Overheating is one of the commonest faults a sea trial turns up. Steve D'Antonio separates two kinds. Chronic overheating comes on above a certain rpm and goes away when you throttle back, often with white, steamy exhaust. Acute overheating comes without warning, regardless of rpm. He calls chronic the more dangerous in a way, because the owner gets used to it, until the day full power is needed to get off a lee shore.
How a surveyor checks it
The sea trial. Watch the temperature at cruising revs and at full throttle. D'Antonio says every engine should be able to run at full throttle without overheating, even though few skippers do. Above 195°F begins to concern; many alarms trip at 205°F.
Water at the exhaust. A steady flow with the engine running. Sail magazine checks the raw water discharge first.
The 20°F test. Measure the seawater temperature and the water leaving the exhaust outlet. D'Antonio says the difference shouldn't exceed about 20°F. A bigger gap points to a restriction in the injection elbow or the water supply to it.
The cooling system, in order. D'Antonio's sequence: strainers, including any on the hull; the raw water pump, impeller, cover plate and cam; the heat exchanger end caps for debris and scale; then the injection elbow. Sail magazine adds the thermostat, the coolant level and the belt.
The coolant. It should look clean and bright, and never rusty, D'Antonio says. Many makers specify a change every two years; lab analysis costs about $30.
What decides the grade
An engine that overheats on the trial is an A, chronic or acute. A creeping temperature at full power, a large exhaust temperature gap or rusty coolant are B. Coolant with no change record on an engine that runs cool is a C.