The symptoms don't sound like a classic head leak any more, but it would be
good to do at least the confidence check. With the engine cold, remove the
radiator cap, start the engine and pinch off the hose to the reservoir.
Place the palm of your hand over the radiator cap opening. If you feel
steadily rising pressure or pressure pulses, the head probably needs to come
back off :-( If not, I'd rule it out. I've never experienced a false
result with that test.
My thoughts are turning toward the radiator cap, a radiator hose collapsing,
or even a leaky heater core... but you don't mention the coolant loss that
almost always accompanies a heater core leak. I'd certainly replace the
radiator cap with OEM on general principles unless you did that recently.
And some peeks at the lower radiator hose to make sure it's still plump are
Radiators are relatively soft. I once saw the neck of the radiator had
become "out-of-round", causing several new radiator caps to blow air bubbles
into the puke can because an effective seal was not formed. A new radiator
fixed the problem. You might check for gunk build-up in the neck which
could cause an incomplete seal.
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