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All internal combustion engines produce gases that sneak past the piston rings into the crankcase. The crankcase must be ventilated so that these gases do not create problems, such as moisture in the oil and excessive pressure.

But the gases cannot be vented directly into the atmosphere because that would increase harmful emissions. That’s why automakers have been fitting vehicles with positive crankcase ventilation (PCV) systems since the 1960s.

Many systems incorporate a PCV valve to regulate the flow of crankcase gases. Like any automotive part, the PCV valve can eventually fail, resulting in problems that you’ll want to address right away.

automotive pcv valve
The PCV system is designed to remove the blowby gases from the crankcase and then recirculate them back to the engine, where they are burned during normal operation.

What Does a PCV Valve Do?

No matter how well an engine is designed, there will always be some combustion gases that sneak past the piston rings into the crankcase. These gases, which are often referred to as blowby, consist primarily of unburned fuel and water vapor.

The PCV system is designed to remove the blowby gases from the crankcase and then recirculate them back to the engine, where they are burned during normal operation. If it weren’t for the PCV system, crankcase pressure would climb until oil was forced past seals and gaskets. The PCV system also prevents the blowby gases from mixing with oil to form engine-damaging sludge.

All PCV systems must have some way of regulating the flow of blowby gases entering the engine. Some designs handle this task with a calibrated orifice or separator—others rely on a PCV valve to get the job done.

How Does a PCV Valve Work?

The PCV valve is a one-way check valve that contains a spring-loaded plunger to regulate the flow of blowby gases. One end of the valve attaches to a hose connected to manifold vacuum, while the other end typically fits into the engine’s valve cover or intake valley.

When the engine is off, the PCV valve’s internal spring forces the plunger to close. But once the engine is running, manifold vacuum begins to pull the plunger open. Then, the open PCV valve pulls fresh air, which enters the engine via a breather tube, through the crankcase. The resulting scavenging effect draws blowby gases into the PCV valve.

After leaving the valve, the gases enter a rubber hose that’s typically connected to the engine’s intake manifold or throttle body. The gases then enter the engine, where they’re burned as part of the normal combustion process.

The PCV valve cannot allow the same amount of blowby gases to enter the engine at all times, though. Instead, the valve must use its internal plunger to regulate flow, as follows:

  • Blowby production is low when manifold vacuum is high. In this state, vacuum pulls the tapered-shaped plunger from its seat so that the gases can only flow through tiny grooves in the plunger’s body.
  • As vacuum decreases and blowby increases, the plunger moves further away from its seat until it’s in a maximum flow position. Crankcase gases can then flow freely into the engine’s intake system.

But wait—that’s not all. The PCV valve also provides protection against backfires if necessary. If a backfire occurs in the intake manifold, the valve’s internal plunger will be forced towards the crankcase, preventing the flame from igniting fuel vapors inside.

air filter housing
Oily blowby gases will begin to build up and eventually get pushed back into the air intake and/or air filter housing once your PCV valve becomes clogged or stuck closed.

Signs of a Bad PCV Valve

Do you think you might be dealing with a bad PCV valve? If you notice one or more of the following symptoms, you might be right.

Note: Other problems can mimic a bad PCV valve. You (or your mechanic) should perform a thorough diagnosis before conducting any repairs.

Oil Consumption

If the PCV valve is stuck open, oil will be siphoned from the crankcase under high vacuum conditions (when the valve would normally be closed). Consequently, a PCV valve that’s stuck open can lead to oil being burned inside the engine and increased consumption.

Rough Idle/Stalling

The PCV valve pulls air and gases through the crankcase and into the engine. If the valve is stuck open, the extra air entering the engine will disrupt the air-fuel mixture, causing problems, such as rough running and stalling.

Illuminated Check Engine Light

On many modern vehicles, the engine management computer monitors the PCV system for proper function. If the device detects a problem with the PCV system, it will turn on the check engine light and store a corresponding diagnostic trouble code in memory. A stuck-open PCV valve can also disrupt the air-fuel mixture and cause misfires that trigger the check engine light.

Oil in the Air Intake System

If the PCV valve becomes clogged or stuck closed, oily blowby gases will begin to build up and eventually get pushed back into the air intake and/or air filter housing.

Oil Leaks

A stuck closed PCV valve can also cause crankcase pressure to increase to the point that oil gets pushed past the engine’s gaskets and seals.

Sludge Accumulation

If a clogged PCV valve is left ignored, moisture will begin to accumulate in the crankcase oil, eventually leading to sludge formation inside of the engine.


How do you test a PCV valve?

Testing a PCV valve is usually fairly straightforward. The video below will give you an idea of what the process often involves:

Can you drive with a bad PCV valve?

You should not continue to drive with a bad PCV valve. A faulty PCV valve can cause additional (potentially costly) problems, such as blown-out gaskets and increased oil consumption.

How often should a PCV valve be changed?

Some (but not all) automakers recommend a PCV valve replacement as part of the vehicle’s regular maintenance schedule. You can find the maintenance schedule in your owner’s manual or supplemental service booklet.

If the schedule does not list a service interval for the PCV valve—and you’re sure your car has one; remember that some do not—a good rule of thumb is to replace the valve every 30,000 to 50,000 miles.

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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