Idling diesel engines


Question: My mechanic friends tell me not to let my diesel engine idle for long. They say that it should either be under load or turned off. I hear a lot of diesel engines in trucks idling at truck stops for hours though. Is there a difference in the two types of diesel engines or are my mechanic friends mistaken?

—C. Henry Depew
Tallahassee, Florida

Professor Diesel: Your mechanic friends are right. Diesel engines (and gasoline engines, too) run better when they are operated under-load and at appropriate operating temperatures. If a diesel is left idling, the internal temperatures will eventually drop, a change the temperature gauge may not register. The drop, in turn, can cause internal parts to cease fitting together as the manufacturer intended.

Pistons and rings, for example, will start failing to match the cylinder walls and other parts of the engine, as will bearings, timing gears, and various valve-train components. Varnish deposits will ensue, particularly on the engine’s cylinder walls, and those deposits can cause the rings to seat incorrectly. Excessive wear or oil consumption, smoke, poor performance, and eventually, engine failure may result.

Moreover, diesels that idle for long periods cause air pollution. That’s why there are state and federal laws restricting idling times. Violating these laws bring fines, points on one’s license, and other penalties.

And remember. Pleasure boating is supposed to be an enjoyable, outdoorsy experience. Sitting in your boat with a smelly engine idling interminably in the boat next door is annoying, particularly when you know the exercise is, at best, pointless.

Most engine manufacturers supply operator’s manuals explaining how to run and maintain their products. All too often, when aboard another guy’s boat, I discover this manual lying forgotten in the shrinkwrap.

I strongly suggest that every boat owner dig up his manual, take it into his engine room, and read the thing while looking at the parts described. It’ll save time, money, and heartache!

Professor Diesel is Larry Berlin, director of Mack Boring’s Training Services division.


This article originally appeared in the May 2011 issue of Power & Motoryacht magazine.