Evaluating the condition of a diesel engine on a boat involves two key steps: a thorough visual inspection and a cold start.

Diesel engines give visual signals. How one looks can tell you just about everything you need to know. A trained eye can see the problems without ever doing a compression check or taking an oil sample.

Seasoned diesel mechanics know that the condition of the engine interior is explained in the color of the exhaust smoke and that much can be discovered about an engine's maintenance history simply by looking to see how clean it is on the outside.

CAPT. ALAN R. HUGENOTThis spotless engine was either never used or carefully maintained. Notice the spare oil and antifreeze stacked neatly on the shelf.Visual tests, which take just a few minutes, will quickly tell you whether the engine is worth looking into or not. If the engine fails these tests, it usually means trouble. Next, start the engine and observe the smoke signals, which will give you the rest of the story.

This approach will give you a good read before hiring a mechanic to do expensive compression checks and look deeper into the engine. If you still want to go forward, you will have some idea of what you are getting into, and the mechanic will have to fix the problems, not diagnose them.

VISUAL CLUES

Is the engine room spotless? If the previous owner regularly changed the oil, then he generally wiped up spilled oil too. Conversely, if the engine room looks like no one has crawled around in years, the chances are that no one has.

Clean engine rooms usually mean clean engine interiors. If the engine room is not spotless, prior maintenance is automatically suspect. If the engine room is filthy, you can almost guarantee there was no maintenance done. (For a story on proper diesel maintenance, see link below.)CAPT. ALAN R. HUGENOTThis engine was fully operational, but it is obvious it was receiving only minimum maintenance.

You can take this visual inspection further. Boat engines depend directly on seawater for cooling. Seawater is, of course, filled with sand and marine organisms that can quickly destroy pumps and impede or stop the flow of cooling water into the engine. Give the hose connections, the pump, and the heat exchanger and a good look. If any show green crusty residue from salt water leaks, there's a strong chance that the engine has internal cooling problems. If the paint has flaked off the heat exchanger, it's another sign of the same problem. If the boat has been sitting unused for several months, it may overheat shortly after reaching operating temperature because the seawater intakes are clogged with marine organisms.

Saltwater leaks are a different matter, and you should also look for signs of work around the ends of the heat exchanger. Determining whether the heat exchanger was allowed to freeze last winter or the thermostat is malfunctioning will require a mechanic's survey. But this visual marker already tells you there is a cooling problem.

Here's another quick test: Is there an engine hour clock and is it working? If there is a working engine clock, and the owner has listed the hours in the ship's log when the oil changes were made, you can feel fairly certain that proper maintenance was done. One exception is auxiliary diesels on sailboats under 35 HP, which often do not have an engine clock.

THE COLD START

Tell the owner that you want the engines to be cold when you come to do the test. Many sellers will run down early and light the diesels so you won't find out how difficult they are to start. When you get there, feel the engine exhaust elbow. If it isn't cold, then the engine has been turned over in the last four hours or so, in which case the smoke signals may not materialize. Postpone the test until the following day when you have a cold engine.

Make sure the boat is securely tied with bow lines, stern lines and both quarter and bow spring lines, so that you will be able to operate the propellers while it is tied up and give the engine a load to warm up on.CAPT. ALAN R. HUGENOTLooks can sometimes be deceiving. The rust and dirt surrounding this diesel generator might indicate poor maintenance. However, closer inspection shows new belts, hoses, and oil filters. The rust was caused by a leak.

Start the cold diesel engine immediately. If it won't start, then you can be sure the prior owner has not been starting it regularly, which is what a diesel engine requires to prevent corrosion on the inside. The by-products of combustion adhere to the engine interior, and without the proper anti-agents that are contained in diesel engine lube oil, the cylinder walls quickly corrode. (For a story on diesel lube oil, see link below.)

If it does start, observe the quantity and color of smoke emitted by the engine. Let the owner start the engine while you observe the smoke from the dock. Position yourself so that you are looking through the smoke toward the sun. It is a good idea to write down the time of start, what color you observe and also the time that the smoke clears.

What you should see is some white or gray smoke when the engine first starts up on a cold morning, which disappears later. This smoke appears because, until the engine reaches operating temperature, the moisture in the air is condensed as steam in the combustion chambers, which appears white. If the smoke is a little gray, it means that the diesel fuel is not getting a complete burn because the engine is cold and the condensation water is preventing it. The black smoke from the unburned diesel combined with the white steam comes out gray. With a new engine, or an older engine that has been well maintained, it should clear up within one or two minutes.

After the engine has been running for five minutes, place it under load. You can do this at anchor by merely placing the engine in reverse. It will pull against the anchor chain but the anchor which will not drag if properly setIt is important to always use reverse propulsion, because this has no steering effect on the boat. She just pulls on the anchor chain or the mooring lines without moving sideways. To warm up properly bring the engine up to between 1,000 and 1,500 RPM. When at the dock, you will need to first put on a quarter spring line leading forward to the dock, to check your reverse movement, and then shift into reverse. It is important to use reverse propulsion, because this has no steering effect on the boat.CAPT. ALAN R. HUGENOTThis Perkins diesel has corrosion at both ends of the exhaust manifold and on the raw water heat exchanger, as well as on the top of the glycol reservoir. These are all indications that cooling has been a problem.

Let the engine run for 30 minutes under load and it will have arrive at its operating temperature, which is usually about 1,000 degrees Fahrenheit. (For a story on engine temperature, see link below.)

Now look at the exhaust smoke again and write down what you see. Smoke can be blue, black, white or gray, and each color points at specific problems if you are not seeing perfectly clear exhaust after a 30-minute warm up under load. Here's a quick run-down of the likely causes: blue smoke is generally an indicator of excessive oil in the upper cylinders; black smoke indicates unburned diesel fuel; white smoke is usually water vapor; and gray smoke is generally water and unburned-but-atomized diesel fuel. Grey smoke often 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 lighting most diesel engines. (For a story on reading diesel smoke signals, see link below.)

You will by now have formulated a pretty good idea of what might be causing this engine's problems. If the engine got a low score on the visual and smoke tests, but you decide to go ahead with the purchase anyway, you've got some work ahead. The next thing to do is to perform the required maintenance and repairs to adjust the engine so that the smoke disappears.