Mercury Optimax Models 135, 150, Direct Fuel Injection., Page 284Get this manual
COOLING
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 temperatureWhen the thermostats are open, water passes through the cylinder heads and exits to the drive shaft housingAt higher RPM (above 1500 RPM) the poppet valve will control the water flowWater that passes through the poppet valve enters water passages in the adaptor platesWater passes through the adaptor plates into the driveshaft housingWater dumped into the drive shaft housing builds up wall of water around the exhaust tubeThis performs functions: Helps silence the exhaust Prevents air from being drawn into the pump Excess water from the wall of water exits around anodes on the gear housingWater that passes through the air compressor exits out the tell tailWater exits through two 18 in(3.175 mm) holes in the lower adaptor plate into the exhaust