Mercury Outboard Cooling System

[Chip S]

I just had the water pump impeller, thermostats, and poppet valve replaced on my 1993 Mercury 150 hp Black Max outboard. The mechanic said the old thermostats and the old poppet valve were constantly open. This might explain why my engine temperature would be 130 degrees when I was idling off plane and 170 degrees when I was on plane.

Looking at the attached water flow path I can't seem to understand how the system works. It seems to me that when the thermostats open the engine gets less cooling water and the temperature should go up, not down. Can someone explain how the thermostats opening increases the cooling of the engine?

I understand that the poppet valve is used to relieve excess water pressure at higher rpms. If the poppet valve is open at low rpms wouldn't this result in higher temperatures since the engine would not be getting enough water? Any insight how this system works would be greatly appreciated.

I'm picking my boat up on Saturday and plan on test driving it. Hopefully with the new water pump impeller, thermostats, and poppet valve the temperature will stay at a constant 150 degrees or so. We'll see.

 

[scoflaw]

With stats closed he water doesn't discharge from engine, same water circulates until it's hot enough to open stats. When they open you are circulating fresh cooling water in around and then out.

 

[Chip S]

Ok, I think I'm beginning to understand this system. My epiphany was when I realized that the amount of water going into the engine must equal the amount of water coming out of the engine. This is what I'm currently thinking.

Thermostats closed, Poppet valve closed: Water enters the exhaust manifold cover > cylinder block > cylinder head and then returns to the entrance point to repeat the cycle. The volume of water passing through the engine is low and the water pressure is low.

Thermostat open, Poppet valve closed: Water pressure is still low but the temperature is high. By having the thermostat open the amount of water that can pass through the exhaust manifold cover and cylinder block increases (amount in must equal amount out). Since the thermostat is before the cylinder head, the amount of water passing through the cylinder head remains the same. The cooling requirements of the cylinder head must not change in this situation.

Thermostat open, Poppet valve open: Water pressure is high. Some of the water from the water pump is diverted through the poppet valve to the water discharge prior to entering the engine to prevent the engine from seeing too much water pressure. A specified maximum water pressure is maintained. Also, the water discharge from the cylinder heads goes through the poppet valve increasing the amount of water going out. This allows more water to go into the exhaust manifold cover > cylinder block > cylinder heads increasing the cooling capacity (amount in must equal amount out). The poppet valve opening reduces the back pressure on the water flow path allowing more water to go through the engine while maintaining the same water pressure.

 

[Chip S]

I picked up my boat and ran it today with the new impeller, thermostats, and poppet assembly. On plane the temperature ran at 148F (down from 170F) and off plane the temperature ran at 130F (same as before). Since the thermostats open at 143F I think it's working like it should. I'm not sure why it's dropping to 130F off plane but I'm not worrying about it.