Thread: 99 25hp Merc thermostat/water discharge?

Are you talking about the P hole? If so that one should be steady if not mistaken.sounds almost as if it's getting air into the system. Check the water intake holes in lower unit for any trash and assuming your using it as a clamp on, make sure it's not mounted to high. Barnicale Bill will see this and he knows more than I on the Merc's.

Yep P hole. This is running on muffs or fast and slow on the lake, makes no difference.

That should have a steady stream. Is the anti cavitation plate even with the bottom of the boat? When is the last time the impeller and thermostat was changed? If you don't know You should change them.

I looked at an exploded view of the 25 and the thermostat is just ahead of the discharge hose and not on the side of the block like the larger engines. As for Impeller and thermostat change I don't know when the last time was. The guy I got it from said he put a new impeller in not long ago but we all know that means nothing. Half tempted to pull the t-stat housing and see if there is even one in there, the owners manual says it is optional to have a thermostat. Then run it without it, all of the old engines survived fine without one.

I have a 2000 25hp merc. It did the same thing, and the water was always very warm. I took the thermostat out, and now a steady stream of cool water. Ran it like that for 3 years.

Pulled the t-stat and cut the guts out of it so it would still have some restriction. Pumps like mad now.

Nice. Glad you got her fixed

If it was me I would put a new thermostats in there. Controlling the water temp/ engine temp helps with fuel burn and can reduce carbon build up.