coolant loss but no internal leakage

I have a 97 crown vic. 4.6l
3 mos ago, the intake manifold was replaced--old one split
about month and 1/2 ago, I notice that the coolant expansion tank was
dry. I have been adding coolant ever since.
I pressure checked the system. the needle holds steady.
No external leakage of coolant noted. water pump was replaced about 5 mos. ago.
notice from time to time, that the heater will blow cold. the temp gauge will climb, then fall back down--heat then felt coming from vents. still happens even after replacing thermostat.
I'm stumped. If the head gasket were failing, or if I had a crack in the block, then I would have seen a loss of pressure when I pressure checked the cooling system.
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phil wrote:

I'll be following this thread closely. (don't look too promising) I've had the exact situation with my 97 4.6 Bird. Seems its always been that way but it might have started after factory replacement of original manifold. I lean towards always. Seems like I've always needed to add water and never known why. The "climb" and "fall" stopped happening a couple years back, not sure why. Runs like a dream otherwise. Fixing to change the manifold (again) and while I'm at it pretty much everything else in the cooling system. New manifold right here in the box, just gotta get off my butt and do it.
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