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When your engine is running, the combustion pressure pushes unburned fuel and gases from the combustion chamber to the crankcase. They pass through the piston rings, where they could leak, resulting in blow-by gases and harmful emissions.

The positive crankcase ventilation (PCV) system prevents issues by recirculating the vapors to the engine’s induction system, which has cylinders that burn them. The crankcase filter is one of the system’s crucial parts.

pcv system diagram
The positive crankcase ventilation system

The PCV is connected to manifold vacuum and may have a valve with a spring and plunger or it may simply be a fixed orifice in the rocker arm cover such as you see on 4.0L Jeep Cherokee engines that have a replaceable rubber PCV orifice at the rear of the rocker arm cover. Some Chevy V8 pickups have a PCV orifice built into the rear of the left hand rocker arm cover.

automotive crankcase and pcv image
Since the PCV is drawing crankcase vapors out, there must be a way for air to enter the crankcase to replace the vapors being drawn out. The closure hose connected to the air inlet tube in the illustration of the PCV system is how this is handled on most all vehicles since the late 1990s. | Image Source: Richard McCuistian

The reason is that the intake stream provides the clean air needed, and since that air is being drawn through the MAF sensor, it is also factored into the fuel trim. If you remove the oil filler cap, PCV air will begin to enter there as an unmetered air feed, and the PCM will skew the fuel trim due to inaccurate mass airflow readings.

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Older vehicles with round air cleaners typically had a crankcase filter that was part of the oil filler cap or they’d have a closure hose leading from the top of the oil filler cap to connect to the nipple on a plastic housing with a foam or fiber filter that was just inside the air cleaner housing. You can see it when you remove the top of the air cleaner.

Older vehicles with round air cleaners typically had a crankcase filter that was part of the oil filler cap or they’d have a closure hose leading from the top of the oil filler cap to connect to the nipple on a plastic housing with a foam or fiber filter that was just inside the air cleaner housing. You can see it when you remove the top of the air cleaner.

Richard McCuistian, ASE Certified Master Automobile Technician

If the PCV side of the system becomes clogged or loses its vacuum, the crankcase vapors naturally reverse course and are funneled into the air cleaner.

What Does the Crankcase Filter Do?

The crankcase filter cleans the blow-by gases or escaped gases to remove contaminants like dust and dirt that will inevitably be drawn to the filter by the crankcase ventilation.

Where Is the Crankcase Filter Located?

The crankcase filter is usually on the breather hose on top of the crankcase. It could also be on the valve cover, depending on the vehicle. See opening text.

Symptoms of a Clogged Crankcase Filter

Dirt, debris, and other deposits can clog the crankcase filter over time. Here are the telltale signs that your vehicle’s filter is clogged:

Oil Leaks or There’s Oil in the Air Cleaner

Oil in the air cleaner is a telltale sign that the PCV side of the system is clogged and the blowby is coming back through the filter and into the air cleaner.

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A blocked or clogged filter builds pressure, which leads to blown seals or gaskets that cause oil leaks. Of course, for that to be the case, both the PCV valve and the crankcase filter must be totally clogged. If one is clogged but not the other, it won’t blow seals.

Illuminated Check Engine Light

If the powertrain control module (PCM) detects any problem in the crankcase filter, it will illuminate the check engine light. Mechanics use a scanner tool to get the on-board diagnostics (OBD) code that indicates the issue from the system.

For example, if the tool logged a P1507 code, which means “Crankcase Filter Restriction,” it indicates that there’s something wrong with the filter.

Remember that other faulty parts can exhibit the same symptoms as a clogged crankcase filter. So be sure to cross-check the other signs or let a mechanic check your vehicle for a proper diagnosis.

Causes of a Clogged Crankcase Filter

If your crankcase filter is clogged, it might be due to the following:

Engine Oil

Engine oil can become thick and sludgy when it’s old or contaminated, which clogs the crankcase filter.

Dirt and Debris

The crankcase filter traps debris to prevent it from contaminating the oil. The dirt builds up over time, eventually clogging the filter.

Carbon Deposits

Carbon deposits are a result of incomplete fuel combustion. They create a wall of black soot that hardens and blocks the passageway.


Moisture can build up in the crankcase filter, especially if the engine often runs at low temperatures. It tends to mix with other contaminants, which clogs the filter.

Why Is It Important to Clean the Crankcase Filter?

Keeping the crankcase filter clean helps maintain high engine performance. For example, if it’s clogged, it could cause oil contamination, resulting in sludging, engine wear, and power loss.

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Does a Clogged Crankcase Filter Cause Rough Idling?

This might be caused by a disconnected PCV valve vacuum hose, but the crankcase filter can’t cause rough idle.

Can You Clean the Crankcase Filter Yourself?

Yes, you can clean the crankcase filter on your own, provided that you’re familiar with your vehicle’s PCV system and have the tools and know-how needed for the task.

When Should I Change the Crankcase Filter?

Experts recommend changing the crankcase filter when it’s damaged or at least once every two years.

How Much Does Crankcase Filter Replacement Cost?

The cost of replacing a crankcase filter ranges between $60 and $180. New filters cost $10 to $80, while labor costs can set you back $50 to $100, depending on the auto shop.

How Long Does a Crankcase Filter Last?

It’s best to replace the filter every 25,000 miles or every two years, depending on which comes first. The lifespan of a crankcase filter depends on the filter type and the conditions of the engine.

Some factors might accelerate the need for replacement, such as weather conditions and the frequency of oil changes. It’s also worth noting that some manufacturers have recommended filter replacement intervals.

Final Thoughts

If you spot the symptoms of a clogged crankcase filter, address them immediately to prolong and preserve your vehicle’s performance and prevent irreversible damage. Take your ride to an auto repair shop as soon as possible.

About The Authors
Richard McCuistian, ASE Certified Master Automobile Technician
Reviewed By Richard McCuistian, ASE Certified Master Automobile Technician

Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

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The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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