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Surveying a Diesel Engine

A clean engine is usually a happy engine. This is what you want to see when you open the engine hatch.
Many sailors become blind to reality during the final stages of buying a used boat. All of us weary of the search and become anxious for the stream of classified ads, calls from brokers, and futile dock walks to come to an end. When the perfect boat drops into our lap, we're so in love that we temporarily lose our sanity. Just when we need to muster the keenest observation of our prospective new boat, we get in a hurry to complete the deal so we can go sailing.

There are many potential pitfalls lurking beneath the fiberglass skin, though, requiring us to be on our toes during the final purchase process. While we expect some minor deficiencies in our new beloved, the main parts—indeed the expensive parts—of the boat deserve our undivided attention before we sign on the dotted line. Major problems with the hull, deck, rudder, and steering system may even cause us to fall back out of love. Defects in the spars, rigging, and sails would most certainly take us back to the bargaining table with the seller. The last item that could put a major dent in the cruising budget is the diesel engine. This one last item, however, is the biggie since it would be cheaper on many boats to replace the entire rig and sails from the deck up than to re-power.

Poor exterior condition is a likely indication of a questionable maintenance record, so inspect closely. 
Starting Cold    When preparing to inspect the boat, insist that the diesel not be “warmed up” for your visit. As soon as you enter the engine room, feel the header tank and exhaust manifold for telltale signs of heat that indicate the engine was run recently. More on this later.

Start with a visual inspection. Heavy layers of grime, streaks of rust, patched wiring, or hoses wrapped in tape may not involve expensive repairs, but they are certainly indicative of the care that the seller gave the engine—if he didn't clean it, how often did he change the oil? Drips of oil or water are of greater significance, and may be a signal of bigger problems. Run a white rag or paper towel under the oil pan, inspecting the contents that emerge.

Next, check the fluids. Pull the lube oil dipstick and check the oil level. A little low is OK, but a little high should send off alarm bells. Run the oil through your fingers, inspect the color and consistency, then smell it to see if it has a burned odor. Especially if the oil is overfull, look for the telltale off-color “milkshake” bubbles that indicate that the oil has been mixed with water, coolant, or transmission fluid. Diesel fuel in the oil is harder to detect, but has a distinctive smell that you'll notice “isn't quite right.”

On some older boats it may be less expensive to re-power with a  new engine than repair the existing one.
Check the coolant. It should be nearly clear, possibly with the distinctive greenish-yellow tint used to dye antifreeze. Major particles or a rusty color should raise at least an eyebrow. Look at the gasket on the underside of the filler cap to the header tank for signs of leaks—a common $10 fix.

Pull the transmission dipstick too. Most transmissions use ATF, although some use lube oil—having the right one is critical, so if you're not sure which it's supposed to be, do some additional research. If ATF, the fluid should drip freely off the end of the dipstick and have a reddish hue. Dark, sluggish transmission fluid with a pungent, burned smell may indicate that an expensive transmission re-build is right around the corner.

While you're there and the engine is still cold, check the condition of hoses, belts, and wiring. While not expensive, these do add up if there are a lot of defects to repair all at once, and they each can ruin a nice sail when they fail. Inspect the motor mounts for signs of delamination or breakage—a flashlight and dental mirror come in handy. Don't neglect to check the exhaust hoses and muffler for leaks, as well as connections to a hot water heater.

Start ‘er Up    If you're satisfied at this point, ask the seller to fire the beast up. Stand directly over the exhaust outlet while he cranks the starter, with every sense on full alert. Almost all of the problems that a diesel engine can have show up in the cold start. Many owners “pre-lube” their engine, that is, they crank it over with the stop applied. This is usually a good practice because it pumps oil to the engine's bearings before the high stress of actually starting. But in this case, don't allow the owner to pre-lube since it can mask engine starting problems. If the engine won't start at this point, of course, your inspection has ended, at least temporarily. Hopefully, it will catch immediately. If it sputters, coughs, spits, or cranks for an inordinately long time before smoothing out, you may need to go to the next inspection stage.

"AlI diesel engines  smoke, but clouds of gray or blue exhaust may indicate real problems for the boat owner.
So why are you hanging off the side of the boat? The reason you are watching the exhaust is to get the first puff of smoke coming out of the engine. All diesels smoke, but clouds of gray or blue exhaust may indicate problems. Excessively gray or heavy black smoke indicates fuel injection or metering problems, while blue smoke signifies an engine consuming oil. A plume of white smoke may indicate water vapor in the form of steam—a possible indication of major mechanical problems. You are also listening to the exhaust sound for any unusual sounds of grinding or clanking.

After the engine has run a few minutes, observe the gauges. Cold start oil pressure in many engines at idle is quite low, so don't be alarmed if you see only 20 pounds or so. Note the idle level on the tachometer—if it is more than 800 rpm, it may have been turned up to mask idling problems. Excessive idle rpm may have also caused transmission problems. Rev the engine up several times into the 2,000-rpm range, checking that the throttle control is smooth, that the engine responds without hesitation, and that it doesn't buck when the throttle is suddenly backed off. Some engines “hunt,” or roam up and down in a narrow rpm range at idle with a minor syncopated rhythm especially if the engine is turbo-charged, but this is usually not serious, unless it is excessive.

Excessive smoke is never a good sign. Some engine problems need a mechanic's expertise to diagnose the appropriate fix.
After checking that the boat is firmly tied in its slip, shift the transmission first into forward for a moment, and then into reverse, noting again the smoothness of the controls. The transmission should drop into gear with an authoritative “clunk”—any chattering or hesitation should be a wake-up call.

With all this done, there is only one thing left to do out on the water. If the seller has the engine manual, locate the manufacturer's recommended maximum rpm, and, after the engine is fully warmed up, run it wide open (yes, wide open!) for three to four minutes. Be careful at first that the tach does not exceed the max rpm for any more than a few seconds—if it does, the boat has an undersized propellor. The rpm should be within 100 of the manufacturer's recommendation. If it doesn't come up to that, the prop is too large, either in diameter, or pitch, or both. Running the engine hard, watch the temperature gauge—if it rises, you may have cooling problems. Also watch the oil pressure gauge to make sure that pressure doesn't fluctuate, which may indicate problems with bearings or the oil pump.

Drop Testing    If you suspect at this point that a problem exists, one simple method of double-checking is with a drop test. Have a friend or cohort stand at the tachometer with the warmed-up engine set to run between 800 and 1,000 rpm. Loosen the first fuel line connector to the injector to disable that cylinder, having your friend record the level to which the rpm falls for cylinder number 1. Repeat this in turn for each cylinder, one at a time. When all cylinders have been tested, look at the numbers recorded. They should all be the same, or very close—if one cylinder has significantly less rpm drop than the rest, it may have fuel injection, or compression problems.

The Professional Survey

If you are still unsure of the iron monster in the bilge, it's time to call in a pro. Since an overhaul or a new engine is such an expensive proposition, a few hundred dollars spent on a professional survey may be cheap insurance. Find a mechanic who specializes in the brand of engine you are working with, if possible. Insist on an oil analysis, since a laboratory report on the trace elements in the lube oil reveals a great deal about the engine's condition. If the questions come down to basic mechanical condition, your engine surveyor may want to perform a compression test with an additional cost.

If the engine proves to be suspect, and you still like the boat, don't hesitate to sit down with the seller or broker and discuss the report from the oil analysis lab and professional survey. Quite often, a price reduction on the boat that is fair to all involved can be negotiated, allowing you to have the boat and enough cash left over to re-build or replace the engine.

Suggested Reading:

Replacing the Diesel Engine by Sue Larry

Electric Motors and Sailing Vessels by Mark Matthews

Diesel Fuel Essentials by Tom Wood


Tom Wood is offline  
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