Steve,
Your symptoms all point toward a leak from the cooling system to the
combustion chambers, almost certainly leaking head gaskets.
I believe the temperature spikes because combustion gasses build up at the
thermostat & reduce heat transfer to the thermostat, causing the engine to
get noticibly warmer than normal before the stat opens.
I have seen the same symptoms, of the sudden high temperatures, poor heat
from the heater & the stubborn refusal of the coolant to return to the
radiator in numerous cars, including my Tiger 260 & my 86 Mustang 302.
It is usually accompanied by mysterious coolant loss, and perhaps a bit of
smoke from the exhaust, but you can go through a fair bit of coolant without
having excess smoke or exhaust odor.
It is almost always cured by replacing the head gaskets.
Ofter you will not have the satisfaction of seeing a visible coolant path
through the gasket.
If the heads are reasonably flat & not cracked (they probably are, but it is
best to have a machine shop inspect them) replacing the gaskets is likely to
cure the beast.
Doug Leithauser
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