COOLING SYSTEM
Overcooling: Debris may prevent the valve from closing completely.
Pressure Relief Valve Components
Thermostats
Thermostats control water flow and operating temperature at lower speeds (below 1800 RPM).
Overheating: A restricted or faulty thermostat typically results in LOW SPEED overheating.
Overcooling: Debris may prevent the thermostat from closing completely.
Thermostat
Block Venting
A fitting and hose from the top of the block allows the constant movement of water and/or air from the block. Circulated water flows through the EMM and Vapor Separator before exiting through the overboard indicator.
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 powerhead damage.
Cylinder Block / Cylinder Head Cooling
The flow of water through the cylinder block and cylinder heads is controlled by two thermostats and one external pressure valve. The pressure valve is connected by hoses to the top of each cylinder head.
- Thermostats
- Pressure valve assembly
The thermostats and pressure valve control the flow of water entering the vertical water passages of the cylinder heads.
At low speed, the pressure valve is against the seat and the thermostats are closed. Warm water from the cylinder block gradually migrates to the thermostat pocket at the top of each 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 and bypasses the thermostats. Hoses route the water flow from the pressure valve to the vertical water passages below the thermostats. All water flows through the cylinder heads to the outlet passages of the block and then exits through the adapter housing.
This pressure controlled water outlet circuit provides a “high flow” discharge of water during high speed operation.
This cooling system configuration provides “balanced cooling” at higher RPM by using one pressure valve to control the discharge of water through both cylinder heads.
Additional Cooling
Two external water supply hoses direct water flow into the exhaust (plenum) area of the cylinder block. The water reduces exhaust runner temperatures and circulates through the cylinder block.
The base gasket limits water flow to the base of the block and diverts water flow to the rear port and rear starboard fittings of the adapter housing. A third external water hose directs water flow from the exhaust housing of the cylinder block to an exhaust passage at the adapter. The water reduces inner exhaust temperatures and cools the propeller hub.
- External water supply hoses
- Exhaust (plenum)
- Inner exhaust cooling hose
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.
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 adapter/exhaust housing.
