Home / Engine / What Is A Positive Crankcase Ventilation Valve? [Explained]

What Is A Positive Crankcase Ventilation Valve? [Explained]


The PCV is simply the abbreviation of positive crankcase ventilation (PCV). The positive crankcase ventilation valve is a simple one-way flow valve and normally introduces re-circulated the unburned fuel, in the outline of blow-by gases, reverse into the combustion engine to dismiss them properly.

Therefore, combustion engines have got a perfect fuel-to-air ratio and in this case it is the “P.C.V. valves” who does the job monitoring the amount of blow-by gases rear into the engine with no upsetting the ratio.

Basically, fuel and air are combined in the intake manifold before injection into the combustion chamber. When this mixture combusts, the force of the combustion drives the pistons creating the driving force for the engine.

The piston rings slide up and down along the piston walls providing a seal to prevent gases from getting past the piston and preserving the power to drive the pistons in your engine.

Unfortunately, the piston rings do not provide a perfect seal, and there is some blowby of air and fuel into the crankcase, which also creates pressure in the crankcase. There is a mix of air, oil, water, and some fuel creating this increase in pressure in the crankcase.

Long-term pressure in the crankcase can cause oil seals and gaskets to fail and other problems for your engine. The PCV or positive crankcase ventilation system valve is the answer to this problem in modern internal combustion engines.

What are positive crankcase ventilation valves?

The positive crankcase ventilation system or PCV valve is a one-way valve that passes crankcase gases from the crankcase to the intake manifold and re-enters the combustion system.

These gases consist of unburnt gasoline, air, and oil that has been vaporized. A one-way valve will only allow the gases to pass in one direction, in this case, only from the crankcase to the intake manifolds and intake valves.

The PCV valve is a major improvement to managing the emissions from our vehicle engines, reducing smog and pollution. Positive crankcase ventilation valves have been in use since the 1960s, making a major contribution to the reduction of smog in many cities and our environment in general.

What do PCV valves do?

New PCV valve for replacement

Before the addition of positive crankcase ventilation valves, the excess gases created by blowby of air and fuel were exhausted to the atmosphere creating much more pollution.

Engineers developed a system to recirculate these gases back into the combustion system and cause the unburnt fuel to burn during the combustion process.

The PCV valve is one way only. Otherwise, air and fuel mixed in the intake manifold could pass back into the crankcase depending on relative pressures.

With a one-way valve in place, the blowby of unburnt fuel and air mixture can only proceed back to the intake manifolds. Oil is separated by oil separators before the mixture is passed through the PCV Valve.

The PVC system is manufactured to reduce air pressure within the engine and assist in the creation of a fuel-efficient and smooth-operating engine.

How do PCV valves work?

The positive crankcase ventilation PCV exit hose connects to the intake manifold. As air is drawn into the engine, a vacuum is created, drawing air through the intake system and also from the hose connected to the PCV Valve.

Since this valve is a one-way valve, it opens and creates a vacuum in the crankcase and draws the unburnt fuel-air mixture through the PCV valve into the intake manifold.

Depending on engine speed, power levels, and acceleration, the vacuum in the intake will increase and decrease. Whenever the vacuum in the intake decreases, the PCV Valve blocks gases from being drawn into the crankcase.

The PCV system has a similar effect on the air-fuel mixture as a vacuum leak. This is compensated for by a fuel injection system, which calculates how much goes into each cylinder at any given time and according to engine demand.

Where is the positive crankcase ventilation PCV valve located?

The PCV valve is usually placed at or near the valve cover. However, it can be located anywhere between the intake manifolds and the crankcase air outlet. Look for a tube exiting the crankcase and follow it until you can observe the valve.

The crankcase also has a breather tube. You will want to ensure you are not mistakenly following this tube. The tube leading from the PCV valve will enter the manifold at some point.

What happens if a PCV valve is disconnected?

Removing the PCV valve or disconnecting the tubes connected to it can have some serious impacts on your car’s engine and the environment. The following are the typical impacts of a PCV valve stuck open or disconnected:

  • Contamination of the oil
  • Sludge buildup
  • Oil leaks
  • High fuel consumption
  • Engine misfires
  • Hard engine starting
  • Rough engine idling
  • Possibly black smoke
  • Spark plugs fouled with oil

It can increase contaminants exiting the tailpipe. The check engine warning light may also illuminate, and the error codes could indicate malfunction of the mass airflow or oxygen sensor, making it difficult to diagnose the problem.

How to test to Verify the PCV valve is bad?

Always inspect the rubber PCV vacuum hose and parts to make sure there are no loose hoses and no cracks in them. Look for cracks and oily deposits around connections. You can also remove the valve and give it a shake. If there is no rattling sound, you should replace it.

If the hoses or the PCV valve are filled with slime, you will need to clean them properly. Some valves have a heater. Check the connections for corrosion. Use an ohmmeter to check the heater coil. If there is no resistance, the coil is damaged and should be replaced.

Positive crankcase ventilation PCV System Diagram Explained

Some small amount of blowby passes past the piston rings into the crankcase, creating positive pressure in the crankcase.

This vapor includes air, unburnt fuel, and vaporized oil as it mixes with the oil in the crankcase. A tube is connected from the crankcase to the intake manifold with a PCV Valve connected at one end of the tube.

As air is drawn through the intake manifold, it creates a vacuum, drawing the vapor in the crankcase through the vacuum hose and the PCV Valves. The crankcase ventilation system may also have an intake breather to bring fresh air into the crankcase breather.

When the engine is revving at high RPM, blowby is at its highest, and the vacuum created by the intake manifold is also high, drawing the vapor from the crankcase through the PCV valves into the intake manifold to take the vapor through the combustion process a second time.

Electrical Heated vs. Coolant Heated PCV Valves

In extremely cold temperatures, the PCV Valve can freeze unless it is heated by either an electrical heater or by engine coolant. Not all PCV systems are heated. PCV Valve heaters are controlled either by a thermal harness or a PCM heater controller.

When the temperature is below a specific value, the heating element is activated and remains on unless the engine is turned off or the temperature rises above the threshold set for your vehicle.

PCM heater-controlled PCV systems turn on when the air temperature is below a specific number. It is also deactivated when the engine is turned off. Water-heated systems circulate engine coolant to keep the PCV system from freezing.

Common problems that PCV valves cause

Every car is different, and the engine emissions control system is engineered differently on all makes and models. However, common PCV valve symptoms many car owners experience are the following:

  • Engine oil contamination with fuel vapors via oil droplets
  • Build up of sludge in the engine, the PCV valve hoses, and even in the valve cover
  • If the crankcase pressure is not relieved, the pressure cause oil leaks to occur
  • Fuel consumption may increase due to engine damage
  • Smoke exiting from the exhaust indicating burnt oil
  • Illumination of a warning light indicating mass airflow or oxygen sensors have detected reading outside the expected limits
  • Rough idle or high idle RPM
  • Excess crankcase vapors

The diagnostic trouble codes for a bad PCV system are P0171 and P0174.

Have your engine checked by a quality automotive repair mechanic, verify the error codes if your check engine light is on, and check all of the hose connections. This is common when doing a PCV maintenance check.


The following are some of the most frequently asked questions related to PCV systems concerning when to replace them, the cost, and some of the most common problems related to the PCV system.

When is it time to replace my PCV valve?

The consensus from many mechanics is that car owners should check the valve every 30,000 miles and clean it up if needed. In addition, if your engine is exhibiting some of the symptoms mentioned earlier, such as:

  • Smoke in the exhaust
  • Engine running rough or idling high
  • Sludge or carbon buildup in the PCV systems
  • The valve does not respond to testing
  • High engine operating temperature
  • Oil droplets found in the crankcase ventilation system

Always replace the valve with one that is recommended for your engine. Replacing the valve with one that is not recommended could allow too much air-fuel vapor or not enough to be recirculated to the intake manifold.

Basically, it should be replaced or checked as soon as you notice idle speed changes, black smoke in the exhaust, or a high engine operating temperature.

How much does it cost to replace a bad PCV valve?

The PCV Valve is relatively inexpensive, ranging from $10 to $15 depending on the make and model and whether it is heated or not. A mechanic may charge somewhere between $35 to $75 to check the hoses and replace the valve.

Note that if one or more of the hoses are cracked, they should be replaced, which will be at an additional cost. If a hose is plugged or dirty with sludge, a mechanic may suggest a replacement valve rather than cleaning the hose.

Of course, if you are doing the work yourself, cleaning the hose is not difficult and can save you a few dollars. Depending on the manufacturer, many PCV valves have a 50,000-mile common replacement interval.


Positive crankcase ventilation is a method of venting the engine’s crankcase gasses that have been heated by the engine. Also, these gases are typically vented into the air intake system in order to be burned off with new incoming air, but there are other methods as well.

The PCV system can help reduce emissions from vehicles and can also increase fuel efficiency. The PCV valve is part of the emission control system in modern engines, reducing pollution in terms of unburnt fuel and vaporized oil.

Plus, the valve controls the recirculation of blowby into the intake manifold created by the intake manifold vacuum and breather tube attached to the crankcase.

A failed PCV Valve and reduce gas mileage, increase oil usage, cause your engine to idle roughly and sludge to buildup inside the engine, potentially causing harm to other parts of the engine. Have the PCV system checked every 30,000 miles as preventive maintenance or whenever you notice any of the above symptoms.

Last Updated on: February 21, 2024

Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments
This div height required for enabling the sticky sidebar