It's not the turbo.... you may have a leak in the coolant lines which run to it, but you'd most likely notice the hoses spraying coolant.
The boost gauge is indeed run by a small sensor which has a single vacuum hose running to it. The sensor is mounted by the cruise control actuator and it's vacuum signal comes from the same vacuum hose that runs to the cruise control actuator.
As the heater core sits higher than the radiator's top tank it's not uncommon for air to get trapped in the system... Despite what what many say they've experienced it really shouldn't take long for it to burp out as long as the coolant reservoir is kept topped off for a few days.
I'm going to have to go with bad water pump or blow head gasket.
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