If you really want to know what’s going on inside your engine, you need to know about oil analysis.

In the surfeit of CSI programs so popular with TV viewers these days, one scene is never absent: The investigators lock eyes with the suspect in the interrogation room and coolly announce that blood samples taken from the crime scene identify him as the perpetrator. Under pressure, he spills his guts, and the case is solved.

Is it possible that the same sort of thing could happen with—of all things—your boat’s engine? It is, but instead of blood, the evidence is found in oil, the lifeblood of your engine. That’s because oil not only lubricates, it also captures combustion byproducts and tiny amounts of engine components as they wear. So like a CSI lab analyzes crime-scene evidence to find a suspect, an oil-analysis lab examines what’s in your engine’s oil and reveals any suspect components or conditions.

The big difference is the process the lab uses to analyze your oil. It’s called spectrographic analysis, and it involves burning a small sample of engine oil and observing the resulting colors in the flame to precisely determine what is being burned and in what amounts. Thanks to the wonders of automation, the process is not only accurate, it’s also inexpensive. In fact, it’s the most cost-effective way to head off engine trouble before it occurs.

Most labs charge around $30 per sample, plus postage. Drawing the sample from your engine is simple and is detailed in the images here. We used PMY senior editor Capt. Bill Pike’s 32-foot Grand Banks Betty Jane and her 135-hp Ford Lehman diesel for the test, as she had had an oil analysis performed 287 engine-hours earlier that would serve as a baseline against which to compare to the new samples. (For a thorough explanation of the process and the results of sample analysis done by both Thompson Tractor and by AV Lubricants, see “No Action Required,” this story.)

Once used mainly in the earth-moving and trucking industry, spectrographic analysis has now become de rigueur for many marine mechanics who use it not only for a periodic health check but to confirm their diagnosis before they begin costly work. I learned from Thomson’s Caterpillar marine field service supervisor Richard Sewell that most newer, electronic Cats have an oil-sample port near the oil filter that allows a sample to be drawn while it’s running at normal operating temperature.

The key to effective spectrographic oil analysis is consistency. Sewell recommends keeping a consistent hour interval between drawing oil samples; he mentions that the most common hour interval for many of his customers, who happen to be big-time fishermen, is 250 hours. For the many pleasureboaters with vessels that will not rack up those kinds of hours, the sample should still be based on consistent engine-hour intervals—as opposed to once per season, although it still may work out that way—and drawn just before you plan on changing your oil and oil filter.

This article originally appeared in the April 2007 issue of Power & Motoryacht magazine.