A smoking engine is a worry, not to mention annoying to ride. The smoke is oil, and it needs investigating asap.
Top 6 reasons an ATV blows blue smoke:
- Excessive oil in crankcase
- Wrong oil Type
- Faulty crankcase breather
- Blown head-gasket
- Oil valve stem seals worn
- Worn rings
In this post, you’ll learn the most likely reasons your ATV is blowing blue smoke. You’ll also learn what you can do to fix it.
This is an easy and common mistake to make. Putting too much oil in the engine can cause excessive crankcase pressure. The moving internal components of your engine need room to move. They create positive and negative pressure as they cycle.
Overfilling the void with oil causes it to be pushed into places it isn’t meant to go. It makes its way into the carburetor via the crankcase breather. Once in the carburetor, it will be combusted with the fuel and seen as a blue fog from the tailpipe.
Any oil that wasn’t burnt by the combustion is forced into the exhaust system, creating even more blue smoke. The excessive crankcase pressure may force the oil into the combustion chamber through oil stem valve seals also, which compounds the issue.
Wrong Oil Type
You think oil type shouldn’t matter, but for some engines, it does. It’s always best to use the oil type your manufacturer recommends. Your engine may be designed to run on heavier mineral oil. Running on a lighter-weight oil such as synthetic may allow oil to sneak past the oil rings.
The oil that makes it past the rings, as you know, will be burnt, turn to white-blue smoke.
What is it? The crankcase breather’s job is to direct any excess crankcase pressure or blow-by to the carburetor where the gases are reused in the combustion mix.
What’s Blow by? Compression gases that sneak past the piston rings and reach the crankcase are known as blow-by. A faulty crankcase breather allows oil to enter the intake breather ducting and onto the engine. The breather assembly incorporates a valve that allows excess pressure out but prevents oil from leaving.
If the intake and breather pipe has a lot of oil present, check the breather system for failure.
A blown head gasket is a pretty common complaint. The symptoms however vary depending on the type of engine and where the gasket actually fails.
What is a head gasket? A head gasket is a graphite material that’s sandwiched between the cylinder head and the cylinder block. Its function is to seal the cylinder and prevent compression, oil, and coolant loss.
Gaskets often fail because they’re old. They’re under tremendous pressure with every revolution of the engine. However, over full engine oil or excessive valve lash will cause them to fail prematurely.
A failed gasket is commonly associated with white smoke, but blue smoke is also common. It depends on where the gasket fails.
A leak-down test will usually show you where your gasket fails.
What’s a leak-down test? A leak-down test sends pressured air into the cylinder via an adaptor fitting to the plug hole.
Air loss is measured over a given amount of time, but you’ll usually hear where the air is coming from.
However, a failure between the cylinder and the cam chain is common. The material is usually narrow here, and failure leads to compression escaping into the crankcase and oil drawn into the cylinder.
Replacing the head gasket is the only repair, but it’s a job the home mechanic can easily do, and while you’re in there, go ahead and replace the valve seals (see below).
Oil Stem Valve Seals
Valve seals are nylon seals fitted over the valve sleeve on the cylinder head. The seals prevent engine oil from slipping past the valve stem and into the combustion chamber. The problem with the seals is, they leak as they age. Hey, we can’t blame them.
These little guys work super hard in really hot conditions. If your engine is ten years old and they haven’t been replaced, they’re probably pretty hard, hard seals leak guaranteed.
Symptoms of leaky valve seals are usually smoke when you gun the throttle and smoke on initial start-up, but that can vary. Replacing the seals is a job for the DIY mechanic too. Ordinarily, you’d need to remove the cylinder head from the engine. But there’s a couple of hacks to get around that.
Using a leak-down tester adaptor, compress the cylinder with air. This prevents the valves from falling into the cylinder when you remove the keepers. The other method is more MacGyver style – stuffing the cylinder with a clean nylon rope does the same job.
The valve springs still need to be compressed and keepers removed and reinstalled, so this method may not suit a first-timer, especially if the workspace is tight.
Worn piston rings aren’t last on this post because it is least likely. It’s last because it’s the most challenging and expensive of all the causes of blue smoke to repair.
The rings around the piston, as you know, are what create the compression needed for your engine to run. Without proper compression, your bike won’t perform.
However, without a proper test, you can’t tell for sure. Some of the early symptoms of ring wear may not be very noticeable. They include:
- Topping up oil more regularly between oil changes
- Oil in the air-box
- Blue/white smoke trail
- Smoke when pushing the engine or when towing
- Misfiring bike
- Fouled oily plug
Worn rings will cause excessive blow-by, and you already know what that is. The excessive pressure forces engine oil into the combustion chamber, where it’s burnt. Fouling the spark plug and misfires is common with worn rings too.
Leak-down testing is the best test for worn rings.
The test, as you know, involves setting the engine at TDC (Top Dead Centre) all valves closed and compressing the cylinder with air. Pressure loss is measured with a gauge, and the loss is usually found by simply listening.
In the case of worn rings, we’ll expect a lot of blow-by to show up as air leaking from the dipstick tube.
Compression testing using a compression tester.
We can use a compression tester too, however, some engine compression release setups (decompression valves) may interfere with compression test readings.
To avoid a bad reading best to confirm ring wear by doing a wet and dry compression test. A large difference tells us we have a ring problem and conversely, no change in the wet and dry readings means the rings are not at fault.
Dry test – Start by testing the compression. Best to test a warm engine if possible.
Remove the spark plug, fit the tester. Hold throttle wide open and choke off, at least six cycles of the engine, and note the max compression (Anything over 100 psi, is normal but you’ll need your model specs).
Wet test – Now we’ll need to perform the wet test. Repeat the process as before, however, first, drop a cap full of engine oil into the cylinder.
Good rings will show little to no change in wet and dry readings. A large difference means the rings are worn.
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John Cunningham is a technical writer here at ATVfixed.com. He’s a Red Seal Qualified Service Technician with over twenty-five years experience. He’s worked on all types of mechanical equipment, from cars and trucks to ATVs and Dirt bikes.