Hmm, that must be the Mitsu 3.0L v6, right?

Sounds like the heater core (or route to it) may be blocked if it has no heat. You'd still get pressure in the radiator hoses and under the rad cap.

If the cooling system is tight (no leaks) and functioning properly, it SHOULD maintain pressure even when the car is off. Well, at least M-Benz's do (based on my experiences of late trying to remedy a coolant leak in my 84 300TD).

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94 E320
71 350SL