure the impeller is not rusted away, i worked on many of these thar ended up with bad heads or gaskets for the cylinder heads.
Flush the heater core and give that a try