I have a Sutphen with a twin 350 setup. Both of these motors have a crossovers. I had the boat out the other day and the waterpump went and the motor overheated. I changed the impeller, thermostat and temp sender after this happened. When i would run the boat before it over heated the temp sould stay around 160-180 and not fluctuate that much. now when i am running the temp on the motor that over heated fluctuates from 160 to almost 220. It almost goes in a cycle from 160 to 210-220 and back to 160 ever 30 seconds. does anyone know why this might be happening after the motor over heated? The other motor stays around 170 running at cruise.

I'm fighting with the very same situation. check to see if the starboard side of the motor is getting hot. In my case water is flowing smoothly through the port side, but not through the starboard side, small amounts of water will surge through the hot side of the block and push hot water over the temp sensor causing the water temp to vary from 100 to 220 degrees in a cycle. If I block off the crossover and pull out the thermostat everything runs cool.

did you flush out the impellor parts ? you may have a clog in the system, oil/p/s cooler ??

Are your thermostats drilled? If your running crossovers with a bypass, often times the bypass water rushing past the t-stat will keep them artificially cooler than the block water temp, until the trapped block water gets so hot that it's finally enough to overcome the cooler bypass water and manage to open the t-stat. Then, once water starts flowing through the block things quickly cool off and the t-stat once again closes. This cycle repeats so on.

The simplest way to overcome this is the drill two to four 1/8" holes in the T-stat.

Works like a champ for me.

I think one of his motors is working and the other has the temp fluctuation. I'm guessing something needs to be flushed.

Ya, that's a definite possibility, but sually with a blockage, the over-temp condition is constant, not cyclical.

Could is be a simple case of a bad t-stat; one that's sticking closed too long?

I'm guessing his old one had holes{And the other motor} and the new thermostat does not.
Mine did that until i drilled it also

Thanks for the responses. I am running one
With a bypass and no holes in the thermostat. I don't think there is any blockage because the impeller came out in one piece. The rubber impeller part became separated from the shaft so it would no longer spin. Should I still drill the thermostat if I am running a bypass?

Absolutely. Without holes in it the T-stat, water doesn't circulate through the block at all until the t-stat opens up. That's not happening until very late because of the cool bypass water hitting the top of it. And by that time, there's excessive heat in your motor. That's precisely why you're cycling from 160-220 in my opinion.

You want some flow through the block at all times so you don't develop extremely hot spots (especially towards the rear of the engine - furthest the water inlets).

And for cheap peace of mind, let's not discount that you may also simply have a sticky thermostat. It won't hurt to get another new one just in case.
It wouldn't be the first time a new t-stat was found to be defective.

Drilled the thermostat and the problem is solved. Thanks guys.