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Have you checked obvious stuff like is your coolant topped up? An air bubble in the system can do weird things with your heater. Are you sure it's not your waterpump?

The gauge moving up to red would worry me.


+1. Air bubble.


HUMMER .25: No need to compensate.

1989 Dodge Raider: 3.0/5
1992 Chrysler LeBaron Convertible: 3.0/A
1994 GMC Safari: 4.3/A