Mechanics

PCV Valve

PCV Valve
PCV Valve

The PCV Valve or "Positive Crankcase Ventilation" valve was one of the first pieces of emissions equipment introduced into the automotive industry. Originally all of the gases that were created inside of the engine were just directed out and towards the ground, these were known as "Road draft" tubes. With the advent of the PCV system, these gases were routed back into the engine to be more efficiently burned, thus eliminating the practice of simply venting these gases into the atmosphere. GM's Cadillac division was the first to implement these valves into production; they also allowed the entire automotive industry to copy the valve and PCV system (Royalty Free) due to the incredible amount of emissions that were reduced.

Internal combustion engines naturally create internal engine pressure due to some amount of leakage past the piston rings during combustion, this is known as "Blow-by". Early engines had oil seals that were designed to withstand this pressure, so the engines leaked quite a bit, but it was considered normal. By using the engines intake manifold vacuum, engineers were able to take the internal pressure off the oil seals and redirect it back through the engine, making it easier to keep them from leaking oil as well as reducing emissions.

A PCV system consists of a PCV Valve, a breather tube that connects to the intake manifold and a fresh air intake (Usually on opposite side of engine) that allows clean air into the engine to help keep it clean internally, this tube usually attaches to the air cleaner. The PCV Valve is the most important item in this system; it is what meters the amount of air that is being re-circulated back into the engine, depending on engine speed (amount of throttle applied). There is a tapered piece in the valve that is held with spring tension, this piece moves depending on the amount of intake manifold vacuum present, which is how the amount of re-circulated gasses is determined. 

When there is the most manifold vacuum (Idle) the valve is at its most restrictive due to the lack of engine blow-by, but as vacuum drops (acceleration) the tapered cone allows more internal gasses to pass through the valve. During wide-open throttle, the valve is at its maximum allowance of flow, which is also when blow-by is at its peak. The PCV valve also contains a check ball, this one way valve keeps pressure from entering the engine should the intake manifold become pressurized (Back-fire or boost from turbo).

On the opposite side of the PCV system from the valve is the fresh air intake. These intakes usually go to the air cleaner box to allow the filtering of the air used to evacuate the crankcase. There are also baffles used to keep possible flame (Back-fires) from entering the crankcase and causing a possible ignition of the internal gasses.

Keeping the PCV system in your vehicle is very important to the life of the engine and for emissions purposes. All hoses need to be inspected regularly to be sure there are no cracks or broken lines and that the PCV valve is clean and operating properly. The valve can be cleaned out using carburetor cleaner, this will ensure a smooth operation of the metering and check ball systems inside the valve. If you are not sure where your PCV valve is located on your specific vehicle, consult a manual, as they are not all in the same place on all vehicles. 

Replacement of the valve is usually just a matter of removing the valve and plugging it onto the vacuum line and re-installing it in the valve cover or where ever it is located. Always use a high quality valve when replacing your PCV valve, cheaper ones are exactly that, cheaper and may not last as long or perform as well as an OEM (Original Equipment Manufacturer) part. 

If further technical assistance is needed, ask our team of certified car repair technicians.

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-08-19)