If the thermostat is operating correctly, the
engine should maintain a steady temperature, whatever the rating for that
particular thermostat is. The only sure way to know what temperature the
thermostat is opening at is to put it in a boiling pot of water with a
thermometer and watch.
I had a overheating problem with my 68 coupe, which
turned out to be a 160 degree thermostat that wouldn't open till 194 degrees!
The car would operate fine, but on stopping, it would boil over a bit
excessively and was an irritation. Having said that , I think that maybe your
heater core may be plugged and could even be on the verge of failure. You can
try back flushing with a garden hose, just keep putting the hose on different
fittings, changing the direction and working the loose stuff out. Only problem
is, if the heater core is corroded internally, you may dislodge enough corrosion
for it to start leaking. This is a real problem on southern cars where the need
for antifreeze isn't as necessary, but when they are ran on water or weak
antifreeze, there is little or no corrosion protection. Also, the fact that the
windshield doesn't clear quickly makes me suspect you have a small heater core
leak already, just enough to mist the inside. Antifreeze doesn't evaporate as
quickly as water, have you noticed any sweet smell coming from the
vents?
Heater cores are a bit of a problem on our vintage
machines,when I got my 68 4 door, it had the
heater hoses disconnected for this reason, I'll replace it when I start
on my 4 door in earnest.
Phil <><
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