leave it hooked up the way it is and run a ground wire to it and run the engine up to temp and see if it moves

Yes.

no.
it just makes sure your sender doesn’t have bad ground thru the threads.

Seems like I've pinned it down to a bad temp sender. Went out to the boat today, put a heat gun on the sender for a while to get it hot, temp gauge never moved. But, at least it heated the thread sealant up enough that I was finally able to remove it. It was in there good. Looks super crusty, right?!

Anyway, the new sender I previously bought is apparently the wrong one, too small to thread in. I connected the wire to it anyway and grounded the body, then put the heat gun to it and the gauge moved appropriately, which seems to confirm my original sender is bad.

Current (bad) sender appears to be a Datcon brand part # 02020-00. For the life of me I cannot an equivalent cross reference for this with Autozone, Napa or O'Reilly, nor online. Anyone ever run into this? Autozone guy I spoke with THINKS their TU201 will work but they dont have it in stock and there are no specs online. Maybe Im stuck buying another Datcon online and waiting for it in the mail. It seems the type I need is a 100-240F with 1/2" NPT threads

Don't know what gauge you have, so let's assume you have Gaffrig water temp gauge with a range of 0 to 260. You have to contact Gaffrig and get the sender to match. You also need to tell them what size NPT thread you want.

Your done.

Im not sure which brand gauge either, as it has no name on it, at least not on the front side. Id have to take apart more of the boat to get to the back of the gauge and look for a brand name. Only my tach and speedo show the brand (Faria), but the rest of the gauges have no name. I suppose I better just order the same Datcon sender that was in it then.

That's odd. Typically manufacture's want the advertisement. There may very well be something on the back.
​Did it ever work? If so and you can get that sender online I would start there. If not I would start with a sender designed to operate in that heat range.

Im sure it worked at some point. The boat is a 1991 model. But, I just acquired it a couple months ago and it has not worked since Ive had it. I ordered the same Datcon sender that was in it but wont have it for a week or so. In the meantime, I am going to pick up the Autozone sender # TU201 tomorrow, then go hook the wire up, ground the body, and put the heat gun on it to see what happens. Hoping I can compare what my dash gauge reads to what the temp of the sender is after applying some heat. I have an IR/laser temp gun to check this. Thanks for the replies!

​​​​​​ You really just care about overheating. AutoZone should be able to tell you the heat range of that sender. That's what you want. One size does not fit all.

Got the TU201 from Autozone but its too small. They also couldnt tell me the heat range, very stupid. Anyway, guess Ill wait for the Datcon since I know its the right size and the range is listed as 100-240, which also matches the range on my temp gauge.

That's crazy, they are just being lazy. With a little effort they could find it out. Did you try NAPA? It was probably 1/8 pipe. They make adapters to step it up to work with your intake.