Why Does My Outboard Motor Smoke?


Have you ever been at the dock and remember seeing that one guy that smokes out the place? That is, when he turns on his engine it’s like mosquito control coming through and blasting everybody out? Have you ever wondered

Why Does My Outboard Motor Smoke? An Outboard Motor smokes because it is burning oil. Two-Stroke Engines burn oil all of the time. Four Stroke Engines burn oil at startup mainly because of oil left in the cylinders or all the time with an oil leak.

There are a couple of tricks that can be done to reduce the amount of smoke that an engine burns and here are your best options!

What Type of Outboard Do You Have?

Why does an outboard motor smokeThe key to this portion is understanding what type of engine you have. Is it a 2-Stroke or a 4-Stroke? There are major differences between the two and huge deciding factors that impact why an outboard engine smokes.

Two Stroke engines burn oil. That is just a fact of life. There is no getting around it and there is no real hardcore fix to change this. The way the motor operates is by mixing oil into the fuel, either at the carburetors or by injecting it into the reed plates; which gets sucked into the crankcase and lubricates the engine.

Without the oil, the motor would not run. It would not have lubrication and would fail. That is how most two strokes “blow up.” They lose oil and the major components inside rub together until they lock up.

A Four Stroke, on the other hand, has an oil pan; where there is an oil pump that pumps the oil up into the powerhead, (What is an Outboard Powerhead), which lubricates all of the components without burning the oil and cuts down on the smoke. We’ll talk about the reason’s a Four Stroke smokes, here, in a little bit. Before that, let’s figure out

What Color Is The Smoke & What Does it Mean

Exhaust smoke is the leftover burned fuel and air mixture that comes from a combustion engine. There are mainly only three different colors of smoke that you will see coming out of the engine. You can see different variations of the colors, but the main colors are Blue, White, and Black.

  1. Blue Smoke is an indication that the engine is burning a lot of oil. When the oil is burned it will give off this blueish or even a grayish tint to it. Note that the gray color can also be part of the white smoke.
  2. White Smoke indicates water or vapor. White smoke is usually not a good thing but it doesn’t always necessarily mean there is something to worry about, (we’ll talk about it in a minute!)
  3. Black Smoke usually comes from a rich burn or carbon build-up. Meaning that it’s burning more fuel than it should be, or the engine is building carbon on the valves.

Blue Smoke

If you see the blue smoke, we know that we are burning oil. So why is the engine burning oil? If the outboard is a two-stroke, don’t worry too much here, this is actually a really good thing! This is what you want to see because it is letting you know that the engine is being lubricated and you don’t have to worry about any issues there!Why outboard engines smoke

If we are talking about a four-stroke, then there are a couple of reasons. A common reason is that if you leave the outboard tilted down after running it, there can be oil build up in the cylinders. Which, as the engine sits, it can leak down into the cylinder and then, when you start it up, you will see smoke! Just trim the engine up to about a 45-degree angle and let it sit for a while. That should resolve the smoking.

Now, there are other causes, like having worn piston rings, a leak at the valves, the valve seats, or even worn cylinder walls. There really isn’t a fix to this. Generally, the engine is beginning to get tired. One thing to remember is that when we are talking about outboard engines, it is not like your car. Outboards are designed to run at 6000 RPMs; where your car can go 80 down the highway at only 2200 RPMs. So the outboard sees a lot more wear and tear and can wear out more quickly.

White Smoke

The dreadful white smoke! Sometimes it is bad, but there are certain instances where you can see white smoke, due to an overheating issue. Why 2-Strokes smoke so muchDealing with an outboard engine that is using seawater to cool it down, if there is a clog somewhere in the cooling system, the engine can heat up and turn the cooling water into vapor. Thus the white smoke.

This could happen in different situations where, for instance, you are running along and you pick up some seaweed or a bag; that clogs up the water pick up screen. The engine heats up and throws an overheat alarm. You slow down and notice the white smoke. Then, when you stop all of the grass or bag falls off the lower unit. Boom! The engine cools down and you are good to go!

The other hand is that there could be a water leak in the cylinder head or the gasket, which is letting water into the cylinder. So, you might want to pay some close attention to your specific situation.

Black Smoke

If the engine has a fuel injector that gets stuck open, it will consistently pour fuel into the cylinder. The engine is designed to only burn a specific calculation of fuel and air to perform properly. So, if there is an overflow of fuel into the cylinder, it can create this black smoke.

There are other issues where there could be carbon build up on the valves, the rings, or the injectors. This build-up is usually due to exhaust build up. This build-up can cause the fuel to improperly burn, which can also cause this.

Carbon build-up most commonly comes from an engine that never gets run hard enough. Meaning, as we talked about earlier, the outboard is designed to run at 6000 RPMs. So, if an outboard spends most of its time at idle, or trolling, it never gets hot enough to burn off the carbon/exhaust. As this goes on, it can build up deposits on the valves.

A quick fix would be to run some Mercury Quickleen or Yamaha Ring Free. Running either of these in your fuel is great for your engine regardless. We completely encourage it. We personally use the Quickleen because it is cheaper than the Ring Free, but both are great products!

What Should Your Main Concern Be?

Causes of outboard motor smokeYou should really only be concerned if you see the white smoke. If you see the black smoke, try the cleaning additive in the fuel and running the engine hard for a weekend. If that doesn’t fix the issue, then chances are, you are looking at getting the injectors cleaned.

The White Smoke, on the other hand, is something to raise an eyebrow at. If the overheating scenario is not fitting your particular situation, then you could be looking at a serious issue that needs to be looked at ASAP.

I wouldn’t recommend brushing it off. Take it to a knowledgeable Marine Technician that can perform some simple tests. Compression test, Leak Down, and running water through the flushing adapter with the spark plugs out of the engine for a few minutes (like 15 or so); to see if water is getting into the cylinder. These will at least give you a direction to go, in deciding what to do next. All is not lost yet, hopefully, you stopped in time to resurrect the engine before it was too late!

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We upload videos every week and we are always creating how-to and service videos for outboard engines and boating repairs; along with some traveling and fun on the water! Don’t be shy and drop us a comment below and tell us about your situation! We’re happy to give out advice and our opinion!

Aaron Hilligoss

Aaron has been working in the Marine Industry for over a decade and holds certifications for Yamaha and Mercury Marine. It is not uncommon for him to own and be working on at least three different boats at any given point in time!

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