Temp Fluctuating 350Mag MPI Raw Water Cooling System
 

I have the strangest issue. On my Starboard engine the temperature has just started to fluctuate from 160 to 185. The weird thing is that the water pressure gauge starts to climb to 30 psi at the same time that it starts to overheat. Then as soon as it hits 185, the temperature drops like a rock to 160 and the water pressure goes down to 12 PSI at the same time. On the port engine I have 1 psi at idle and 7 WOT and the temp stays at 160. Looking at both thru-hull exhausts the flow looks to be the same.Tried changing out the thermostat on the engine with the issue. same exact result...no change??? Removed the thermostat and it runs consistently at 110 with similar water pressure characteristics as the Port engine. Any ideas? Going to start removing hoses tomorrow and back flush coolers but this is weird to me. FYI the water pressure is taken from the manifold and the water temp is taken from the thermostat housing.

Raw water pump hose collapsing on the suspect motor or perhaps slightly plugged?

Found chunks of an old impeller (previous owner might have lost an impeller) stuck in the thermostat housing. I'm pulling my impeller assembly apart tonight to make sure it wasn't the one I recently installed. These chucks were obviously plugging the exhaust ports in the thermostat housing. Ran it after clearing and the water pressure is back to normal subsequently, I don't think any other pieces made it to the exhaust.

Not an impeller I installed. Got to wonder how long those pieces were circulating in there.