How PCV Valves Work and what they do

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PCV Valve stands for Positive Crank Ventilation Valve. All cars with an internal combustion engine have a PCV Valve. So how do PCV valves work, and what function does it have? The PCV valve routes partially burned gases from the crankcase back to the combustion chamber. Even though it is a small and inexpensive part, it can play a critical role in an engine’s health.

What the PCV Valve System Does

The PCV system reduces crankcase emissions by recirculating air and fuel to the combustion chambers. All engines have some amount of unburnt fuel that makes it past the combustion chamber called “blow by”. This “blow by” will collect in the crankcase of the engine creating sludge which can be very harmful to the engine. The PCV valve system reduces the sludge build up and emissions by routing the unburnt fuel and gases back to the engine intake manifold so that they will get another chance to ignite in the combustion chamber. PCV valve also has a constant flow of air through it, so it is able to eliminate moisture in the oil system. If too much moisture gets into the engine, it can start to wear down components and cause corrosion.

How PCV Valves Work

A PCV valve is made from very few parts: a housing, a plunger, and a spring. When the engine is not running or when the engine backfires, the plunger will close the hole in the housing and stop air from running through the valve.

PCV Valve Closed

PCV Valves work by using a simple force balance on the plunger. During normal operation air from the crankcase will exert a force on the plunger. This force will overcome the force of the spring which holds the plunger closed. Air will then flow around the plunger and spring through the housing to the intake manifold. This is how PCV Valves work.

PCV Valve Opened 

Although the operation of the PCV Valve is simple, it can have a serious effect on an engine’s health

Symptoms of a Bad PCV Valve

There are several symptoms of a bad PCV valve:

  • Check Engine Light
  • Oil Leaks
  • Reduced Fuel Economy
  • Milky Cloudy Film on the PCV connecting hose
  • Low moaning or whistling noise
  • Increase in internal engine pressure
  • Black smoke

All of these can be symptoms of a PCV valve failure. PCV valve system will have its own code if there is a check engine light usually they are P0171 and P0174. A bad PCV valve will cause a sludge buildup in the crankcase and this can cause oil seals to fail. Reduced fuel economy can be cased by the PCV system not recirculating unburnt fuel to the combustion chamber. A milky cloudy film on the PCV valve hose line indicates that their is moisture in the system. If there is moisture in the system make sure to change the oil along with the PCV valve. A faulty valve could have many symptoms or none at all. Even if your vehicle has just one of these symptoms, it is a good idea to replace the PCV valve.

How to tell if your PCV Valve is Faulty

There is one easy trick to tell if your PCV valve needs to be replaced. Remove the PCV valve and shake it. You should here the spring and plunger rattling around freely. If you here a deeper clunking sound or no sound at all while shaking it, the PCV valve is bad. However, a PCV valve could past this test and still be faulty. A visual inspection is also a good idea. Inspect the valve for soot, contamination, or any obvious damage.

Even if your PCV valve is showing no symptoms of failure, it should be replaced at every major service. Consulting the owners manual should provide the PCV valve replacement interval. If you don’t know how long the PCV valve has been on your engine, it is a good idea to replace it. PCV valves are generally inexpensive and very simple to remove and install. The PCV is a simple inexpensive part that plays a critical role in your engine’s overall health, and a preventative replacement can you save you a lot of time and money down the road.