I have a 1994 Pro line with twin 5.7 Mercruiser (not original) that are sea-water cooled. Neither possess closed cooling systems. I recently installed new Sierra 142 degree thermostats each and one new temperature sender to starboard motor. The gauges are original, as best that I can tell. With outdrives hooked up with muffs, my gauges read 180-185 after motors heat up. This seem high. I thought 160-165 was normal, after they open? So, I inserted thermometer in-between hose and thermostat housing and tighten hose clamp around hose. This insertion would be very close to temperature sending unit with solid connection. The other end of hose connects to riser. I tested temperatures for both motors using this method. Either motor reads between 142-146 via thermometer. AND, the motors feel cool, regardless after running for several minutes each. My concern is whether gauges are calibrated correctly. My guess is that they are not. Both becoming defective seem a bit remote? The difference between the temperature at the thermostat housing and gauge seems too much.