PCV Valve - What is a PCV Valve? How Do PCV Valves Work?

PCV Valve – What Is A PCV Valve?

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An engines PCV valve is an important piece of emissions equipment. The positive crankcase ventilation system helps trap harmful gases, using the PCV valve to reroute them back to the engine to be burned off, improving emissions. The valves are however subject to a lot of dirty gases which can cause them to clog up and fail, leading to a number of performance related issues.

I’m going to cover everything PCV in this guide. From understanding the function of the PCV system and the valve, to symptoms of valve failure, to how to replace a bad valve. Additionally, I’ll discuss catch cans and their benefit in reducing engine blow-by. I run a catch can setup on every vehicle of mine, and you should to.

PCV Valve - What is a PCV Valve? How Do PCV Valves Work?

Understanding the PCV System

PCV valves are an integral part of the positive crankcase ventilation system (PCV) system. It’s also known as the crankcase ventilation system for short. This system removes unwanted gases from the crankcase. These gases are a result of blow-by that occurs when fuel, oil, and exhaust vapors slip by the piston rings. All internal combustion engines produce blow-by gases.

The PCV systems goal is to re-route these gases to the intake tract where they mix with fresh intake air and go through the combustion process again. Re-routing blow-by gases helps reduce emissions. It was actually among the first emissions control devices with California requiring them in 1961. Within a few years most new cars in the US featured a PCV system. They then quickly became standard worldwide on automotive engines.

Part of its rapid implementation was due to emissions. However, PCV systems also help keep the engine internals clean by removing blow-by gases before they form sludge or carbon build-up. Additionally, the positive crankcase ventilation system keeps oil cleaner and increases oil life-span.

How the Valve Works

To understand how the PCV valve works it’s important to layout the basics of the crankcase ventilation system. Most engines use the following parts for the PCV system:

  • Crankcase Breather
  • PCV Valve
  • PCV Hose

Crankcase breathers start the process of removing unwanted blow-by gases from the engines crankcase. The breather and breather hose route fresh air into the crankcase. Many crankcase breathers pull fresh air directly from the intake manifold, but the location varies by design. Regardless, the idea is that you need fresh air to enter the crankcase as the blow-by gases are removed.

Then, the PCV valve applies intake manifold vacuum to the engines crankcase. Vacuum pulls the blow-by gases from the crankcase while the crankcase breather replaces it with fresh air. Crankcase gases exit through the valve, and travel through the positive crankcase ventilation hose back into the intake manifold. From there, the blow-by gases go through the combustion process again.

In essence, the PCV valve applies the necessary vacuum to remove blow-by gases from the crankcase. It actually controls the the flow – or the amount – of crankcase blow-by gases entering the intake. Nothing too complicated. However, it’s an incredibly important part that can cause a handful of issues if it fails.

Where it’s Located

The valve is typically located at the top of the engine around the valve cover(s). For V engines it may be on the back of either the driver or passenger side valve cover. Inline engines only use one valve cover, and the PCV valve is usually at the back. Anyways, look for a hose that runs from the valve cover to the intake manifold. That is typically the PCV hose and the valve will then be inside.

On almost all engines you can replace the positive crankcase vent valve without removing the valve cover. The hose should simply pop off and the valve can then come out. It’s usually designed to be a fairly easy, straight-forward repair. We’ll circle back to this in the topic about replacing the PCV.

Bad PCV Valve Symptoms

  • Seal & gasket failures
  • Oil leaks
  • Whistling sounds
  • Sludge build-up
  • High oil consumption
  • Misfires
  • Rough idle

When the valve fails you may have high crankcase pressures. That pressure will try to escape somewhere, which can cause seals or gaskets to fail due to the pressure. Oil leaks can then begin or oil may simply seep past the seals or gaskets even if they don’t totally fail. High pressures also cause whistling noises from the engine, which is one of the most common symptoms.

Sludge build-up is another concerning issue as the blow-by gases will stick to internals and for deposits. It’s an ugly sight and doesn’t help engine operation or performance. Another symptom of a bad PCV valve is high oil consumption. It can also reduce oil quality as blow-by gases mix with engine oil. Misfires and rough idle are less common, but do happen sometimes.

Replacing the PCV Valve

PCV valves are considered maintenance items. They are, after all, subject to a lot of blow-by gases traveling through them which can sludge the valve up. As such, most manufacturers make them easy to access and replace. Again, it’s often on the rear of the valve cover where you see a hose running to the intake manifold. The PCV valve will be inside that hose typically attached right at the connection to the valve cover. Some simply attach to the hose rather than lying inside.

Anyways, replacing the PCV valve on most engines is as simple as the following:

  1. Locate the valve
  2. Disconnect hose
  3. Twist PCV counter clockwise
  4. Check the hose, clamps, grommet, etc
    1. Replace if anything looks in poor shape
  5. Install new PCV valve

The valve usually runs about $10-30 and most people willing to DIY shouldn’t have any trouble replacing the PCV. Sometimes it can be a tight area to work, though. If you end up at a repair shop then PCV replacement can add about $50-100 of labor to the bill.

Testing Valve Flow

Fortunately, the positive crankcase ventilation valve is easy to access on most engines. That makes it easy to test its functionality. A few methods to check the health and operation of the PCV valve include:

  • Disconnect valve and visually inspect. If you notice a lot of slime/gunk/sludge then it’s probably a good time to replace the valve. Cleaning is an option, but it’s a $10-30 part so we think it typically makes sense to simply replace the PCV.
  • Warm up engine to operating temperature. Disconnect valve and cover the end of it with your finger (while the hose is still attached). You should feel the vacuum/suction from the end of the positive crankcase valve. Also visually inspect the valve while it’s off.
  • Remove the oil filler cap and put a piece of paper over the hole. With a functioning PCV valve the paper should be sucked against the hole. Ensure you have a stiff sheet of paper or take alternate steps to avoid the paper being pulled all the way into the oil filler opening.

Engine Oil Catch Cans

PCV System - Oil Catch Can - What is an Oil Catch Can?

The idea of the oil catch can (OCC) is to attach it to the PCV system. It then filters out the oil vapors to slow the process of carbon build-up on intake valves. Catch cans aren’t 100% effective, but they do have some benefits in at least slowing the process. We’ll leave it there for now and follow up with a more in-depth post regarding oil catch cans in the near future.

Frequently Asked Questions

What Does PCV stand for?

PCV stands for positive crankcase ventilation. It may also be known simply as crankcase ventilation.

What is the Purpose of a PCV System?

They were actually originally designed to allow military craft to travel through water to help ensure a tight seal on the engine. However, the purpose of a PCV today is to reduce harmful emissions. PCV systems also have the added benefit of improving oil life and keeping engine internals clean.

What are the Symptoms of a Bad PCV Valve?

There are a lot of potential symptoms of bad PCV valves. Seal or gasket failures, oil leaks, whining sounds, and higher oil consumption are a few of the most common symptoms. Refer to bad PCV valve symptoms section above for additional info.

How Expensive are PCV Valves?

A positive crankcase ventilation valves often costs about $10-30. It’s inexpensive maintenance, so don’t overlook replacing the valve.

How Do You Replace a Bad PCV Valve?

Replacement on most engines is very simple. Locate the hose at the back of the valve cover, remove the valve, and install the new one. Labor at a repair shop may be in the $50-100 ballpark on most engines.

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