4 Causes of Oil in the Air Filter

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Ever done some routine maintenance on your air filter and found oil where you shouldn’t? While the oil in itself might not be a bad thing, it could be indicating other problems with your engine. Oil in the air filter is most commonly caused by blow-by.

oil in air filter
The Air filter protects the engine from dirt, dust, and other contaminates.

So then, what is blow-by? Blow-by occurs when the air-fuel mixture or combustion gases escape from the combustion chamber and into the crankcase at the bottom of the engine. Most commonly, these gases or air-fuel mixtures escape the combustion chamber between the piston and cylinder walls.

Signs of Engine Blow-by

If your air filter has oil in it, it could be from engine blow-by. There are some common signs your vehicle could have if its suffering from blow-by. Among these symptoms are: engine sputtering, blue clouds from the exhaust, white exhaust fumes, coolant-in engine oil, and even engine knocking.

Blow-by can reduce fuel economy and engine performance. This is due to air and fuel being pushed into the crankcase and air filter on each combustion and exhaust cycle.Excessive blow-by leads to reduced fuel economy and horsepower numbers, since some of each combustion cycle is wasted venting the mixture into the crankcase and the air intake.

Other problems that are caused by blow-by is that it affects the fuel mixture. This makes it harder for the ECU to supply the correct fuel mixture to engine lowering engine performance. This could also possibly cause pre-detonation or knocking.

What Causes Engine Blow-by

Piston rings are what seals the air, fuel, and combustion gases from entering the crankcase. The piston rings don’t create a perfect seal, so some blow-by is normal, but large amounts will be detrimental to engine life.

The causes of engine blow-by are related to three things: the pistons, the piston rings, or the cylinder walls. A lot of times the issues are caused by broken or damaged piston rings. The piston itself can also be damaged causing a lack of sealing. The cylinder walls can become scratched creating leak paths. Although these issues are all common causes of engine blow-by, they causes aren’t limited to the ones discussed above.

Faulty PCV Valve

A clogged Positive Crank Ventilation (PCV) Valve is a common cause of an oily air filter. A PCV valve functions to recirculate blow-by gases back into the air intake. The gases then travel back into the combustion chamber to get a second chance at ignition. To learn more about PCV Valves check out our article on how they work here. PCV valves were implemented to reduce emissions, before their use engines dumped blow-by gases back into the atmosphere.

When a PCV valve gets clogged, pressurized blow-by gases can be forced through seals or other gaps the gases find. If the PCV valve gets stuck open it could allow oil to enter the intake and contaminate the air filter. All of these issues can be avoided by simply changing the PCV valve at its required interval. Most manufacturers state replacement intervals for this valve, and if you don’t remember when you last replaced it, its been too long. The good news is this valve rather inexpensive.

Worn Piston Rings

Worn or damaged piston rings is the second likely cause of oil in the air filter. Like we’ve discussed in the blow-by section, the piston ring, located in between the cylinder wall and the piston itself, seals the gases in the combustion chamber. A poor seal will reduce engine power by allowing gases to escape the combustion chamber without pushing the piston back down.

Piston rings function in a tough environment so the higher mileage your engine is the more wear the piston rings will likely have. To diagnose if you have faulty piston rings, you can check cylinder pressure. The pressure should be similar between all cylinders. Refer to the manufacturer’s specification on what the pressure should be.

An easy way to increase piston ring life is to ensure that you frequently change the oil. Also using synthetic oil can be beneficial for your engine.

Clogged Oil Path

Oil and oil filters can last for thousands of miles. However, they won’t last indefinitely. Its very important to change the oil along with the filter before exceeding the manufacturer’s recommended interval. Old oil or a failed filter can wreck entire engines from the inside out.

In addition to oil flushing hard particles through your engine, old oil creates sludge build up. This sludge can clog oil ports and pathways. Sludge can also accumulate in places like the air intake.

You could notice a loss of engine power, loss of lubrication, loss of oil pressure, and oil in your air filter.

Aftermarket Air Filters

Air filters that function in dirty and dusty environments such as a dirt bike’s, commonly come coated in a thin layer of oil. This oil is meant to be on the air filter as the oil helps stick particles in the air to the filter.

Some automotive filters are also reusable. Generally these reusable air filters also come coated with a thin layer of oil and require re-oiling at certain mileage intervals. The apply of the oil should be thin. If you notice oil inside the filter it could be that too much oil was applied last time the filter was re-oiled. Another key thing to look out for is that the oil should never be black. Dark black oil indicates that the oil is actually from the engine.


If you still don’t know what is causing the oil to get into your air filter. Consider changing the PCV valve and the engine oil. These are relatively simple maintenance jobs that can easily be done at home. If the problem still persists it could be something more serious like the piston rings.

While the piston rings are very simple parts. Replacing a piston ring requires a lot of disassembly of the engine.