Thanks for the advice guys. Impeller was last change is unknown but it's been at least a year if not much longer. It does this "spray" at any rpm. I am mechanically inclined but have no real experience with boat motors. I will research on poppet and t-stats and impeller. I watched a video on YouTube of a guy doing an impeller and the thought of missing a step or not putting on a gasket right makes me nervous. I will research and try to find the t-stats and poppet and see if that will fix things and work my way on to impeller.
Texasmark, I tried the search for that thread but didn't find one by that tittle. Did you say anything dramatically different than what others above have stated?
Not but a few posts from you for the latest iteration: 2008 Mercury Running Hot
On 2 strokers, know not about 4s, there are 2 valves: 1 thermostatically controlled up to 2500 rpm set to start opening at 143 F with the engine OT alarm set to 195, and a second larger pressure regulated pop-off that allows improved, non regulated, flow above that rpm...stated number in manual, actuals may vary somewhat on opening rpm. If your pop-off doesn't open you can experience what you are experiencing.
On 2 stroke I engines both valves are under the same cover on the rear of the engine block, easily visible as a bolted on protrusion from the water jacket cover. If you are going to pop the cover you might as well change the stat too while there. On V engines, that I have seen, there is a stat at the top of each bank but the popoff is below the block fed from a hose connected to the top, rear, center, of the block.
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Complete water pumps are expensive. I don't change just to be changing, especially on expensive parts. One year meets a lot of the criteria of a lot of folks on here. Mine went 10 years and was still working fine but figured it was time to do it. It had a set, slight wear but was still keeping a tough enough pee stream to shoot about 1.5" below the water's surface at idle.
Personally I'd do the popoff and stat as they are easy to do for a beginner. Just remember where things were when you took them out. The money you could have put into a water pump will buy you time on the site Seloc www service manual and help you get them changed out correctly the first time. They sell stats and pressure valves on here too.