COMPONENTS
General Description
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.
IMPORTANT: Restricted or inadequate water flow through the outboard reduces cooling system performance and may lead to severe power head damage.
Water Pump and Intakes
External water intakes mounted in the gear case housing collect water and must supply the inlet side of the water pump with an unrestricted and un aerated water supply. Water is drawn into the water pump through a hole in the lower plate of the water pump assembly. All cooling water to the power head is provided by the water pump.
• A nylon wedge (impeller key) is used to engage the impeller bushing and driveshaft.
• The nylon impeller housing with stainless steel liner must seal against a separate stainless steel water pump plate.
• The pump operates as a positive displacement pump at LOW speeds (below 1500 RPM) and
as a centrifugal pump at HIGHER speeds.
• The bottom plate MUST seal to gear case housing. Refer to WATER PUMP on p. 288 for water pump servicing.
(Impeller key removed for clarity) 000697
- Water inlet
- Water outlet
- Seal
- Plate
- O-ring
Water Supply Tube
The water pump outlet connects with the water supply tube located in the outboard’s midsection. Grommets seal the water tube to the water pump housing and the adaptor housing. The water tube fastens to the inner exhaust housing with a water
plate and screws. Water supplied to the water tube provides all cooling water to the adapter housing and cylinder block.
Adapter Housing
• Adapts the cylinder/crankcase assembly (power head) to the exhaust housing and the water
tube.
• Provides water passages which route incoming and outgoing cooling water.
• Provides exhaust passages which connect to the inner exhaust of the outboard’s midsection.
Pressure Relief Valve
A pressure relief valve is used to control water flow and operating temperature at higher speeds (above approximately 1800 RPM). Pressure relief valve activates or “blows-off the seat” as water pressure increases. Spring tension is used to set the “blow off” pressure.
Overheating: A restricted or faulty valve typically results in HIGH SPEED overheating.
Overcooling: Debris may prevent the valve from closing completely.
Pressure Relief Valve Components DRC8113
Thermostats
Thermostats are used to control water flow and operating temperature at lower speeds (below
1800 RPM).
Overheating: Restricted or faulty thermostats typically result in LOW SPEED overheating.
Overcooling: Debris may prevent the thermostat from closing completely
Thermostat DSC02335
Refer to THERMOSTAT SERVICING on p. 223.
Block Venting
A fitting and hose connected to the top of the block allows the constant movement of water
and/or air from the center water passage (top) of the block. Circulated water flows to an outlet fitting and exits the block through the adapter/exhaust housing.
