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Positive Crankcase Ventilation Valve | Bad PCV Valve Symptoms


The PCV is simply the abbreviation of positive crankcase ventilation (PCV). This 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. PCV 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 valves 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.

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.

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.

Symptoms of Bad Positive Crankcase Ventilation Valves

The PCV valve can become stuck in either the closed or open position and cause several conditions that contribute to poor engine operation, increased pollution, and other issues. If the engine surges or there is black smoke coming from the tailpipe, this is an indication of a failed valve. Less noticeable is the buildup of moisture and sludge in the engine.

Increased oil consumption and oil leaks

An increase in oil consumption is usually a symptom of a PCV valve stuck in the open position. The valve is open, allowing gases to exit the crankcase in all situations – idling as well as high power situations. Vaporized oil is passed through the valve into the combustion manifold and burned to cause black smoke to exit from the tailpipe.

If one of the tubes connecting the PCV valve to the crankcase and the intake manifold is loose or disconnected, the blowby gases are exiting into the atmosphere. You may hear a whistling sound, and the engine compartment can become abnormally dirty with oily blowby exiting the hose. An oil leak or an intake manifold gasket leak are common problems found when the PVC system is not working correctly and can cause the engine to run rich or lean.

Contaminated filter

Filters become contaminated over time for a variety of reasons. Most people are familiar with the filter on the air intake systems for your engine. This filter removes dust, dirt, and insects before the air enters the engine and should be checked when servicing the engine.

Some vehicle engines have a wire mesh filter just below the PCV valve to filter out oil and sludge. In addition, there is a crankcase fresh air intake filter which can also become clogged disrupting the flow of unburnt gases from the crankcase through the valve and back to the manifold. Again these should be checked at servicing to make sure they are not clogged.

Overall poor engine performance

Poor engine performance can be caused by any number of sensors malfunctioning, fouled spark plugs, dirty injectors, a bad PCV valve, and more. If you are experiencing poor engine performance, check trouble codes generated by the car’s computer and begin checking some of the easier items.

A PCV valve can easily be checked. Carefully disconnect the PCV valve from the PCV hoses and shake it gently up and down. You should hear a clicking sound which indicates it is not stuck closed or open. While you have it out, clean it and then reinstall. If this is not the source of the problem, other areas will have to be explored.

Check engine light is on

The check engine warning light may be on because the car’s computer has detected an issue with one of the systems in the engine. The first step is to check the error codes and follow these indications to determine what is causing the light to illuminate.

A PCV valve that is not operating properly can cause the mass airflow sensor or the oxygen sensor to trigger an error and cause the check engine light to illuminate. Before replacing either of these sensors, you should confirm the PCV valve is clean and operating properly.

Smoke coming out of the exhaust

Several issues can cause smoke to come out of the exhaust. These situations include the following:

  • PCV System fails
  • Engine coolant due to a blown head gasket
  • Burned oil due to damaged cylinder head or cracked engine block
  • Condensation built up in the exhaust system, which quickly disappears

The PCV Valve opens and closes to allow blowby from the pistons into the crankcase. When it stays open, unburnt gas and air along with a small amount of oil are fed back into the combustion chamber. Burning oil can produce smoke in the exhaust system and is an indication of a PCV failure.

Rough engine idling or high idle

Smooth engine idle speed depends on everything working as designed, including the PCV Valve. Many factors can contribute to rough engine idle or high idle, including:

  • Failing PCV Valve
  • Dirty or bad spark plugs
  • Bad ignition coils or wires
  • Vacuum leak in the emission system
  • Dirty Carburetor or bad injectors
  • Dirty air filter
  • Bad air control valve

A warning light may illuminate in many of these situations, and the error code will indicate the appropriate device that may be causing the problem. Although some error codes may indicate sensors are malfunctioning, such as the mass airflow control or oxygen sensor. Check for a clogged PCV valve or one that is staying open or closed PCV valve.

Car Engines misfire or Engine backfire

Alterations of the air-fuel mixture and the ignition timing are common reasons that will cause an engine to misfire. In many cases, the check engine light will turn on, identifying error codes that should be checked out.

Misfiring engines can cause damage to the engine, which can become expensive to repair. Here are some of the reasons an engine will misfire:

  • EGR system and positive crankcase ventilation PCV system failures change the air to fuel mixtures
  • Failed fuel system injectors not igniting the air-fuel mixture
  • Ignition system sensors, spark plugs, etc. failures
  • Mechanical issues with the engine valves, engine vacuum – these are considered severe issues
  • Failing sensors that cause the ECM to alter the air-fuel mixture incorrectly
  • Corroded or poorly connected circuits connecting the ECM with all of the sensors
  • The hose that connects the throttle body to the valve is loose

Excessive carbon buildup inside the engine

Carbon buildup in engines can occur in both modern and older engines due to deposits from the ignition process not being fully clear away during the exhaust process. The deposits may look like soot or ash inside your engine and on surfaces exposed to unburnt gases caused by blowby around your piston’s rings.

Regular oil changes along with a filter change using high-quality motor oils can reduce this problem and keep your engine clean. Follow the manufacturer’s recommendations for oil changes and use oils with detergent cleaning properties to keep the engine clean.

Engine oil sludge in the PCV housing

Engine sludge inside the valve can cause the PCV Valve to malfunction and even become plugged. When you have an un-plugged PCV valve, moisture and blowby vapors cannot be removed, and sludge builds up inside the crankcase and also creates pressure inside the crankcase, causing oil to leak past gaskets and seals, increasing the burning of oil inside the engine.

With no airflow through the PCV Valve, the air-fuel mixture is richer, increasing fuel usage and increasing emissions as well. Your car may not pass an emissions test if the OEM PCV Valve is plugged with sludge.

Lean air-fuel mixture

PCV Valves that are stuck in the open position or even if drivers install the wrong valve, which allows too much airflow through the valve, can cause the air-fuel mixture flowing through the intake manifold to be lean.

Leaking tubes attached to the PCV Valve can also cause similar problems to occur. Drivers should check for vacuum leaks and also test the PCV Valve to make sure it is operating properly.

With the PCV valve removed, a slight shaking should trigger the valve to open and close. If it is dirty and coated with sludge, either clean it or replace the valve with a new one.

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.


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.

What are the most common problems that many 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.


Positive crankcase ventilation is a method of venting the engine’s crankcase gasses that have been heated by the engine. 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. 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.

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