Outboard Motor Smoke: What Do the Colors Mean?

If your outboard motor starts blowing out a ton of smoke while you’re out on the water or when you fire it up, you know there’s a problem, but you might not know why the smoke came out the color that it did. 

It’s normal for most outboards to puff out small amounts of smoke, especially on cold mornings or when the motor has sat unused for a while. But anything more than a standard puff of smoke or vapor can be alarming.

Two-stroke motors are expected to smoke some due to the delicate balance of fuel and oil they require. On the other hand, four-stroke outboards, particularly the more modern ones, aren’t expected to smoke at all. There are several different colors of smoke you may see coming out of an outboard engine, mainly blue, white or black. Here’s what those outboard engine smoke colors might mean.

Blue Smoke

Smoke coming out blue from an outboard means it’s burning a lot of oil. It’s normal for an older two-stroke to blow out some smoke, as it’s a sign the engine is being lubricated.

Blue smoke isn’t necessarily bad in a two-stroke, but it could be the fuel ratio is incorrect if more smoke than usual is coming out. If you pre-mixed the fuel and oil yourself, it’s easy to end up with too much oil in the mix, which the motor will burn off and thus produce a cloud of blue smoke. In that case, you may have to dump the fuel out and correct the fuel-to-oil ratio. However, if excess blue smoke keeps occurring when running the motor, you might have a more serious oil problem on your hands.  

On a four-stroke outboard, blue smoke is more of a concern. One reason is that if you leave the outboard tilted down after using it, oil can build up in the cylinders and when it sits, it can leak down into the cylinder. So when you start it up, you might see blue smoke. Trimming the engine up and letting it sit for a while could solve the problem. However, if you have a 4-stroke motor that produces excess amounts of blue smoke, you may have an oil leak or some worn out parts.

White Smoke

This color smoke is usually indicative of an outboard overheating problem. For an outboard engine that uses the water it’s in to cool it down, a clog in the cooling system can heat up the motor and produce white smoke.

Sometimes the issue resolves itself, where you stop the engine and whatever is clogging it (seaweed, trash, etc.) falls out on its own. However, white smoke could also mean something more serious like a water leak in the cylinder head or the gasket. Or the motor might not be getting enough water for cooling, causing it to overheat. Small amounts of white smoke are normal, but excessive amounts could indicate more major issues like cracked cylinders or bad valves. 

Black Smoke

A fuel injector that gets stuck open can mess up the fuel-air ratio the engine is designed to burn up, causing it to blow out black smoke.

Carbon build-up on the valves or piston rings can also cause the fuel to burn incorrectly. This type of build-up can happen to an outboard that doesn’t run hard enough, which means it never gets hot enough to burn off the carbon. The outboard might sputter and cough up a cloud of black smoke to purge out the excess fuel.

Fuel additives plus running the engine hard can often fix the issue, as could cleaning the fuel injectors. Black smoke can also mean the motor is burning more fuel than it should be, which is known as running rich. Other causes of black smoke include a faulty fuel pressure regulator or carburetor float.

Carbon buildup on the outboard’s spark plugs can also produce black smoke. Inspecting the plugs should be part of regular maintenance anyway, so don’t wait for black smoke to happen to get the plugs inspected and replaced. 

Also Keep In Mind

There are a number of tests you can perform to find leaks in an outboard engine that could be producing smoke, including a leak down test and a compression test. 

Other things to keep in mind if your outboard engine is blowing out smoke include:

• Checking the owner’s manual to make sure you’re using the proper air-fuel mixture or fuel-oil blend.
• Making sure the fuel and oil mix you’re using in the outboard isn’t old.  
• Switching to higher quality synthetic oils instead of using cheap conventional oils. 
• Not confusing smoke with vapor, which dissipates rapidly. If it lingers for more than half a minute, then it’s likely smoke instead of vapor.

Finally, besides making sure the engine isn’t running too rich due to excess fuel, make sure water isn’t getting into the fuel; the fuel-water separator is working properly; and that the fuel filters are being changed out as needed.