Mercury's are known to suck up more sand thru their water intakes than most all other brands. Check your intake line and possibly inlet itself for any partial blockages, and as mentioned earlier, check the impellar. If sand got in there, the impellar might not have a snug fit anymore and you just can not supply the motor with enough water to keep it from over heating.

Some models of Merc's require you change the impellar every 100 hours of use.

longball Said:
I changed the impeller.

Allen Said:
The question I have for the owner is:

Does the motor lose circulation if you have it sitting still and just run it up to 3k rpm, or is this something that is only noticed when the boat is moving?  We don't know yet if it's an rpm problem or a water intake to the lower unit issue.  Are you sure you got the tube re-inserted correctly into the water pump outlet, or that it's not disconnected and bouncing around the base of the engine?  If I remember right, that tube has a rubber end and you may not have it properly placed in there, or the end could be damaged.  I had this on an old Mariner but the difference is it would pump water through the motor when revved up but not when idling.

Someone else mentioned a possilbe blockage in the system.  Have you tried to reverse the flow through the engine to flush it out?  My older Mariner had issues with sucking sand and leaves up that would plug the discharge on the side of the motor.  Probably shouldn't have, but I got tired of it happening and eventually reamed out the hole so it would pass small rocks.


eyexer, the impeller is the important part of the water pump, but the few boat motors that I have done the water pump replacement on had a kit you can buy of all the asundry gaskets and housing.  If a person is sucking up a lot of sand, the housing itself for the impeller may be damaged.