my mercury marine engine at times over heats. the temputure guage keeps jumping.up and down?

Question

the engine at first over heated,about 200 degress. i then installed a new circulating pump. now the engine gets hot at fast speeds, as i slow down it cools the temp, guage mygo down to about 100 degrees any help would be great

Answer 1

try replacing the pump in lower unit. Sounds like a weak impeller

Answer 2

You failed to give us the most important clue.... Is it an Alpha, Bravo, Straight or Vee?

If it's an Alpha, then the first answerer was correct in saying you might try to replace the impeller on the lower end. If this was replaced they might have installed the gaskets and plates wrong.

In a Bravo you do not have pickup pumps for water located in the drive but there is an additional pump located on the lower engine block (two all together) this might need to be changed as it seems the engine is starved for water.

If it's a Straight or Vee, I'd check the sea water strainer. Seems to me like the engine is asking for water to cool, but not provided so it overheats. When it's slow the demand for water is also declining but keeping the engine cool.

Next would be of course to see what happened to the impeller that went bad. It might have self destructed and got stuck in one of the many passages in the cooling system. Try a high pressure back flush, see your Merc Mechanic for this one (it is set up like a closed cooling system and will clear our anything that is obstructing the cooling system.)

I hope this helps.

Dutch

Answer 3

Check your exhaust elbows if it an older engine than it a chance the water holes have closed up sometime you can drill them out . Pull the hose off the ex an look inside between the SS ring an the outside.there are small holes.Or the manifolds could be plugged with rust. Thermostat might be bad?

Answer 4

Okay, hear me out. Now, the fuel gauge will jump up and down with each wake you hit. Not the temperature gauge though. If your engine is overheating, it has nothing to do with the wakes or water. Check it out to make sure it doesn't have a blown head gasket or a cracked block. It really isn't that hard to figure out.. I figure if you are man or woman enough to own anything that moves like that, especially water craft designed for fun, you better know how to fix it?????? , you better figure out all the intricacies that go with owning it. It can be deadly and make for a screwed up day on the lake if you don't. :)