8 Symptoms of a Bad PCV Valve (And Replacement Cost)

PCV valves are inexpensive and often overlooked parts of your engine. When neglected, a faulty PCV valve can cause many issues that result in much more expensive repairs.

What is a PCV Valve?

“PCV” stands for Positive Crankcase Ventilation. As your engine runs, some of the combustion gases get pushed past the piston rings and into the crankcase (the part of the engine block that surrounds the crankshaft). This leakage into the crankcase is called blow-by.

Combustion gases in the crankcase contain air, atomized fuel, and oil. The pressure from these gases needs somewhere to go. Old PCV systems used to vent this pressure to the atmosphere.

In an effort to reduce emissions, modern PCV systems recycle crankcase gases back into the intake manifold, where the gases eventually make their way back to the combustion chamber.

Where Can I Find My PCV Valve?

pcv valve

The PCV valve is usually attached to the intake manifold or valve cover, but is sometimes bolted to the engine block. Some PCV valves are visible immediately when you pop the hood, and others are more hidden in the engine bay.

Why Do PCV Valves Go Bad?

PCV valves are simple mechanical parts, but they still go bad after many miles. Most PCV valves fail because they are clogged and can no longer open and close freely.

A PCV valve may become clogged when oil, sludge, or carbon build up. This limits the range of motion of the internal valve.

Bad PCV Valve Symptoms

PCV valves can get stuck in the open or closed position. If your PCV valve is stuck, the symptoms may vary depending on the valve’s position. Here are some examples of symptoms you may experience with a PCV valve failure.

1) Rough Idle

rough idle

If your PCV valve is stuck open, you may notice a rough idle due to misfires or a lean running condition. This is because more air is able to enter the intake via the PCV system than would normally be allowed to.

This is similar to what you might experience if you have a vacuum leak. In the case of a bad PCV valve, the excess air is coming from the crankcase instead of the atmosphere.

2) Fouled Spark Plugs

dirty spark plug

If there is too much oil flowing through the PCV system, the spark plugs may become caked in oil and other carbon deposits. Once the spark plugs are fouled, they may not ignite the air fuel mixture as cleanly.

Fouled spark plugs could cause misfires and a rough running engine. Cleaning or replacing the spark plugs may be necessary if this happens to your vehicle.

3) Black Smoke From Exhaust

black smoke from exhaust

A PCV valve that is stuck open will allow a lot of oil through to the intake. When this oil burns in the combustion chamber, you may notice some black or blue smoke out of the exhaust.

You will probably notice more black smoke when you step on the gas, rather than when you’re just cruising around on the highway.

4) Oil In Your Intake or Air Filter

clogged air filter

Ever go to change your engine air filter and notice some oil in your intake, or even on the air filter itself? This could be caused by a bad PCV valve.

You can try cleaning out your intake to get rid of the excess oil, but make sure you find and address the root of the problem if the oil comes back after a short time.

Air Oil Separators and Catch Can Systems

Air oil separators (AOS) and catch cans are designed to connect to the PCV system between the PCV valve and the intake manifold. Air oil separators and catch cans filter out oil before the air is returned to the intake manifold.

Engine oil doesn’t burn as well as fuel. Vehicles with air oil separators have much more reliable and complete combustion, which reduces knock and increases efficiency. This may also give your vehicle more consistent performance, which is why air oil separators are popular in performance vehicles.

5) Increased Oil Consumption

low oil pressure symptoms

If you have a bad PCV valve, you may notice you have to top up the engine oil more frequently. This is because the PCV valve is stuck open, and is allowing more oil to pass through the engine than is necessary.

It’s always a good idea to check your oil every time you visit the gas station. This way you can check oil consumption issues early before they turn into oil starvation, a much more serious concern.

6) Oil Leaks

oil leak

If your PCV valve is stuck closed, your engine may see increased pressure within the crankcase. Sometimes this pressure is great enough to push oil past the rubber seals throughout the engine. This can cause oil leaks.

7) Failed Engine Seals

rear main seal

Excess pressure in the PCV system could be strong enough to not only cause oil leaks, but push out of your oil seals. If the engine oil seals were already weak or not installed correctly, the excess pressure from a closed PCV valve may be enough to force them out of position.

A failure like this would cause a very large oil leak, and should be addressed as soon as you can.

8) Moisture Under Oil Cap

oil cap

If you notice moisture under your oil cap, your PCV valve may be stuck closed. Excess pressure from the crankcase will try to take the path of least resistance. This could create oil and moisture residue underneath the oil cap.

How Often Should a PCV Valve Be Replaced?

A PCV valve should be replaced whenever it goes bad, but most manufacturers don’t provide a specific interval for this little component. Many sources suggest cleaning or replacing the valve every 30,000 to 60,000 miles.

How to Test a PCV Valve

PCV valves are simple devices, and it’s really easy to tell when you have a good one. Once you remove the PCV valve, give it a little shake back and forth.

If you hear a rattle, that means the PCV valve is probably good. If you don’t hear a rattle, or the rattling sound is very faint, it’s time to clean or replace the valve.

Cleaning vs Replacing

There are many times when you can salvage a PCV valve by cleaning it out. If you hear a faint rattle, try spraying the PCV valve with some brake parts cleaner, then shaking the valve back and forth. Repeat this step a few times and see if there’s any improvement. If the rattling sound gets louder, you may be able to reuse the PCV valve.

If the PCV valve is seized or looks filthy, there’s no need to bother with cleaning it. PCV valves are incredibly cheap, and it’s often better to just buy a new one and replace it.

PCV Valve Replacement Cost

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Replacing a bad PCV valve won’t be too costly, and many owners can replace this part on their own. The part itself costs about $10 or $20, while the labor costs to replace the valve will be about $80 on average.

You can expect to spend about $100-$200 for a new PCV valve if you take your vehicle to a mechanic to fix it. If you notice the symptoms of a bad PCV valve and you continue to ignore the issue, it may cause much more expensive problems down the road.


4 thoughts on “8 Symptoms of a Bad PCV Valve (And Replacement Cost)”

  1. My 2012 Mercedes GL550 is bringing out blue smoke. I have changed the PCV valve located at the back of the engine at the passengers side, when it was change the check engine light went off but after a few miles drive the check engine light came up again and the smoke still smokes. I have noticed when I turn on the car in the morning the engine steams and fog with smoke and drip water from the exhaust. I mean the normal smoke that comes out of a new engine but smell like burning oil. I don’t see any oil leakage and I don’t know what to do. The engine is as quiet as a mouse. Please I need some advice from an expert.

    • I’d do a leak down test to see if you’re losing oil past a worn piston ring, or something like that. A mechanic can do this test for you. They will tell you the leak down percentage in each cylinder and should be able to tell you where the excess air is leaking from. If you hear air in the PCV system with this test, it indicates you have one or more bad piston rings.

    • Does it sound like marbles in a can? I wonder if the sound you’re describing could be engine knock?

      If you only hear the sound at a specific engine speed even at light engine load, I am inclined to think it is a rattling heat shield though.


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