wot overheating

[KRISTIAN1979]

1996 75hp 2 stroke

took it out for the first time after putting it on my boat
it will idle all day and run all day half throttle when I go to WOT takes about a minute and the high temp buzzer goes off (solid buzzing) when I take it back to half throttle a few seconds goes by and the buzzer shuts off and it never stops peeing water .

any help would be greatly appreciated
Kristian

 

[Texasmark]

Re: wot overheating

The engine has a thermostat and a popoff valve. The stat is to control engine temperature at engine speeds below 2500 rpm. Above that the water pressure forced into the power head by the impeller forces the much larger diameter popoff valve open. This allow a great deal of cooling water to circulate through the engine which is needed at the higher rpms.

I think your popoff is stuck shut.

Pop the cowling. Stand behind the engine. Look at the plate that the spark plugs stick through. Look up in the upper left hand corner of that plate. There is a cover plate there, held in place with 2 screws. The upper circular part of it is smaller than the lower part. The upper has the stat beneath it and the lower the popoff. They probably sell them on this site and while you are getting a new one, you might as well get a new stat too. The opening temp of the stat is stamped on the bottom of the pellet and it is probably 143F. Ensure that the replacement is the same temp. The popoff isn't marked as it is a pressure valve.

As you are removing all that, record where things are so that you can replace things as you should during reassembly.

Mark

 

[KRISTIAN1979]

Re: wot overheating

I have already replaced the t-stat and poppet valve the guy I bought the motor from just had the impellor replaced and I also pulled the lower unit and checked the quality of the impellor my self and it was still pliable and soft I am going to pull the lower unit again tomorrow to check it again.

also does anyone know if they make a higher flow water impellor assembly for this motor?

 

[KRISTIAN1979]

Re: wot overheating

I was reading another thread on here with other engines overheating at wot and someone said for them to pull the power head off and replace the gaskets where the midsection meets the power head I was wondering how do I test to see if I need to do that

Thanks
Kristian

 

[KRISTIAN1979]

Re: wot overheating

Another thing I thought of Is the over temp alarm will go off on muffs at home but not when I put it in a barrel and run the motor

 

[Texasmark]

Re: wot overheating

I was reading another thread on here with other engines overheating at wot and someone said for them to pull the power head off and replace the gaskets where the midsection meets the power head I was wondering how do I test to see if I need to do that

Thanks
Kristian

My guess would be to look for water coming out around where the block and the midsection meet. I'm looking at a parts breakdown for the interface of the midsection to the block and it shows the water tube going into the exhaust tube upper plate, not directly into the block. There is a gasket between the exhaust tube upper plate and the exhaust plate assembly and will be another between the exhaust plate assembly and the engine block. If one of the gaskets is blown internally, you might not see it. Since you did the other things, barring the block being full of corrosion, I guess that's where you head next.

Side note. Pee comes from the exhaust manifold and has nothing to do with the block cooling around the cylinders which the stat and popoff control. The exhaust manifold has to be cooled as long as the engine is running due to the hot gasses present.

Considering the complexity of the things I mentioned above, aka the shape of the gasket, it is highly possible that part of it has deteriorated and made an opening where there shouldn't be one. But for it to overheat that fast, seems to me it would take a big hole. Other thing is that the seal at the top of the water tube which is held to the exhaust tube plate assy by a clamp and 2 screws may be defective, or the seal at the water pump outlet. I think I'd check them before I tore the whole shebang apart.

Finally, at WOT, are all your water pickup holes under water?

Mark

 

[KRISTIAN1979]

Re: wot overheating

At wot it looks that the water line is at the cavitation plate just like on the motor I took off the boat which was a 1996 evinrude 50hp and it ran fine .

I don't know if it has anything to do with this but wot on new motor is only getting to 4000 RPM .

 

[KRISTIAN1979]

Re: wot overheating

Are u able to get to the bolts for the water tube from the bottom

 

[KRISTIAN1979]

Re: wot overheating

After work went out and started the motor on the muffs and im not getting any water out tell tell so I pulled the lower unit and impellor looks perfect checked to make sure impellor wasn't slipping on the shaft and i couldn't budge it .

Looked up the part number on impellor and it is a Sierra part number
Looks like on this motor I have to pull the power head to check the water tube gasket.

 

[KRISTIAN1979]

Re: wot overheating

UPDATE
I pulled the exhaust plate today and I think I have located the problem
The exhaust tube where the water tube bolts to was broke and put together with a BAD JB weld job and the clamp for the water tube was missing and someone tried using RTV to attach the water tube to the opening and half of it was gone .
Also found in the LU that the plate under the water pump was very pitted and cracked and JB welded up .
So hopefully this solves the cooling issue

So parts ordered today are
Exhaust plate
Exhaust tube
Water tube ,gasket ,clamp
All the bolts I need
Mercury water pump assembly
And the plate that bolts to the LU that the water pump housing bolts to