I have a 2005 Weekender 245 with...

[Weekender]

I have a 2005 Weekender 245 with a 350 Mag MPI that I keep on a trailer. When I first start the engine, either on the hose or in the water, it will overheat if I leave it at idle. It won pick up water at idle. If I give it throttle it will pick up the water and run at 160 all day at all speeds.  T-stat and impeller have been replaced and water pump checked. There is no blockage from the outdrive. Anybody know what might cause this?

[SunRunner4]

If the impeller is new is the housing scarred? If the tolerances are too far off it won't self prime at idle and weakly at RPM. I had to replace entire raw water pump housing. I went with an aftermarket SS with wear plates.

[jr_schantz]

I had something like that occur but don't remember the exact symptoms but here's something to consider. I think it was called a bypass valve. It is located in the thermostat housing. It consists of a rod, spring, and ball. Mine (great lakes) became fouled by zebra mussels. In the diagram it's items 3 thru 6. I have a 1989 5.7L so it may differ from your engine/issue. Just an idea to throw in the pot.