Mercury Optimax Models 135, 150, Direct Fuel Injection., Page 290Get this manual
COOLING
Problem Diagnosis
Condition Pressure below specification idle Recommended Range 1.0 3.0 PSI (6.8 20.5 kPa) Possible Cause Poppet valve spring defective (weak, broken, missing) Defective poppet valve seal Thermostat stuck open Severe internal leak Low output water pump Inlet restriction Strainer screen for air compressor water supply is restricted Plugged poppet by-pass passage or fuel cooler Wrong poppet valve spring Low output water pump Inlet restriction Poppet valve vent hole plugged or restricted Severe internal leak Defective poppet valve seal Wrong poppet valve spring Low output water pump Inlet restriction Broken diaphragm in poppet valve Severe internal leak Defective poppet valve seal Inlet restriction Engine mounted too high on transom Engine trimmed out too far Configuration of boat bottom interfering with adequate flow of water to coolant inlets Severe internal leak Low output water pump Plugged strainer screen for air compressor water supply Outlet water passages restrictedSteam pocket has formed at top of powerhead due to lack of cooling water