Erratic temp. readings...

[DoctorJD]

Might it be a grounding issue? Does the sending unit ground through the block? And maybe the gauge grounds through a common wire in the gauge cluster? That's where I would start.

The sending unit simply grounds to the engine block. I'll start looking at an issue with the grounding of the guage cluster. The only reason I originally doubted this theory is that the temperature guage is the second in line of three guages that daisy-chain a common ground. My fuel guage and oil pressure guages are working.

That being said, I have had some grounding issues on some other accessories on the boat as of late.

[wotan2525]

Sounds like a really weird issue.... if it were me I'd pull out the thermostat and run it... see if that gauge moves at all. It shouldn't go up past 100degrees. This will tell you if the engine is actually heating up too much or if the gauge is failing.