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Why Is There Oil in My Car Air Filter?

Uhhhh. That’s not supposed to happen, right? No, probably not. Let’s figure what can cause an oily car air filter, and what you can do about it.

The air filter on most modern cars is located within the air intake housing, which is mounted at the top of the engine. It is connected to the fuel injection system (or the turbocharger). It is designed to clean air on its way to the fuel system to be combined with fuel before it reaches the combustion chamber.

The primary use of the air filter is to eliminate soil particles, debris, dust, and other contaminants before the air is combined with liquid gasoline (or diesel) to form a vapor. When the air filter is clogged with impurities, the engine's fuel efficiency and power output can decrease. If oil gunks up the air filter, the engine output may be significantly affected.

The air filter is meant to trap contaminants — but not oil. Every now and then, however, when you use your local service mechanic to change your air filter, the technician will indicate that engine oil has been detected inside the air filter housing or inside the used filter.

While this is not a sign of engine failure or a reason for you to panic immediately, it should certainly not be ignored - it can be a pointer to potential future problems.

Engine Blow-By

When the engine is working, the cylinders are sliding the piston rings into the crankcase. When rings wear out, they can become loose and can trigger 'blow-by' oil—usually demonstrated by blue smoke from the car's tailpipe while driving. In the early stages of worn piston rings, excessive oil blow-by can cause severe pressure to build inside the crankcase, which, as mentioned above, sends more oil through the PCV valve and eventually into the air intake

Usually, blow-by begins as carbon forming in the top ring grooves, which causes the cylinder to leak - which in turn causes some oil to burn off. The burning of oil produces more ring deposits and more blow-by, as well as more carbon build-up. A little blow-by is perfectly natural as the piston rings can not be completely sealed against the walls of the cylinder. Excessive blow-by can, however, cause problems.

Blow-by will eventually damage your engine – and you might not be able to detect the effects right away, but you can certainly do so with time. After the blow-by has caused its harm, you will find that the blow-by will mask your oil and gas intake and reduce your engine performance. Blow-by can also condense into the crankcase and settle between your cylinders and head, decreasing the effective octane level of your gasoline.

You might also hear the engine knocking, which is a warning of pre-ignition. Pre-ignition happens when the mixture of air and fuel is ignited prematurely. The octane level recommended by your manufacturer is always the appropriate level to avoid pre-ignition, but this is negated if extra fuel is added to the cylinders as a proximate result of blow-by.

The impact on the engine is extremely damaging, and, over time, it can also damage the spark plugs. An oil-coated spark plug can misfire and possibly cause an accumulation of fuel. This is an unsafe situation that needs to be handled immediately.

A Clogged Positive Crankcase Ventilation (PCV) Valve

Old vehicles used to vent blow-by into the atmosphere, but these pollutants were destructive to the environment. The PCV valve was introduced to control this emission by redirecting blow-by back into the air intake.

The PCV valve is attached to the air intake housing by means of a (usually) rubber vacuum hose utilized to release the vacuum within the engine crankcase. Usually, this part is located on the top of the cylinder head valve cover, where the pressure flows from the bottom half of the engine through the head of the cylinder and exits through the air intake. The PCV valve is similar to the engine oil filter in that it inevitably gets clogged with unwanted debris (in this case, engine oil) and should be replaced as recommended by the manufacturer of your car. If the PCV valve is not replaced as required, the excess oil will blow through the PCV valve and reach the air intake system.

Aftermarket Air Filter

Image of aftermarket air filter Alt text: Some aftermarket filters are oily by design

This one is the most common reason for oil in your air filter. It’s also completely normal. Some aftermarket air filters brands, including popular brands like K&N, Volant Primo, Injen High Performance, and S&B Intake, offer reusable filters. These reusable air filters are oiled from the outside of the filter (the side facing away from the engine) before use, in order to trap smaller particles.

Clogged Oil Passages

Another potential explanation for the engine oil to find its way into the air intake system and ultimately clog the air filter is blocked oil passages. This problem typically occurs when the engine oil and filter are not replaced as prescribed, and occurs from excessive carbon deposits or by the creation of engine sludge within the crankcase. If the oil does not flow efficiently, a lot of engine oil pressure will be produced, and extra oil will be forced through the PCV valve and into the air intake.

What’s the Next Step?

If you have reached this point and you are still uncertain about what caused the oil in the air intake to occur, consider consulting a professional mechanic. If you can drive the car, go to the garage as normal. If you can’t, or are afraid to, the technician can come to your house to complete an onsite inspection so he can correctly identify the reason why there is engine oil in your air filter.

Consulting a professional can save you a substantial amount of time and money for major repairs or even replacing your car.

As for home air filters? We got you covered there. Filter King’s online store has just about every size of home AC filter there is, with several different strengths and prices for each size. Check it out to discover why they call us the king!