Maybe
The 5411 exhaust system is a little different from other boats and somewhat deceptive. It's set to run at 145. What's different is that, as it warms up, the warm water coming from the engine is directed back to the inlet, where it mixes with new inlet water (there should be a T fitting near your through hull where the two hoses join). It's only after the engine warms up to 145 that the thermostat lets much water out through the exhaust and even then it's sporadic. Prior to that there's just a trickle to cool the exhaust. Get used to well meaning observers telling you that you don't have water flowing through your exhaust. You also may notice steam coming out of your exhaust which is normal but different from most boats that always have a full flow of water through the exhaust. I actually got nervous enough to call the Universal distributer but was re-assured that all was normal as long as my exhaust hose wasn't getting too hot. What doesn't sound right is it dropping all the way to 0. Mine oscillates between 130 and 150 or so. The colder the water the more dramatic the temp drop. You might want to test your thermostat in a pan of water on the stove. It should trip at about 145. If I remember right, the thermostats are expensive at $40 or so. Check a Kubota tractor dealer to get the best price. Diesels are actually designed to run much hotter that that but if they did, salt would precipitate out of the cooling water and clog up the water passages so don't use a hotter thermostat.
