Diesel Exhaust Smoke: What Black, White and Blue Smoke Mean

Get off the boat (dock or dinghy) and watch the exhaust outlet during a cold start, then again once the engine is warm, at idle and under load (in gear, tied securely at the dock or underway). Write down the color, when it happens, the RPM, and the smell: diesel, sweet (coolant) or burning oil.

A wet exhaust always makes some vapor, especially on cool, humid days. Steam disappears within a few feet; smoke hangs in the air and smells. Check the water flow too: it should spit out steadily with the exhaust. Lots of steam with little water means a cooling problem (clogged strainer, worn impeller): shut down and see the raw-water impeller guide before anything else.

With the engine off and cool, check the dipstick and the coolant tank (on freshwater-cooled engines).

• Oil level above full: too much oil can be thrown into the cylinders and burned (blue smoke). It can also mean fuel or water is getting into the oil, especially if it smells of diesel or looks milky.
• Milky or gray oil: water or coolant in the oil. Stop running the engine and call a pro.
• Coolant dropping with no visible leak: coolant may be going into the cylinders (white smoke). Call a pro.

Open the air filter or intake silencer and look at the element. A dirty filter, a rag, plastic bag or insulation sucked against the intake will starve the engine of air. Clean or replace the element as your manual says. BoatUS notes a blocked air cleaner can cause black smoke, overheating and loss of power, and a fully clogged one can stop the engine.

Diesels need a lot of air. Make sure the engine room vents are not blocked by gear, covers or closed hatches. A quick test: if black smoke drops when you open the engine hatch while running at the dock, the engine is short of air.

If black smoke appears mainly under load and clears when you ease off the throttle, the engine may be working too hard. Common causes are a fouled bottom, barnacles on the prop, a bent or wrong-size prop, a heavily loaded boat, or running at full throttle for long periods. Clean the bottom and prop and compare. Black soot on the transom is a typical sign. On a generator, the same thing happens when you ask it for more electrical load than it is rated for (several big appliances at once). The opposite also causes trouble: Northern Lights warns that running its generators at very light loads plugs the exhaust elbow with carbon, leading to smoke and overheating, and suggests keeping them at roughly 40 to 50% of rated load or more. If it continues, worn injectors, a turbocharger problem or a carbon-choked exhaust elbow are next; those need a mechanic.

White smoke that lasts well after a cold start, often with hard starting, usually means fuel is not burning completely. Check that you are using the preheat (glow plugs or intake heater) as the manual says and that it actually works (see the no-start guide). Poor or old fuel can do it too. Change the fuel filters and look for water in the separator bowl. Westerbeke adds an engine that simply runs too cold (long light-load idling, or a thermostat stuck open) and injector or timing faults to the list of white-smoke causes.

A pattern owners often ask about: lots of white smoke at idle that disappears once you raise the RPM, with no coolant loss and no water in the oil. That usually means one cylinder isn't firing at idle, often a faulty injector that only works properly at higher RPM. A diesel mechanic can find which one by testing the injectors one at a time. Have the oil checked as well; a misfiring cylinder can wash fuel into the crankcase and raise the oil level.

Persistent white smoke once the engine is fully warm, especially with a sweet smell, coolant loss, rough running or power loss, can mean water or coolant is getting into the cylinders. Common causes:

• A corroded exhaust mixing elbow (where water joins the exhaust). These can rot inside while looking fine outside and let water back into the engine.
• A leaking head gasket or a cracked head.
• Water backing up through the exhaust (poor exhaust design or long cranking).

Stop running the engine and call a pro. Running an engine that is taking in water risks hydrolock and major damage.

Blue-gray smoke with a burning-oil smell means oil is being burned. First rule out the easy one: an overfilled crankcase (step 3). On low-hour engines the most likely cause is valve stem seals; on high-hour or neglected engines, worn piston rings. Engines that idle for long periods with little load (charging batteries at anchor, for example) can also glaze their cylinders and burn oil. Track how much oil you add between changes.

On a turbocharged engine, heavy blue smoke can come from a leaking turbo seal. Have any oily turbo checked by a pro right away, before it gets worse.

In rare cases, oil leaking into the intake can make the engine "run away": it races and keeps running on its own oil, and the key or stop control won't shut it down. This is an emergency. Get everyone away from the engine, put on life jackets and call for help (USCG on VHF 16 or 911). The way to stop a runaway is to starve it of air, and your engine manual describes how for your installation. Never put your hand over the air intake: the suction and heat can cause serious injury. Ask your diesel mechanic to show you, before you ever need it, what your boat's emergency shutdown plan is.

After running, look behind the exhaust for an oil sheen or black soot film. A sheen that keeps appearing points to oil getting into the exhaust (or a leaking oil cooler on some engines). Wipe the inside of the exhaust outlet with a white rag: dry black soot, wet oil and clean water each tell you something. Never let oily water drain overboard.

Log the color, conditions and what you checked. If a simple fix (air filter, prop, overfill) cleared the smoke, keep an eye on it for a few outings. If smoke persists, give your notes to a diesel mechanic. They may do a compression test, check the injectors, or pull the mixing elbow.