jrhendo;141596 wroteI hadn't thought about the location of the gauge sender, but from memory of an old coolant flow diagram (maybe one of Ron Kwas'?), the coolant flows to the heater from the elbow and returns via the long pipe. If that's correct, then opening the heater should just channel some of the hot water to the heater, rather than dumping cold water from the heater into the vicinity of the sender.
Yeah, it was Ron Kwas' page. Lots of great info there.
http://www.sw-em.com/Cooling_System.htm#flow_routing_of_cooling_system
Of course, when the heater tap is open, there is extra coolant flow from the rear of the head back to the water pump. It would be interesting to see the results of temporarily bypassing the heater core and seeing if you're getting similar temp readings to that when the heater core is engaged.