Knowing how to read exhaust smoke can tell much about your ATV engine’s health. It’s almost as if your engine is trying to tell you how it’s feeling.
- Black exhaust smoke usually means the engine gets more fuel than it can ignite.
- Blue smoke occurs when oil enters the combustion chamber.
- White smoke is typical when starting a cold engine, but excessive white smoke indicates an internal coolant leak.
This post will help determine whether the smoke is normal or if there is an issue causing the smoke, and whether the issue is something you can fix yourself or if it is something you need professional help to address.
Is All Exhaust Smoke Bad?
If you’re anything like me, any new sound, strange smell, or behavior of the ATV will need a good explanation before it can be left alone and not be of further concern.
Oftentimes these things turn out to be nothing. It’s worth remembering that not all smoke is bad. Suppose you have a general idea of what is expected and what is not; it’s easier to live with all the different «symptoms» our machines make from time to time. It may also help you discover potential issues that need addressing before they evolve into something more serious.
Let’s have a closer look at the different types of smoke and what they can indicate. By following a few simple steps, you can examine the issue further and, in many cases, even fix unwanted smoke from your engine.
Black Smoke
Black or darker smoke is usually nothing to worry about. This happens if your ATV, for some reason, gets too much fuel, and the combustion becomes out of balance.
Your engine needs a certain ratio between the amount of air and fuel it gets to function correctly. This is called the air/fuel ratio.
- On a carbureted ATV, The air/fuel ratio must be set manually by adjusting the carburetor.
- The fuel/air mixture is set automatically to the optimum ratio on fuel-injected engines.
If there, for some reason, is more fuel or less air than the optimal ratio, the fuel mixture becomes overly rich; in other words, the air/fuel mixture becomes too low.
The spark can only ignite a certain amount of fuel per combustion cycle. Any fuel the engine cannot burn in the combustion chamber will be pushed out and will not be ignited before it enters the exhaust system.
Excess fuel that burns after the combustion chamber will not incinerate optimally, causing black smoke.
You can start by cleaning or replacing your air filter, depending on what type you have. A dirty filter will prohibit clean air from making it through to the combustion chamber.
This means there will not be enough air to combust the fuel inside the cylinder, which will make for less efficient combustion with black smoke as a result.
If this does not solve your problem, you should have a look at some of these other causes of why your motor gets too much fuel:
- A blocked or damaged fuel return pipe
- A faulty fuel injector
- A broken O2 (oxygen) or airflow sensor
- The fuel pressure regulator may be stuck in the closed position
Blue Smoke in Two-Stroke ATVs
It’s worth noticing that a two-stroke engine will have blue smoke for some time after starting a cold engine. This is perfectly normal and should not be of any concern.
This happens due to leftover fuel and oil from the last time the engine ran, making it run a bit rich in the beginning. Using the choke will also add to this.
When warm, on the other hand, a properly jetted two-stroke engine should not smoke much. If it does, it’s probably running a bit rich and needs adjustments made to the carburetor.
Blue Smoke in Four-Stroke ATVs
If your exhaust smoke on your four-stroke is blue-looking, this is likely because unwanted oil contaminates the air/fuel mixture. The oil will then combust together with the fuel and air, creating blue-looking smoke.
If the engine is burning oil, you should notice a decrease in the oil levels over time, and you may need to top off with new oil from time to time.
This may or may not be something you need to address, depending on what causes it and whether it’s something you are willing to live with. Some causes need addressing not to develop further and, in time, possibly ruin the engine completely.
You will need to start by identifying why the oil is mixing with the air and fuel. In some cases, this can indicate more serious engine issues, but it may also have a simple explanation and solution.
Not So Serious Cause: Stuck Piston Rings
If your ATV has not been ridden in a longer period of time (months or even years), the piston rings may have become stuck.
This may also happen if all the riding you do is a daily trip to the mailbox and back. The engine will not reach its operating temperature, and the oil will gum up and seize the piston rings.
If this is the case, the rings won’t work as intended, and oil from the crankcase will squeeze its way up, past the pistons, and up into the combustion chamber, where it will burn and create blue smoke.
Sometimes the rings may come loose and start working properly again just by riding the ATV normally for some time. To speed up this process, you can do a couple of things.
Try adding some quality engine cleaner like Marvel Mystery oil to the gas and run like this for a couple of tanks of gas. This should free up the stuck rings in no time with little to no risk of hurting the engine.
If this doesn’t work, you can try to change to synthetic engine oil if your bike is currently running in mineral oil. This will also have a cleaning effect.
It’s always a good idea to talk to your dealer before you try this.
More Serious Cause: Engine Wear, Bad Seals, or Engine Damage
Over time, any engine will wear. Worn or damaged cylinder, piston, piston rings, worn valve guides/stems, and/or bad seals is usually the reason why oil ends up inside the combustion chamber where it’s not supposed to be.
Not following the recommended break-in procedure or not following the manufacturer’s service schedule with a regular oil change may cause premature engine wear.
Another common reason for premature engine wear is dust and dirt getting past the air filter and into the engine.
The small sand particles will pass through the engine, acting like sandpaper and wearing down the different components. To prevent this from happening, you must make sure the air filter is always fitted correctly and replaced or cleaned when getting damaged or too dirty.
Many skip this crucial part of maintenance which may result in big expenses later on.
Tests to Find Why the ATV Has Blue Smoke
There is a couple of different testing methods you can use to give you an indication as to where your leak is occurring. But you won’t get the definitive answer to what’s wrong until you take apart the engine to inspect it visually.
Testing for Unwanted Pressure in the Crankcase
This test is not too accurate, but if you are experienced or have a similar vehicle to compare with, it can indicate what is wrong.
While the engine is running, take off the oil filler cap and feel with your hand if pressure is present. Compare to the healthy engine of a similar bike.
If you feel any significant difference, this may indicate that the piston rings or the piston itself are worn or defective. You can also measure if you have the correct equipment.
Compression Testing
If you have a compression tester available, you can continue by testing the compression of the cylinder. The compression tester will measure the engine’s ability to generate pressure.
However, the test will not be a good indicator for smaller leaks because of how the compression device works.
You fit the tester in place of the spark plug and crank the engine until you get a stable maximum reading. Compare to factory spec.
The device has a one-way valve preventing air from flowing back between the engine strokes. So even with a small leak somewhere, you still should be able to build up good compression when you crank the engine for a long enough period of time.
If you cannot generate compression, this would indicate a severe issue like a damaged piston ring or a crack in the piston. If air is leaking past the rings, there is a good chance that oil will also leak past.
If the reading is lower than it is supposed to be, the engine may need a rebuild. A new piston and rings and a honing of the cylinder may be sufficient, but it may be a good idea to replace the bearings and the oil pump while you are at it.
Consult a mechanic before you attempt a procedure like this on your own.
If you get good compression readings, you can move on to the test that is most likely to pinpoint where your problem is located.
Leak-Down Test
A better way to test what is causing your bike to burn oil and smoke blue smoke is by performing a so-called leak-down test.
If your bike is a four-stroke, you can use a leak-down tester intended for automotive use. You can find these testers for cheap in almost any automotive supply store. For a two-stroke, you will need a slightly different tool. We will not be going into this procedure in this article.
While the compression test tells you your engine’s ability to generate pressure, the leak-down test tells you its ability to hold pressure.
The way the test works is by injecting pressurized air into the cylinder, measuring the amount that is leaking over a given time, and then finally listening to where the leak is coming from.
In short, the way you perform the test is by removing the spark plug and turning the engine so the piston is at the top-dead-center (TDC). Use a screwdriver to feel when the piston is at its highest point. In this position, the valves should be closed.
To turn the engine, you have to pull off the clutch cover and turn the primary clutch manually. Never turn the engine backward!
Make sure you’re doing the test with the engine at operating temperature for the most accurate results.
Connect the tool as shown in the included instructions and proceed with the test using compressed air.
You should be able to read from one of the two gauges whether there is a leak or not.
If you register a leak with the combustion chamber still pressurized, remove the oil dipstick and oil filler cap and listen for escaping air. If you hear air escaping, the leak is likely in your piston rings.
Then remove the airbox cover and listen for escaping air. If so, the leak should be in your intake valve.
Also, listen for air escaping through the exhaust system. If so, the leak is likely from your exhaust valve.
Afterward, you can try performing the same test as a «wet test.» This is done in the same way, but before you fit the test gauge in the spark plug hole, you add a tablespoon of engine oil into the cylinder. Then perform the test as normal.
In theory, the oil should act as a seal around the piston, and if your recorded percentage of air loss is significantly improved compared to when doing the test with only air, the leak is likely from the piston and/or rings. If it’s the same, the leak should be from the valves.
Do not add too much oil, as the reduced cylinder volume will affect your readings.
Fixing Serious Issues Causing Blue Smoke
Hopefully, after performing these tests, you will have a general idea of why oil is entering the combustion cycle, creating blue smoke.
Using thicker-weight engine oil or an oil additive designed to reduce oil leaks might work as a temporary fix to reduce the amount of oil leaking into the cylinder.
But the only way to truly fix the issue is by performing a complete teardown and rebuilding the engine.
White Smoke
White smoke is probably not smoke at all but steam. And fear not; white steam does not necessarily indicate that something is wrong.
From a cold start, most engines will generate some amount of white steam. This is nothing to worry about and is formed by vapor coming from the combustion process.
But when the ATV engine is at its operating temperature, with the outdoor air temperature not near freezing, and you still see a lot of white steam, something is not right. You could be looking at head gasket failure or even a cracked cylinder head or cylinder block.
A split in the head gasket will send coolant into the cylinder. The engine’s attempt to combust the water will usually result in massive amounts of steam and potentially severe engine damage.
Usually, you will have no problem telling the difference between normal steam from combustion in a cold engine and the amount of steam that appears from a damaged head gasket or cylinder/cylinder head.
If this happens, stop your bike immediately to prevent further damage, and order an appointment with a qualified mechanic.
Check this post for a more in-depth guide on troubleshooting an ATV with white exhaust smoke.
