I'm immediately suspicious if the engine doesn't seem to show the characteristic signs that would match the gauge info. A possibility: Run the car a bit, til the engine's warm but by no means hot; around the block might work. Then take (for example) an electronic oven thermometer that has a metal probe to monitor temp of cooking food (or a darkroom thermometer might work), put the probe in either the expansion tank or the radiator, and compare readings as the engine warms up at idle. If there's a significant inaccuracy in the lower 200s (F), there should also be a disparity in the 150-190 range, where you'll still be safe with the caps off, especially since you've got a permanently open thermostat.

No major mechanical expertise claimed, just a bit of inventiveness in experimenting, and I'll still gladly defer to an actual knowledgeable person!

__________________
Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)