Water Flow; Description - Mercury 225 EFI Service Manual

COOLING

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 pas-
sages 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 engine tem-
perature. 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 performs 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
interconnected. Small passages are incorporated to allow the cooling system to drain.
Page 4B-6 90-884294 OCTOBER 2001