OPERATION
All models use a two-stage cooling system design. The cooling system is dependent on water pump pressure and controlled by thermostat and pressure valve operation.
Restricted or inadequate water flow through the outboard reduces cooling system performance and may lead to severe power head damage.
Cylinder Block / Cylinder Head Cooling
The flow of water through the cylinder block and cylinder head is controlled by a thermostat and a pressure relief valve. The pressure valve is located in the top of the block next to the exhaust cover.
- Thermostat
- Pressure valve assembly
The thermostat and pressure valve control the flow of water entering the vertical water passages of the cylinder head.
At low speed, the pressure valve is against the seat and the thermostat is closed. Warm water from the cylinder block gradually migrates to the thermostat pocket at the top of the cylinder head. The thermostat opens when the water temperature reaches approximately 143°F (62°C).
When the thermostat opens, water flows down through the cylinder head to a passage in the cylinder block. Water flows through the block to the exhaust housing and then out of the outboard.
At higher speeds, water pressure opens the pressure relief valve at approximately 1800 RPM. Water flows through the valve to the cylinder head and bypasses the thermostat. All water flows through the cylinder head to the outlet passage of the block and then exits through the exhaust housing.
EMM and Vapor Separator Cooling
Cooling water is routed from the top of the cylinder block to the inlet fitting of the EMM water cavity. Cooling of the EMM helps to stabilize the temperatures of internal components.
IMPORTANT: Improper EMM cooling will activate service codes 25 and 29 and the Engine Monitor warning system. Refer to the EMM Service Code Chart at the back of this manual for specific service code information.
Cooling water from the EMM is routed to the water inlet fitting of the vapor separator water cavity. Cooling the vapor separator fuel chamber minimizes fuel vaporization.
Cooling water from the vapor separator is routed to the overboard indicator.
- Overboard indicator
