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Modern gasoline and diesel engines use a Positive Crankcase Ventilation (PCV) system to help improve fuel economy and reduce toxic tailpipe emissions. The PCV valve is one part of this system and can be an easy repair job for the amateur mechanic, depending on your vehicle. 

Understanding The PCV System and PCV Valve 

What does a PCV valve do? When your engine is running, the pistons move up and down in the cylinders, propelled by the explosions inside the cylinders. The tolerances inside those cylinders are very tight, but there has to be a bit of gap between the cylinder wall and the piston in order for the pistons to move. It’s inevitable that some vapors produced by the combustion cycle will sneak past the pistons into the crankcase, where they could potentially contaminate the oil and contribute to corrosion. 

To combat this, engineers developed the PCV system, with a hose connected to the top of the valve cover, drawing those vapors from the crankcase, and recombining them with fresh air and fuel to burn up in the cylinders. 

When the piston moves down in the cylinder, it creates a vacuum in the cylinder and when the valve opens, that vacuum draws air in through the valve. The PCV system uses that vacuum pressure to pull the vapors from the crankcase into the air intake. 

The PCV valve function is to close that hose when there is no vacuum present to suck vapors out of the crankcase. On most engines, it is a valve with a spring inside that shuts the valve to allow air movement in only one direction. 

Signs of a Bad PCV Valve 

Your vehicle should still run if it is showing these symptoms of a bad PCV valve, but it is something you’ll want to address before long. There are two different categories of PCV valve symptoms. 

If your PCV valve is stuck closed 

  • One very common sign of a bad PCV valve is a whistling or hissing sound coming from the engine. This can indicate that there is a slight blockage in the PCV valve. 
  • A Check Engine warning light can be another sign that you need to replace a PCV valve because the PCV valve impacts vehicle performance and emissions. 
  • Poor fuel economy can signal the existence of a PCV valve issue. 
  • Oil leaks are another sign of a bad PCV valve. When the valve is clogged with deposits it doesn’t allow the blowback pressure from the cylinders to dissipate, over-pressurizing the crankcase and potentially leading to oil seepage. 

If your PCV valve is stuck open: 

  • If your engine has a rough idle or is misfiring, that can indicate that you have an issue with your PCV valve function. 
  • The existence of oil in your PCV valve or hose can indicate that the valve is stuck open. 
  • If you have trouble starting your engine, it can be as a result of a failed PCV valve. 
  • Black smoke in the exhaust, or oil-fouled spark plugs can indicate that your PCV valve is stuck open and is allowing oil to enter the combustion chamber through the air intake. 

Damage to Your Car Due to a Bad PCV Valve 

When your PCV valve is malfunctioning, it throws the air to fuel mixture out of balance and can lead to poor performance and rough idle, as well as misfires. In the very short term, these issues should not damage your car, but if you don’t fix the issue, you can do lasting damage, so it’s best to get it fixed promptly. 

How to Test Your PCV Valve 

First find the PCV valve and hoses. It should be coming out of the valve cover, the intake manifold, or the side of the block. Some vehicles don’t have a PCV valve, and on others it is just a simple breather hose. If you have trouble finding the PCV system and its components, do an online search or purchase a repair manual

Once you’ve found your PCV valve, start your car and bring it to operating temperature and conduct these tests: 

  • Disconnect the PCV hose from the valve cover and place your finger over the end of the hose. If things are operating correctly, you should feel slight suction from the vacuum of the engine and the engine idle RPMs should dip for a second and then stabilize. If you don’t feel the suction, your PCV valve or hose may be blocked with deposits.  
  • You can also pinch the hose and idle speed should drop in the same manner. If you notice a much bigger drop in RPMs, your PCV valve could be stuck open. 

Maintaining the PCV System 

Most of the parts in the PCV system are made of plastic or rubber, while some aftermarket PCV valves are constructed of machined aluminum or other materials. Over time, plastic and rubber can become brittle, so it is a good idea to visually inspect all the hoses, grommets, o-rings, and fittings to ensure they are intact and pliable. If you notice that rubber parts are starting to crack or become hard, do some preventative maintenance, and replace them right away. 

If your PCV valve contains a mesh filter under the valve, you’ll want to check the filter for deposits and either clean it up before reinstallation or replace the filter altogether.  

Check all your hoses and other fittings to make sure they are clean and free of debris, using a hose cleaner to clear out any gunk.  

If your PCV valve is accessible, replace it every 20,000 to 50,000 miles as preventative maintenance. PCV valves are typically inexpensive and will probably save you your investment in improved fuel economy. 

Whether you decide to replace your PCV valve or just clean it up nicely, AutoZone has all the parts and tools that you need to get the job done right.  

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