Got the boat out yesterday with the borrowed pump on it. Ran pretty much all day without much of an issue. She was running a little warm, around 180, but she seemed to hold there. Even running WOT for a mile or two, no issues.

On the way back to the ramp, I noticed the temp gauge fluctuating, then it started going over two hundred... quick! Then the alarm went off. Shut everything down and went to see what was up. I could hear water boiling. Not good.

We checked to see if anything was caught on the leg, didn't see anything. Got back in the boat, fired her up waited. Watched the temp guage drop, and then the alarm quit. With it running, I pulled the inlet hose from the tstat housing, and found plenty of water. Tested it through about 3k rpm... filled the bilge pretty quickly.

I'm going to pull the fitting for the transom hose connection to see if there's a restriction. I'm thinking there isn't.

I'm baffled. Could it have been a coicidence that we snagged something and blocked water flow?

I have water pressure gauges on my Fountain. One time I noticed the starboard engine drop from 15 to about 5 psi. I slowed down and shut off the engine. I restarted it a few seconds later and I the engine was back to normal pressure. When I back-flushed the drives I noticed I blew out some seaweed. Now I back-flush the drives after I flush the motors. I also use Salt-Away. It's a small additional price to pay to ensure clean engines and drives. BTW, do you have closed or raw water cooling? 180 seems "on the edge" to me either way. After you flush everything if your temps do not go down you might want to consider a new thermostat. I would recommend a high flow unit as the stock Mercury ones do not always keep boats at their rated opening temperatures. Good-luck.

Ive had this happen before....driving along and buzzer goes off. Shut it down and wait a minute, fire it up and all is fine. For me I think it was pieces of impeller in the oil cooler. There were enough pieces in there to completely block the flow if organized just right. I would say it was like shaking up a box of pencils...some times they would be all criss crossed (which would allow water flow) and about 1 out of 40 times they would all line up which would cut the flow too much. If you ever lose an impeller do not just back flush the system...IT WILL NOT GET ALL THE PIECES OUT....take the hose off of the oil cooler and fish the pieces out.