Causes of Engine Oil in Air Filter Housing and Fixing

Are you wondering why there is engine oil in air filter housing? Air filter improves air quality and enhances the fuel economy. Oil in the filter indicates air-fuel mixing.

Are you wondering why there is oil in the air filter? The primary purpose of an air filter is to prevent dust, sand, and insect from entering the engine. It ensures clean air reaches the engine because a good mixture of air and fuel provides excellent efficiency and performance. Generally, the filter housing is a casing of the filter cartridge. Sometimes oil is present at the filter housing that indicates a problem like clogged PCV, damaged piston rings, and blocked oil passage.

Usually, oil in the filter housing is known as blow-by. Blow-by occurs when air-fuel mixture escapes from the combustion section into the crack case. Finding oil in the filter indicates some alarming signs about your engine.

In this article, I will guide you on the causes of engine oil in the air filter housing.

engine oil in air filter

What is the purpose of an air intake filter?

An air filter is a porous material strainer that eradicates dust, debris, bacteria, and pollen from the air. Usually, it is compulsory where air quality is essential. It is installed on the top of the combustion engine. The main purpose of the fast moving parts is to transport clean air to the fuel system. Where the air is mixed with the fuel and then enters the combustion chamber. When the filter is not clean, it affects the fuel efficiency and decreases engine output.

Regular maintenance of the air filter increases car performance, decreases fuel consumption, and avoids black smoke.

 Why is engine oil in air filter housing?

The air intake system allows clean air to enter the car engine. Oxygen is essential for combustion, and a sound air intake system allows a continuous flow of dirt-free air into the fuel system. However, oil in air intake causes sputtering noises and exhaust smoke.

Generally, the blow-by is because the air-fuel mixture slips through the piston rings or cylinder walls into the intake system. You may face oil pressure drop while driving. Let’s look at the reason for oil in the air filter.

1. Blocked PCV (Positive Crankcase Ventilation) Valve

Generally, the PCV valve monitors emissions and connects to an air intake housing. The role of the PCV valve is to intake filtered fresh air into the crankcase and burn them in the combustion chamber. After burning, these gases are expelled from the engine through exhaust. Commonly, the PCV valve uses a vacuum to extract gases from the crankcase and reuse them in the intake manifold system. Like engine oil passage,  PCV also gets clogged, and leakage may occur.

A clogged PCV valve leakage causes excessive oil to enter the air intake system. Blocked PCV causes pressure on the crankcase, and oil penetrates into seals and gasket. Moreover, oil in the intake manifold is due to excessive carbon components or sludge. When the sludge is inside the crankcase, motor oil doesn’t flow effectively and leads to oil leakage.

2. Damaged piston rings

A piston is one of the main parts of a combustion engine. It is a moving disk that moves up and down in the combustion chamber. Moreover, piston rings seal the piston and protect it from touching cylinder walls. The piston ring doesn’t allow oil to enter the combustion chamber. However, a damaged piston ring is a major reason for engine oil leakage. Once oil enters the combustion chamber, it’s burned or can enter the exhaust manifold or air intake filter. The result of engine oil leakage is grey-blue exhaust smoke. Generally, mechanics detect damaged piston rings by compression test on every cylinder.

3. Obstructed oil passages

Another reason for clogged oil filters is blocked channels. Car requires regular maintenance, and sometimes engine oil and filter requires replacement. Generally, excessive carbon deposits and sludge deposits in the crankcase create an obstruction that prevents oil from moving effectively. This exerts pressure on the PCV valve and air intake filter. In this way, extra oil enters the intake housing. The reason for oil in the intake tube is mostly because of clogged oil passages.

How to fix the oil in air filter?

Generally, the major reason for air filter oil is a clogged PCV valve. By cleaning  or replacing it with the new one. This problem is solved. Secondly, another reason for oil leakage in the intake housing is broken piston rings. Mechanics usually perform a compression test to diagnose this problem. They install a compression gauge on each spark plug and check the compression. If the compression is low, it means the piston ring is damaged. Piston ring replacement is an expensive, and time taking process and it’s better to seek professional mechanic help.

Regular car maintenance is important by changing engine oil, replacing the dirty air filter, and resolving the PCV valve problem. Moreover, regular oil change increases fuel economy and eradicates dirt and debris from the system. Additionally, the best engine oil increases car performance and efficiency.

FAQ (Frequently Asked Question)

1. Does an oiled air filter damage the engine?

 The oiled air filter has a thin oil layer that prevents dirt from entering the engine. Oiled filters may lead to MAF (Mass Airflow) sensor damage. A bad MAF sensor causes a car to consume more fuel and reduces its performance. However, by regular filters change, this problem is avoided.  Both air and oil filters prevent dirt and debris from entering the engine.

2. How long do oiled air filters last?

Regular maintenance of a car is necessary it includes routine oil change, air filter replacement, and tire rotation. Most experts suggest after 30,000 miles of drive, air filter should be replaced. It increases fuel efficiency and improves car performance.

3. What happens if you over oil your air filter?

Over oiled filter restricts the airflow and causes dirt and debris to be attached to the air intake housing. It became tacky and limit the efficiency. Expert refrain from oiling the outer surface of the air filter because dirt gets attached and airflow is disturbed. However, a small amount of motor oil doesn’t harm the engine.

engine oil

4. Which is better dry or oiled air filter?

An oiled filter is a cotton material strainer with a thin oiled layer that allows air flow. Mostly oiled strainers prevent 98% of dirt, bacteria, and debris from entering the engine. Meanwhile, the dry one prevents 99% of dirt from entering the system. Although both filters are the same, users consider the dry air filter is more effective than an oiled one.


The primary purpose of an air filter is to prevent dirt and bacteria from entering the engine and improving fuel economy. Generally, engine oil in air filter housing is not severe. However, it indicates problems like damaged piston rings, blocked oil passage, and obstructed PCV valve. Damaged piston rings are hard to replace and require professional help. On the other hand, the problem of clogged oil passage and PCV valve are avoided by regular vehicle maintenance.


Mr. Rubel is an Engine Mechanic. He has more than 15 years’ experience on repair and maintenance of different kinds of vehicles. He is also providing training to mechanics and works as a Senior Mechanic of a well-known organization. He is sharing his knowledge to support the car lovers to solve the vehicle issues.

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