Quick background: The entire cooling system is near new (<5000mi.): radiator, fan, switches, stainless pipes, hoses, expansion tank, and expansion tank sensor.
Motor has run nice and cool and the temp gauge has never gone past 5/8ths even hill climbing in near 100F heat.
Recently the water pump started making an ugly racket so I decided to replace. I took out all the coolant pips to refurbish and paint them w/ high temp primer & enamel. Also pulled both the expansion tank and the reserve/fill tank to clean up and make room for the job. I pulled the water pump and the thermostat housing as a unit and took the opportunity to clean up the thermostat housing a bit (no soap or chemicals - just some wire wheel brushing). I did have some issues getting one of the bolts loose (too rusted & actually snapped) and during the "battle" there may have been some untoward pressure put on both sensors - however, neither appeared to be damaged at all.
Everything went back together nice - new gaskets, thin coats of Reinzosil where applicable, new hoses betw. the block and TH and betw. the lower crossover pipes.
While bleeding the system I initially had difficulty getting coolant to appear from either bleed point (engine or radiator). I did eventually succeed in getting some activity at the radiator but unfortunately ran out of coolant so I had to stop. Meantime, when I went to shut off the motor I noticed the temp gauge was SPIKED and the red light blinking. Even after letting the van cool for an hour between bleed attempts, and making sure the expansion tank was full or near full, within seconds of starting the motor the needle would spike and the red light blinks.
Bad sensors on the thermostat housing?
Did my brand new expansion tank sensor go afoul?