4 Common Causes of Oil in Your Air Filter
Have you ever noticed when changing your air filter that it was a little oily? Perhaps you recently bought a used car and are wondering if there’s a problem lurking under the hood? Motor oil in your air filter isn’t necessarily a bad thing, but it could indicate an underlying problem. Perhaps blow-by is the culprit.
Common Causes of Oil in the Air Filter
1) Clogged Crankcase Ventilation Valve (PCV)
Older cars emitted blow-by into the atmosphere, and these emissions were harmful to the environment. Introduced as one of the first emissions controls, the crankcase ventilation valve (PCV) redirects blow-by oil back into the air intake instead of polluting the environment.
The PCV valve, like the oil filter, needs to be changed periodically. Excess pressure in the crankcase needs to go somewhere.
A clogged PCV valve can allow blow-by oil to leak between seals, seal rings, or other small gaps in the engine. It can also cause the valve to get stuck in the open position, allowing more oil into the inlet than intended.
2) Piston Ring Wear
Piston rings provide a seal between the cylinder wall and the piston, maximizing the force exerted by the burning of the air-fuel mixture in the combustion chamber. The more power you have, the more power your car has.
Piston rings can wear out over time before they reach operating temperature due to high mileage, improper maintenance, poor engine design, or engine overuse.
As the quality of this seal deteriorates as the car ages and the piston rings wear, you may experience a gradual decrease in performance and a gradual increase in blow-by.
As the amount of blow-by increases, you may notice more oil in the air filter housing, possibly even in the air filter itself. To diagnose piston ring wear, run a compression test on each cylinder to check the pressure.
Cylinder pressure should be relatively even on all cylinders, and your factory manual will provide the manufacturer’s specifications for acceptable cylinder pressure ranges. Replacing worn piston rings requires an engine overhaul.
3) Aftermarket Air Filters
Some aftermarket air filters, such as the popular K&N brand (such as this one), are reusable. These reusable filters require a film of oil to be applied to the outside of the filter (the side away from the engine) before use. If you notice oil on the inside of your air filter, it’s possible that you or the previous owner applied too much oil. Follow the filter manufacturer’s recommendations for cleaning and preparing the filter before use or reuse.
4) Clogged Oil Channels
Modern engine oils last a very long time, but they don’t last forever. If you ignore or forget oil change intervals, the old oil will thicken and form ugly sludge. This engine sludge can clog the oil passages that keep the engine lubricated and allow the oil to flow through the engine as intended.
When these passages become clogged, oil and sludge can build up in areas of your engine where they shouldn’t, such as the intake. This can reduce performance and fuel economy, and if left unchecked for long periods of time, can cause your engine to fail completely.
What causes engine blow-by?
When your engine is running, pressure naturally builds up in the crankcase from the air, fuel, and oil flowing past the piston rings. This is called blow-by. It’s totally normal for some blow-by to occur because the piston rings can’t completely seal against the cylinder walls. However, excessive blow-by can cause problems.
Additionally, blow-by burns differently than pure gasoline, so it can be difficult for sensors to measure, and it affects the air-fuel ratio, effectively lowering the gasoline’s octane rating.
This can result in pre-ignition (also known as knocking). Knocking can cause engine damage if it occurs periodically or when the engine is under heavy load. If your vehicle is experiencing excessive blow-by, there are several possible causes.
Prevention is the key
If you have read this article to the end and are still unsure of the cause of the oil in your air intake, consider installing a catch can or air-oil separator on your vehicle.
Most collectors and air-oil separators are relatively inexpensive and easy to install, so they provide plenty of peace of mind for vehicles that you plan to store for long periods of time.