How to fix a boat engine that smokes black?
A boat engine is the essential tool for all ships: from outboards to tenders. Even sailboats often need a 2-stroke or 4-stroke engine to go out or come into port. It is therefore crucial to have a means of propulsion that works. Sometimes, despite impeccable maintenance, boat engines have problems. It is then necessary to repair the malfunction immediately so as not to put yourself in danger. In this guide, we explain how to fix a black smoking boat engine. Indeed, the black smoke that comes out of an outboard (or inboard) engine can come from several causes. We explain how to identify the problem and how to fix it. Just follow our instructions for your outboard or inboard motor quit smoking black.
When is it normal to have a boat engine that smokes black?
It may seem counterintuitive, but there are three specific cases in which it is normal to see black smoke coming out of your exhaust pipe.
First, if you inherited an old marine engine built before 1990, then there is a good chance it will smoke black. Do not panic ! All outboard motors of this era were made like this.
Then, it can happen under certain conditions that the start-up fumes from your outboard motor are dense and black. Again, there is nothing to be alarmed about. Diesel and gasoline boat engines can smoke black on start-up. But the smoke should quickly turn into colorless and odorless gas. On the other hand, you have an engine problem if the black smoke does not disappear after more than 5 minutes...
Finally, it is possible that your boat engine smokes gray when you accelerate. This smoke which is not completely black is normal if it fades once the acceleration is finished. However, you have a problem if the outboard motor keeps smoking black/gray once cruising speed has been reached for several minutes.
Determine if your outboard motor smokes black or blue
First of all, you have to be sure that your boat ship smokes black and not blue. Correctly analyzing the color of the smoke is crucial in determining the causes of the problem. Sometimes dark blue smoke can look like black smoke. To be sure of the color, we advise running your boat engine for 4-5 minutes. Indeed, a cold marine engine will release start-up fumes which do not provide valid clues. Once the boat engine is warm, if your smoke is blue, you can refer to our article on how to fix a smoking boat. If your smoke is black you can continue reading this article. Know, that it is better to have a black smoking boat engine what a blue! Blue smoke almost always requires you to take your marine engine to a mechanic. While a problem of black smoking outboard motor can be adjusted most of the time without outside help.
What causes a boat engine that smokes black?
NB: the causes of a boat engine that smokes black are the same for all brands: Yamaha, Mercury, Suzuki, Johnson, Honda, etc. And whatever the power of your marine engine: 6hp, 15hp, 25hp, 30hp, 50hp, 60hp, 75hp, 100hp, 150hp, 250hp, etc.
In 99% of cases, a boat engine that smokes black is due to faulty tuning. You have to trace the chain of events to identify the cause of the problem. The black color comes from soot. Soot is a cloud of tiny carbon particles. The release of soot from the exhaust pipe colors the exhaust gas black. This soot is produced by a boat engine when it does not burn gasoline or diesel completely. It is therefore necessary to find out why your marine engine does not consume all its gasoline or diesel. This happens in two situations:
- When the boat engine lacks combustion air
- When there is poor fuel injection into the engine cylinders.
Causes of a Black Smoking Boat Engine Related to Lack of Combustion Air
A boat engine that smokes black due to a lack of combustion air can be caused by several problems:
- la black smoke from your marine engine often comes from a dirty air filter. This is particularly the case when black vapors appear on acceleration. As you rev up, the boat's engine needs more air and with the turbocharger's air filter clogged, the engine chokes.
- If the problem is not with the air filter, it may also be with the turbocharger turbine blades. Check if the blades are not damaged or dirty.
- Also check the cleanliness of the turbocharger nozzle ring.
- Continue your exploration by going up the air suction chain. The turbocharger inlet grille may also be blocked.
- The problem may not be with the turbocharger but with the compressor itself. Also check that its blades are not damaged or dirty.
- The last suspects to check are your marine engine's scavenge ports and valves. They may be faulty or damaged.
NB: if you are sailing on a boat with an inboard engine, it is also possible that the problem comes from the blowers in the engine room. If they are clogged then your boat engine will smoke black.
Causes of a Black Smoking Outboard Motor Related to Bad Fuel Injection
Un black smoking boat engine due to poor fuel injection can come from several problems:
- Often, black smoke is simply from poor gasoline or diesel that is too rich for your marine engine.
- Another possible cause is a problem with the injection pump. An injector may be incorrectly adjusted and send too much fuel into the combustion chamber. This happens if the spring pressure on the injectors is incorrectly adjusted.
If it's not an injection or fuel quality problem, then your problem may be more rare. In this case, the other possible causes are:
- Poor timing of fuel pumps
- Faulty injection valve lift
- Fuel injected into the engine too cold
- The injectors are damaged and continue to drip once the combustion is finished
- The injector holes are damaged and they are now too wide. As a result, they let more fuel through than intended.
- Your governor isn't working properly and it's overheating your boat's engine.
- In extreme cases, the entire motor may be thermally unbalanced.
The Final Cause of a Smoking Black Boat Engine: Overloading
The problem may not be due to a lack of combustion air or poor fuel injection. If the black smoke occurs suddenly while sailing, then the problem is an engine overload. Generally this overload of the motor comes from an obstruction on the propeller (fishing net, plastic waste, algae, ends, etc...). But it could also be that your boat is overloaded, that the hull is dirty or that your propeller is not suitable.
Finally, if your boat is equipped with a multiple unit diesel engine, one or more units may be faulty. As a result, the remaining units are overloaded.
Solutions to repair a boat engine that smokes black
The Most Common Fixes for Fixing a Black Smoking Boat Engine
So what to do if your boat engine smokes black ? Here is a list of the most frequently used solutions:
- In the event of a lack of combustion air, check the condition of the air filter, particularly in the event of a loss of power during acceleration. If it is clogged, you will need to change it. Replacing an air filter is very simple – you don't even need a tool on most outboard motors. (You will find many spare parts on Amazon)
- In the case of bad fuel injection, replace the petrol or diesel in your tank if the problem appeared after refueling. The fuel is very likely to be either of poor quality or too rich.
- In the event of an engine overload, you must check that nothing is obstructing the propeller. If so, all you have to do is free the propeller from a stuck fishing net or seaweed. A dirty propeller can also cause black smoke. You will have to put the engine in neutral and clean the propeller.
Other solutions to repair an outboard motor that smokes black
If the problem of black smoke on your boat persists, you can apply these solutions depending on the causes:
- Clean the turbocharger compressor with water. If the blower still does not draw enough air, you will need to open the compressor to clean it thoroughly.
- Properly adjust the fuel pump timing.
- If the problem is with the injection valves, test their pressure after the boat engine is turned off and correct the setting.
- If the fuel is too cold, warm it up according to your engine manufacturer's instructions.
- If the motor overload problem persists, check that the bottom of the boat is clean. If not, clean the hull (see a good special hull cleaner). Also check that your propeller is suitable for your boat. If not, change it by following our guide: 'which engine to choose for my boat'.
For all other problems (damage to the injectors, holes that are too large, leakage after injection, etc.), we advise you to contact a marine mechanic. Only he can carry out the necessary repairs.
The solutions to repair a multiple unit diesel engine that smokes black
If your multiple unit diesel engine is smoking black, you will need to isolate the faulty unit. To do this, follow this protocol:
- Start by measuring the temperature of the exhaust gases from each unit.
- If one unit has a lower temperature than the others (usually a difference of more than 50°C) then it is faulty.
- Inspect this unit's air filter as well as its fuel pump.
- Unfortunately, the problem may be due to several units. It will then be necessary to test them one by one by isolating them from the others for a few minutes.
If your multiple unit diesel boat engine still smoking black after checking everything, so contact a competent mechanic.
Conclusion on outboard motors that smoke black
In most cases, a black smoking outboard motor is not a major incident. A few basic tweaks should be enough to fix the problem. However, there is a chance that the black smoke is due to the wear of one or more engine parts. In this case, it is more reasonable to go to the nearest mechanic. Be aware that good maintenance is essential to avoid having a black smoking boat engine. We strongly recommend that you follow all the procedures to keep your engine in good condition – especially during winter storage.
Learn more about boat motors:
How to Fix a Boat Engine That Stalls at Idle
