Put Your Motor to Work

Idling and slow speeds increase the wear and tear on your diesel engine, so don't be afraid to run it hard. "Hands-On Sailor" from our September 2009 issue

September 16, 2009

motor work 368

Too cool engine oil can cause varnish to build up (left) on internal engine parts. Use an infrared pyrometer (right) to determine the temperature of engine components, including crankcase oil as it passes through the filter. In this case, the oil temperature is acceptable. Steve D’ Antonio

It seems counterintuitive that the lighter the load on your diesel engine, the greater the likelihood that it’ll develop problems. It’s true, though. When a diesel engine is chronically underloaded, several phenomena occur to shorten its life and increase the need for maintenance and repairs.

The environment inside a diesel engine’s combustion chamber is a hellish one indeed. The temperature can reach over 1,000 F, and the pressure may be many times that of the atmosphere outside. Interestingly, this is how a diesel engine is designed to operate, at comparatively high temperature under relatively high load. The high pressure found within the combustion chamber represents the very philosophy of the diesel-ignition process: Compressing the air increases its temperature, which in turn enables it to ignite the subsequently injected fuel.
Ideally, a diesel engine should be run at approximately 80 percent of its output capacity for 80 percent of the time. In doing so, the temperature within the engine ensures maximum efficiency and longevity.

While discussing proper operating temperature, we should note that the different regions within a diesel engine may be operating at different temperatures while under different load conditions. For instance, when you start your engine and run it at idle or at low rpm to charge the batteries, you may notice that the temperature gauge-which measures coolant temperature-doesn’t move very much. If it’s graduated in numbers, as it should be, it’s unlikely that the needle passes 140 F. When a load is applied, on the other hand, say when you’re motoring hard to make port before a weather systems descends upon you, then the needle should hover around the engine’s maximum-designed operating temperature, which for engines with closed cooling systems is typically between 160 F and 195 F.


In light-load conditions, when the coolant temp is low, the temperature of the combustion chamber is also low. This leads to the formation of excess soot or carbon, which gets deposited on the piston rings, injectors, and valves, reducing their efficiency and working life. Cylinder-wall glazing, which exacerbates what’s known as blow by, also occurs, especially early on in the motor’s life. Blow by-a small amount of which is normal-is the leaking of combustion-chamber gases past the piston rings into the crankcase; the gases carry with them some of the water that’s part of all diesel exhaust.

When the engine runs at the optimum temperature, its oil is hot, about 180 F, and is able to vaporize water that accumulates because of blow by. But in an engine running under moderate load, even when the coolant reaches normal operating temperature, the oil temperature often remains cooler. The consequences of running an engine with “cold” oil are an increase in sludge and varnish production within the oil and water contamination.
Sludge, often brown or tan in color, is a combination of water, carbon, and other contaminants, and it impedes oil flow. Varnish is a much harder precipitate, and it adheres tenaciously to metal surfaces within the engine. Both of these contaminants starve vital engine components of lubricating oil.

The bottom line? Avoid chronically underloading your engine. If you must do so, run it at up to 80 percent of its full load for 15 minutes out of every four hours to stem the sludge, varnish, and carbon tide.


Steve D’Antonio offers services for boat owners, builders, and the marine industry through Steve D’Antonio Marine Consulting (


More How To