2008 Mercury Running Hot

[realjet]

I have a 2008 Mercury 4 stroke outboard. I noticed that the faster I go the water temperature goes up. Is this normal? Yesterday coming back in from a storm I was running wide open and the temperature went up to around 185? and the alarm went off. I slowed down and the temperature went down and it seemed ok. The motor has just over 400 hours. I replaced the water pump at 300 hours (October 2013) and the old impeller didn?t look bad but still replaced it.
Any ideas what may cause this? Is the temperature supposed to rise as one goes faster? I thought maybe the thermostat. It has the original.

 

[Texasmark]

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.