Broke in a new 498 blower motor today with a Bravo One with no ciculating pump, a crossover, and new impeller in the raw water pump. Timing, fuel ratio, oil, fluids, all look good. Here is my issue. I run a Merc 140 degree thermostat, same as I did on the last setup which was the same motor except for the heads and a few cubic inches. Now the temp goes from 150 to 200 and back to 150 in about 15 seconds. It then stays at 150 for about 5 minutes and then starts the process all over again. Thermostat? Sender?
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Are you running a circulating pump or a cross-over?

Crossover, thanks!

Sorry, I missed it in your first sentence. (It's been a long day.)

Make sure someone didn't put the thermostat in upside-down.
Don't laugh, it happens!

Thermostat could be defective. Try another one.

Yup, I thought of the backwards thing but I was actually showing someone how to put one in at the time so I know it's in correctly!

does the t-stait have three small holes drilled in the outer plate?these holes i am told are there to help keep the engine up to temp and at a constant rate.

Thermostat most likely.
Other less likely possibilities. Collapsing intake water hose or head gasket failure.

Also check for an air leak before the seawater pump, such as a loose freshwater flush fitting if the T-stat shows good.

No, it does not have the holes. It is a stock Merc thermostat came withoiut them and my last one had no holes either.

ghittner,
The holes are suppose to be there to eliminate this particular problem. When I bought a Stainless Marine cross over 15yrs ago.. the holes were there. I am still running that stat today. I havent seen it in a long time, but they are 1/8" I think.. Try drilling three evenly spaced holes in the flange. If it helps, but doenst cure it.. drill them to 3/16"... that should do it.!!
Or call Stainless and see what they recommend..

Hope this helps.

D*ck