Mercruiser 350 MAG MPI Seawater pump replacement / overheating

[captnvinny]

Have a 2007 Merc Bravo 3 , 330hrs. Seawater pump bearings rusted out and blew the front seal w/ shaft off the pump housing. Took it to a shop I had work done before and they told me their motto..... B.O.A.T. So I decided to grow some and do it myself. Removal and installation 46-862914T10 (w/o air fittings) went fine. The pulley was tricky to swap to the new one but went on ok. I did turn the pump clockwise and it was tight (from the new impeller) and turned. I put back and watched my temp rise at idle w/ muffs on. Increased rpms and the temp decreased. Idle is the issue causing the overheat at this point. So with the rust in mind I decided to replace the thermostat, which I already knew from doing a few vehicles b4, these things rarely fail. I pulled it off, tested and it was fine, but I decided to replace. I assembled it correctly and re-tried. Now I watch my gauge go right to 200 w/ alarm after 5 min, IDLE or LOAD.
I had a guy say that at the bell housing there is a gimbal housing nipple that could have corrosion and could have a restricted area. Yet when under load, the impeller is pulling more water and thus it would cool down. I watched the muffs from start up and the waterpick up seems to be fine. I kind of now suspect the raw water is getting to the pump from the bell housing and just not doing the job? When I put my hand on the heat exchangers, they seem ok, but the thermostat housing does get rather hot. Where do I go from here?

 

[captnvinny]

Re: Mercruiser 350 MAG MPI Seawater pump replacement / overheating

ahhh the rust....

 

[tpenfield]

Re: Mercruiser 350 MAG MPI Seawater pump replacement / overheating

Get yourself a manual for that engine, it will help greatly in understanding the cooling system.

Replacing the thermostat is one of the 'canned answers' that you will always get when asking about an overheating problem.

since you replaced the sea water pump . . . move onto the next thing in line. The oil cooler. That is usually the collector of junk that gets sucked up. If you remove the inlet hose to the oil cooler, you should be able to backflush it with the hose that leads up to the thermostat housing.

See how that goes.

I did have on 1 occasion a barnacle that grew inside the outdrive water duct and then detached and got stuck in the narrower passage within the bell housing, as you had mentioned. you could remove the outdrive for a quick look.

 

[captnvinny]

Re: Mercruiser 350 MAG MPI Seawater pump replacement / overheating

this is the bell housing where the water inlet is attached.

doesn't look to have an issues