5.7 mercruiser thermostat poppet valves problem

[mp3mike05]

What are these two spring loaded ball check valves in the thermostat for? Maybe I am a bit lost but water comes in through the bottom and to the waterpump when the thermostat is closed, no water is up by these poppet valves? Then when the thermostat opens water forces the balls open so water goes to the exhaust manifolds.

I am in the process of replacing the entire cooling system (have done sender, waterpump, impeller, thermostat) and noticed mine is basically missing the springs so this valve is always open.

I don't even know where to go about buying a replacement part, can I just buy some springs from ACE and throw them on? Anyone know how stiff they are from the factory? 1990 Skier Euro F3.

[martinarcher]

What are these two spring loaded ball check valves in the thermostat for? Maybe I am a bit lost but water comes in through the bottom and to the waterpump when the thermostat is closed, no water is up by these poppet valves? Then when the thermostat opens water forces the balls open so water goes to the exhaust manifolds.

I am in the process of replacing the entire cooling system (have done sender, waterpump, impeller, thermostat) and noticed mine is basically missing the springs so this valve is always open.

I don't even know where to go about buying a replacement part, can I just buy some springs from ACE and throw them on? Anyone know how stiff they are from the factory? 1990 Skier Euro F3.

You are correct - they are simply a check valve to allow water to only flow out of the t-stat housing and through the exhaust and not the opposite way. Looks like the nylon balls could be replaced as well. You can find all the parts you need here. The springs are Merc PN 24-75533 and the balls are Merc PN 30-98475.

Attached is the pdf of your t-stat housing for reference if you need other PN's.

T-stat.pdf

Edited July 26, 2011 by martinarcher

[coldwater-echelon]

Now I have to ask, are these check valves required? I have a '93 echelon with the same 5.3 motor and thermostat housing and my housing does not have these check valves installed. I have run the boat for 8 years with no problems. I can't tell you though if the previous owner removed them or maybe when he had the boat serviced the marina removed them. Any thoughts guys?

[martinarcher]

I can't imagine they are required, but I know on your 90' the way the springs are they aren't doing much.

My guess was the Merc engineers were worried about the exhaust gas pressure possibly pushing exhaust and cooling water back into the cooling system from the mixing point in the exhaust mani. It's probably a legit concern, but certainly not a typical situation if your exhaust back pressure isn't abnormally high.

[Steve B.]

I have a 95, and I think my housing is the same, or extremely similiar, and mine doesn't have those, um, balls, ha.

Steve B.

[mp3mike05]

I can't imagine they are required, but I know on your 90' the way the springs are they aren't doing much.

My guess was the Merc engineers were worried about the exhaust gas pressure possibly pushing exhaust and cooling water back into the cooling system from the mixing point in the exhaust mani. It's probably a legit concern, but certainly not a typical situation if your exhaust back pressure isn't abnormally high.

Thanks for the link to the parts, that is exactly what I needed. I just put it back together for now as is and will run it until the parts come in the mail. The good news is my temperature gauge is finally working, running better with new plugs and wires, now I just need to find out how to remove the damn fuel tank to replace my fuel sender. Those hoses sure are on tight, I may resort to cutting them off and buying replacements.

Edited July 27, 2011 by mp3mike05

[martinarcher]

Replacing 21 year old fuel line sounds like a good idea to me.

As far as the fuel tank, there should be tabs on the stainless tank that screw it into the transom and floor.