5.7 Mercruiser Thermostat Housing Confusion

[Michael Gaeta]

Hello, I have a 2002 300SD Sundancer repowered with 5.7 Mercruisers in 2020. the boat has a raw water Cooling system. This haul out i replaced both thermostat housings. after much searching, i installed the thermostat (160 degrees) in the housing in a different location than what was installed when i observed as installed when i pulled the housings.

Previously, the thermostat was placed directly on the engine outside of the plastic sleeve. based on every image i have seen, the thermostat was placed according this the following:
first the cork gasket, then the thermostat with the spring and charge cylinder visible, then the plastic sleeve, then the gasket. this is what it looks like after the "proper" install.

However, when I installed the thermostat this way (on both engines), the engine ran too hot (stopped the engine when it appeared to go north of 180).

I then changed the location of the thermostats to outside of the plastic sleeve, same orientation, but resting on the engine. in this configuration, the engines ran cold (roughly 100 degrees).
Given the options, i will take a cold engine instead of one that overheats; although none of these are desirable.

I feel it is highly unlikely that both thermostats were faulty, but neither setup makes sense. I would appreciate any help with this.

 

[Scott215]

If I remember correctly when I did mine the plastic sleeve holds in the thermostat so that is the right way to install it That's how mine was

 

[hughespat57]

That thermo is upside down

wait are we looking at the bottom?

 

[Fill Ess]

Did you make sure the holes in the plastic sleeve line up with the chambers in the housing? I think there is a "nub" so it lines u properly.

Do you have this style housing? If so, this is the proper orientation -- so the water only goes out to the top/smaller hoses when the stat is open:

 

[Michael Gaeta]

Yes, the sleeve was lined up properly, i paid VERY careful attention to this and did notice the nubs. and placed the sleeve in the correct orientation, and aligned the nubs. thanks for asking...

 

[Michael Gaeta]

And yes that is the configuration, but the results do not work, it is really odd

 

[Michael Gaeta]

Theoretically, it should work, i guess i could test the thermostats by placing in 170 degree water and observe the valve..

 

[Scott215]

It looks to me like it's in right The sleeve is holding the thermostat in how it shows that's weird why it would overheat like that I don't get that at all

 

[hughespat57]

Give is the serial number off one of the engines please.

 

[Michael Gaeta]

I think i MAY have found the issue, on my boat, after the exhaust manifold i have an extension below the riser. water flows out of the housing (when the thermostat is open) and into the extension, then out of the riser and subsequent exhaust.
i had recently replaced the gaskets on the extension and the risers, when i did that i placed the flow restricting gasket above the extension (between the extension and the riser) and the non restricted gasket on the portion where the manifold attaches to the extension.

in this configuration, when the thermostat opens, the water flow out the housing into the extension out to the riser is obstructed because of the gasket configuration.
i will test the tomorrow however i am fairly certain that this will solve the problem.
in the image below, the red line indicates where the restrictor gasket is currently placed, Mercruiser manual shows the restrictor location is on the blue area, if this is correct, hopefully someone else will not have to make the same mistake....