The exhaust from a diesel engine can tell you a lot about its health. Smoke can be blue, black, white or gray, and each color can be an indicator of specific mechanical problems.

Diesel exhaust should be perfectly clear after a 30-minute warm up under load. If the smoke is a little gray at first, it usually means that the engine is cold, causing condensation that prevents the diesel fuel from getting a complete burn. It is common to see this for the first minute or so after ignition.

But other colors can point to problems. Blue smoke can be an indicator of excessive oil in the upper cylinders; black smoke may indicate unburned diesel fuel; and white smoke is often water vapor. The exhaust may not tell you exactly what is wrong, but it is a useful benchmark when taking a new boat out for a sea trial or just monitoring the health of your own power plant.

- Capt. Alan R. Hugenot