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Electronically governed diesel engines monitor themselves to warn of problems before a failure occurs. They also provide skippers information otherwise seen only on mechanical engines when an engine surveyor is aboard — information that helps with day-to-day operation. Real-time fuel consumption, for example, dials in the best economy for cruising conditions, and exact load percentages help gauge engine health. But where, exactly, do those numbers come from and how accurate are they?

Let’s start with engine load — something only guessed at, based on exhaust gas temperature, in mechanical engines. “The electronic control module (ECM) is calculating load based on the amount of fuel injected into the engine versus the maximum amount it could inject at that rpm,” said Ryan Kamphuis, a senior marine sales engineer for MTU.

“If a 1,000-horsepower engine reads 90 percent load at cruising speed, and that rpm represents about 90 percent of total engine power at full throttle, then the engine is actually producing around 810 horsepower,” he said — 90 percent of 90 percent.

Knowing exact engine load through a wide range of rpm helps judge propellers. “I might see an engine overloaded as the boat comes up on plane, or I might see a propeller overloaded before the engine reaches its maximum torque,” said Greg Platzer, whose company, Platzer Marine Propulsion, creates custom propellers. “I can adjust those props to better match the engines.”

But load readings are theoretical calculations, not actual torque turning the propeller. Platzer sometimes uses strain gauges to measure minute twist in propeller shafts and accurately calculate actual engine horsepower.

“In general, electronic engines’ load calculations are accurate within a few percent, but once an engine exceeds its rated rpm, those algorithms often aren’t correct,” Platzer said. The result might be a boat that looks like it’s propped correctly, yet actually could carry a bit more pitch in the propeller. “Fuel burn is an accurate indicator.” How fuel consumption increases through those highest few rpm should indicate actual load.

A change in engine load at cruising speed likely indicates a developing problem. One or 2 percent variance between engines is normal. Since both port and starboard engines rotate in the same direction, port transmissions typically impart 1 percent higher load to turn that shaft in the opposite direction. Bent or misaligned shafts also increase load slightly, but large variations demand attention. “If an injector isn’t working right, or there is some other problem within a cylinder, the ECM won’t know that cylinder isn’t providing all of its torque,” Kamphuis said. The engine compensates with more fuel to all cylinders, increasing indicated load at a given rpm and lowering maximum rpm at wide-open throttle.