A marine diesel that shows a bit of smoke on start-up is probably nothing to worry about, but if it keeps on smoking after a few seconds of run-time, or starts smoking after it warms up, or when you throttle up, that engine is screaming to you to get something fixed. But what’s causing the smoke, and just what needs fixing?

When we talk about diesel engine exhaust smoke, we’re looking at a pretty small spectrum of color—blue, black, and white. Each has its own subset of possible or probable causes. Let’s review these one by one.

This boat has a bad case of black smoke, a sure sign of excess, partially combusted fuel. It may take some detective work to figure out the cause.

This boat has a bad case of black smoke, a sure sign of excess, partially combusted fuel. It may take some detective work to figure out the cause.

Blue Smoke

Most people who have owned engines have probably heard that blue smoke is generally indicative of oil burning within the engine’s combustion chambers. The big question here should be “where’s the oil coming from?” In the old days of two-cycle outboard engines, before computer controls and super-precise oil injection systems, outboards could be smoky beasts. With modern engines, either two- or four-stroke, there should be no reason to see smoke from the exhaust.

As for oil smoke in a diesel, you shouldn’t expect to see any of that until the engine has many, many hours of run time and is nearing the end of its life expectancy. If you do see blue smoke, here are some of the possibilities.

With a high-hour engine, things like valve seals and piston rings that are worn out can be a cause for excess blue smoke emission from your exhaust. In either case, an experienced mechanic will be needed to make a definitive diagnosis. Your engine may be near the end of its useful service life.

Did you overfill the engine’s crankcase with oil? Check the oil dipstick. If the reading you get is above the full mark, you or someone else may have added too much oil to the engine. This needs to be corrected ASAP, otherwise engine crankshaft seals are at extreme risk of damage as crankcase pressure could end up being high enough to invert these seals and cause major damage.

Is your crankcase breather system plugged or blocked? Any engine needs to breath. If the vent system is restricted for any reason, this can build up excess crankcase pressure that will force oil up past the piston rings and cause oil burning and the resultant blue smoke. Most breather systems today recirculate into the air intake system for the engine or direct the recirculation back into the engine via a hose connected to the valve cover at the top of the engine. Pull the hose off while the engine is running and you should feel a strong pulsing air pressure coming from the lower part of the engine. If this same hose is oil-filled it can mean that piston rings are letting compression into the crankcase area of your engine and allowing oil to blow out of the crankcase and into this recirculation system.

Another potentially frightening scenario is a leaking turbocharger seal. If your engine is turbocharged, the air-intake side of the turbo, as well as the exhaust side of the turbo rotor, has a high-pressure oil seal installed. Remember that these turbocharger rotors often spin at speeds more than 20,000 rpm, so lubrication for the bearings that support the shaft is of paramount importance. But sometimes the seals wear out and allow oil accumulation in the air intake system. This is a dangerous situation that needs immediate attention, because a diesel engine can run on lubricating oil. With a pressurized oil supply to your turbocharger and a leaky seal on the air-intake side of the shaft the turbo blades are mounted on, you will end up supplying raw engine oil to the combustion chambers of the engine. In this scenario, even with the fuel supply to your engine completely shut down, the engine could go into a runaway situation. A runaway diesel running on the engine’s crankcase oil will run until the engine oil is depleted and the engine seizes due to lack of lubrication.

The only way to shut down an engine in this situation is to block the air supply needed for combustion. Some diesel engines come equipped with emergency-stop controls to facilitate shutting down the air supply. If your engine is so equipped, be happy. The truth is, most are not so equipped. So, alternatively, discharging a CO2 or Clean Agent (Halon or one of its replacements) fire extinguisher into the air intake may be enough to starve the engine into shutting itself off. Never use a dry chemical extinguisher for this purpose, as extensive engine damage could be the result. (The powder it emits could build up on the top of one of your engine’s pistons and cause the piston to effectively smash into the cylinder head. In a nutshell, this could destroy your engine.)

The bottom line on blue smoke is not to procrastinate about fixing it. The cure could be as simple as an engine oil change if you address the problem early. Also, understand that the age of your engine plays a big role in the probability of things like broken piston rings or worn-out valve guides. Simply put, on an engine with low hours, blue smoke is probably due to an overfilled crankcase, and that will almost always be due to human error.

A dirty, clogged air filter can change the air-to-fuel ratio and inhibit combustion—one of the most frequent causes of black exhaust smoke.  Doug Logan photo.

A dirty, clogged air filter can change the air-to-fuel ratio and inhibit combustion—one of the most frequent causes of black exhaust smoke. Doug Logan photo.

Black Smoke

Black smoke is caused by excess, unburned or partially combusted fuel, or conversely by inadequate air supply to your engine. In either case the fuel-to-air ratio for your engine has been altered significantly. The trick, of course, is figuring out where the excess fuel or restriction in airflow is coming from.

Also, it’s important to understand that in some cases, a little black smoke can be considered a normal condition. Let’s start with that—the possibly normal.

Your diesel engine is equipped with a governor that helps control the amount of fuel that gets delivered to your engine’s cylinders depending on the load your engine is experiencing. The occasional puff of black smoke that may occur when you suddenly push your throttle to the max is really nothing more than the governor shooting some extra fuel into the injectors before the engine catches up and comes up to rpm. With the advent of electronically controlled diesels this anomaly is beginning to fade away fast.

But let’s say the cutlass bearing on your propeller shaft begins to seize, or you run over a lobster pot and wrap the line around your prop and shaft, or you catch a load of sargasso grass in your propeller. Your governor will sense this new heavy load, and will offer up more fuel — and now black smoke will be a part of the picture. The key here is whether the smoke goes away in short order or not. If it does, and the engine climbs immediately to a normal rpm and boat speed seems normal, then the smoke was caused by the over-reaction of the injection pump’s governor to a sudden rapid change in throttle position. Not a big deal. If the smoke continues and engine rpm is lost, a trip overboard may be in order to try and clear the propeller.

The cutlass bearing can be tricky, as it may not necessarily cause excess drag and resistance with the engine in neutral. I had a case once where the excess load only occurred when the engine was shifted into gear. At that point the engine and shaft shifted just enough for the broken flute on the cutlass bearing to flip onto its side and lock that shaft tightly. This of course tricked the governor into sending more fuel to the engine, due to a perception of increased load.

Don’t forget air restrictions. An air-starved diesel is going to lose power and emit more black smoke than normal. In that case ask yourself what’s changed. A collapsed ventilation hose? Maybe a filthy air filter element?

If the black smoke is also coinciding with poor engine performance and/or rough idle, you probably have one or more fuel injectors that are leaking. Unless you have experience in these matters I recommend calling in a pro on this one.

White Smoke

White smoke can be caused by either excess fuel or an internal coolant leak in your engine. This is a case where you’ll probably want to call in the diesel pros, but here’s a diagnostic tip:

While the engine is smoking, hold your hand over the exhaust outlet for 20 seconds or so. Don’t restrict the exhaust, just attempt to coat your fingers with the smoke. Then hold your hand up close to your face. If you smell a strong diesel odor, the white smoke means the extra fuel is in such excess that it can’t even begin to ignite (as compared to black smoke, where partial ignition has occurred). Typically, this white smoke indicates a serious fuel injection problem.

If you don’t smell a strong diesel fuel odor, touch one of your fingers to the tip of your tongue. If you get a sweet taste in your mouth, stop right there: An internal coolant leak is indicated. (The sweetness is the antifreeze in the coolant, which is poisonous if you were to drink it.) This is also going to require the diesel doc unless you’re very handy yourself. What’s indicated is a blown head gasket or, in a worst-case scenario, a cracked cylinder head.

So, the bottom line here is that any excess of smoke from your diesel exhaust is indicative of a problem. Don’t ignore it!

Written by: Ed Sherman
Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.