90-824052R3 JUNE 2002 POWERHEAD - 4B-3
Water Flow
Description
Cooling water enters the cooling system through the
lower unit water inlets. The pump assembly forces
water through the water tube and exhaust adapter
plate passages filling the power head central water
chamber (located behind the exhaust cavity). Water
enters the exhaust cover cavity through 2 holes near
the top of the exhaust cover.
Water exits the exhaust cover cavity through 4 slots
(2 each side) filling the water passages around the
cylinders. Water flows around each bank of cylinders
to the top of the cylinder block.
Water flow exiting the cylinder block is controlled by
the thermostats (1 in each cylinder head) and the
poppet valve (located at the bottom starboard side of
powerhead). At low RPM (below 1500 RPM), the
thermostats control water flow depending upon en-
gine temperature. When the thermostats are open,
water passes through the cylinder heads and exits to
the drive shaft housing. At higher RPM (above 1500
RPM) the poppet valve will control the water flow.
Water that passes through the poppet valve enters
water passages in the adaptor plates. Water passes
through the adaptor plates into the driveshaft
housing.
Water dumped into the drive shaft housing builds up
a wall of water around the exhaust tube. This per-
forms 2 functions:
• Helps silence the exhaust
• Prevents air from being drawn into the pump
Water exits the engine in 3 locations:
• Excess water from the wall of water exits around
anodes on the gear housing.
• A portion of the water that passes through the
thermostats exits out the tell tail.
• Water exits through two 1/8 in. (3.175 mm) holes
in the lower adaptor plate into the exhaust.
To allow complete passage filling and to prevent
steam pockets, all cooling passages are intercon-
nected. Small passages are incorporated to allow the
cooling system to drain.
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