PCV Valve

PCV Valve

PCV Valve

A positive crankcase ventilation (PCV) valve is an engine’s safety feature as well as an emissions feature that helps to protect the crankcase from excessive pressure buildup while reducing engine emissions. As a rule, it features a spring-loaded mechanism which restricts or allows engine gases to leave the crankcase. Its failure usually means an array of engine related problems, including the following:  

Rough idling or stalling

Since a PCV valve redirects some engine gases back to the combustion chamber, their volume may cause a lean air-fuel mixture if the valve gets locked in the open position. As a result, you may experience rough idling or even engine stalling.

Your check engine light is on

In case your engine is equipped with an oxygen sensor, it will let the engine control module know when an air/fuel mixture becomes too low or too rich due to a failed PCV valve. Once the problem is detected, the engine control unit will turn on the check engine light.


The primary job of a PCV valve is to prevent excessive pressure buildup in the crankcase. If the valve gets stuck in the closed position, the pressure will rise, causing oil leaks through seals and gaskets or, even, the air cleaner assembly. 

Sludge buildup

A malfunctioning PCV valve may lead to sludge buildup when blowby gases mix with the oil, which can destroy the engine.

Poor fuel economy and increased emissions

A bad or clogged PCV valve can restrict the gas flow to the intake manifold, making the engine run too rich. This will affect your fuel economy and emissions.

Low performance and abnormal idling

A clogged or always closed PCV valve leads to a lean air/fuel mixture, which, in turn, means poor acceleration and idling as well as backfires.

What is a PCV valve?

Regardless of the condition of your engine, there is always a minute gap between the piston rings and the cylinder walls. This gap allows some combustion gases to leak down into the crankcase when the engine is running. Known as blowby, these leaks may cause serious damage to the crankcase and other related components if they are ignored. To avoid this problem, manufacturers use a positive crankcase ventilation valve to maintain optimum pressure inside a crankcase. Usually found under the engine valve cover, it redirects some crankcase gases to the intake manifold or throttle body via a hose.

In addition to its primary function, a PCV valve also allows for reducing your emissions as it routes harmful combustion gases back to the combustion chamber. Thanks to a built-in spring-loaded plunger, your PCV valve changes the flow of the gases to the intake manifold in response to engine operation. 

When your engine is running at full throttle, the pressure inside the crankcase significantly increases causing the spring-loaded valve to open, allowing for maximum gas flow to the intake manifold. In some cases, the amount of gases can exceed the PCV’s carrying capacity, so some gases can be rerouted to the air cleaner assembly. However, this is not the best scenario, since your air filter can quickly get clogged.

At low RPMs, your engine creates a lot of vacuum, allowing for the minimum combustion gases loss. In this case, the PCV valve remains almost closed, preventing a gas flow to the intake manifold. 

Your PCV valve also serves to protect the crankcase in case of backfires. When a backfire occurs, a lot of excessive pressure and backfire flame that reaches the intake manifold tries to leave it via the PCV hose. These forces move the valve plunger towards the crankcase, causing the valve to close, which protects the crankcase from the backfire flame and prevents it from possible damage.

Can I fix a bad PCV valve?

A faulty PCV valve is not repairable, so it must be replaced. In most cases, it is easy to do by just removing the old one and installing a brand-new valve. However, there are PCV valves that are built in a valve cover. In this case, the entire valve cover assembly must be replaced. You should leave this job to a professional mechanic.