Temperature Gauge went crazy

Hello. I have a 96 fourwinns 200 and while out for the seasons first trip yesterday, the temperature gauge did something unusual. To start out, I just had the seasonal services done by my local marina and they replaced the water pump. They have a tank and ran it after repair and said all was good. Yesterday at the lake I was cruising out for about 15 minutes, all gauges seemed fine. Then I slowed to an idle and within a few seconds the gauge went up to around 220 degrees. Then started dropping about 10 degrees/second until it got down to 130, then the gauge just dropped to below the lowest reading of 120. Almost seemed as if gauge went out. But when I shut it off gauge drops lower , so it's still responding to ignition power. I checked to see if I had water flow coming out at the back and it seemed so , and engine didn't show any signs of actually being over heated . So I kept a close eye on it and was out for several more hours. Gauge never worked again. Boat ran fine. Is there a wire that could have got bumped near the water pump or was it just coincidence that the gauge went out. It's a ford 302 V8, OVC. I appreciate any ideas. Thanks

What you saw at first sounds like a stuck thermostat that then popped open with enough heat (at 220.) It may be that your t-stat is now stuck open, so appears not to work. First thing to do is to pull the t-stat. Check to see if it is stuck open. If not, put it in a pan of water over a burner and heat the water and see if it opens the proper distance at the specified temperature. The fully opened temperature and the distance of opening should be in your operators manual. If there is any doubt, replace it. Most any auto parts store should carry the proper t-stat for that engine, assuming you have a closed cooling system. If not, you will need to get a marine-type t-stat. BTW - it's not a good idea to run that engine with a t-stat that is stuck open, thus an engine that is running at a low temperature. Gas will seep down pass the rings and get into and dilute your oil. Pull your dipstick and see if you small gas. If so, you will need an oil change once the problem is fixed.

It is also possible that you just have a bad connection with either the "signal" circuit or the ground. First, find the sending unit on your engine. Typically, it is screwed into the water jacket or manifold. Check the wire coming out of top of the sending unit to make sure it is tight and corrosion free. The sensor relies on resistance to give a signal to the temp gauge. As the temperature goes up, the resistance goes down. I don't know the exact resistances for that engine, but you can look them up and test it with an ohm meter. If that all checks out, then pull your gauge. Check connections for tightness and corrosion. Clean as necessary. Use your ohm meter again to test whether you have a good ground at the gauge - it will be a black or green wire attached to the "G" terminal on your gauge. Test resistance between the ground terminal and a known good ground source at your instrument panel. Resistance should be very near zero.

Hope this helps. Let us know what you find.