PCV Valve – How Does It Work – Failure Signs – How To Test It

Positive Crankcase Ventilation (PCV) System - What Does It Do
Positive Crankcase Ventilation (PCV) System - What Does It Do

Knowing how the PCV valve actually works, is the first step in diagnosing a bad or failing one.

When your engine is running, thousands of powerful explosions take place; to release the fuel energy, producing highly toxic and harmful gases.

So, this simple emission control system, uses engine vacuum to pull blow-by gases, out of the crankcase; pushing them down the intake manifold and back into the combustion chambers, where they are reburned.

As a result, in 1961, the PCV valve system was introduced, to deal with this problem.

The PCV Valve System
The PCV Valve System

So, after each combustion process, the exhaust valve routes these gases into the exhaust system; where the catalytic converter turns them into much less toxic fumes, before releasing them into the atmosphere.

Still, a small amount of the gases in the combustion chambers, find their way into the crankcase. Because, of pressure leakage, between the piston rings and the cylinder wall.

So, if left on their own, these vapors and fumes, will play havoc with your engine. Because, blow-by gases contain; hydrocarbons (unburned fuel), carbon monoxide (partially burned fuel), particulates, water, sulfur, and acid.

Also, together, these substances will corrode any engine metal component they touch; dilute engine oil, build up harmful sludge that accelerates parts wear; and plug small passages and hoses.

Signs Of A Failing PCV Valve

You don’t realize how important the PCV system is, to the well being of your engine; until you understand how a failed PCV valve; or any part of this system; disrupts engine performance and internal components.

Basic Operation At Different Engine Conditions
Basic Operation At Different Engine Conditions

So, a failed PCV valve or related component, can produce a number of symptoms. For example, if the valve gets stuck in the closed position or clogs; you’ll notice one or more of these symptoms:

  • Increase, in internal engine pressure.
  • Failure of one or more, oil seals or gaskets.
  • Engine oil leaks.
  • Moisture and sludge buildup, inside the engine.
  • Engine surges and possibly black smoke.

But, if the PCV valve gets stuck open, or a system hose gets disconnected or ruptured; producing a vacuum leak; you’ll notice one or more of these symptoms:

  • Engine misfires, at idle.
  • Lean air-fuel mixture.
  • Presence of engine oil, in PCV valve or hose.
  • Increased, oil consumption.
  • Hard, engine start.
  • Rough, engine idle.

In addition, a PCV valve stuck open, can trigger the “check engine” light, due to increased air flow.

A PCV Valve Stuck Open Can Trigger The "Check Engine" Light
A PCV Valve Stuck Open Can Trigger The “Check Engine” Light

However, a diagnostic computer, may point to a Mass Air Flow or Oxygen Sensor failure instead; making it harder for you to detect, the real source of the problem.

PCV Valve Testing

Unfortunately, many car manufacturers are not strict, about PCV system maintenance. As a result, some suggest servicing the system, every 20,000 or 50,000 miles. However, a more frequent system inspection; helps prevent costly repairs and keep the engine running smoothly.

Locate The PCV Valve
Locate The PCV Valve

So, to start checking the PCV system in your vehicle; first locate the PCV valve and its related components. Also, depending on your particular model; you may find the valve on a rubber grommet on the valve cover; on a breather opening around the intake manifold; or to one side of the engine block.

Also, keep in mind that some new models, don’t have a PCV valve at all. Instead, you’ll find a simple vacuum hose; going from the valve cover to an air inlet duct. Others may have a simple restrictor in place. Still, you can check the restrictor, hoses and other components.

So, if you are not familiar with, the PCV system in your vehicle; or can’t find the valve; buy the service manual for your particular vehicle make and model, from a local auto parts store.

PCV Valve Flow Operation
PCV Valve Flow Operation

Luckily, It Doesn’t Take Much To Check The System:

  1. Check PCV system parts. Rubber components like grommets; O-rings, and hoses swell and turn hard and brittle; after constant exposure to high temperatures. They begin to leak. Replace one or more of these components, as necessary.
  2. Carefully disconnect the valve and any system hoses and visually inspect them. If you find the hoses filled with slime; clean them with solvent or lacquer thinner and replace the valve. Or, simply replace those components, along with the PCV valve.
  3. Many engine models use a simple, inexpensive valve. As a result, many car owners just replace it, every service interval. Other valves incorporate heating elements and cost more. Regardless of the type of PCV valve your engine uses; always buy a quality brand valve. Because, it’s more likely to have a more precise calibration, for your specific engine model.
  4. On some engines, you’ll find a mesh filter, underneath the valve. Some car manufacturers recommend, replacing the filter, every 30,000 miles or so.
  5. Most vehicles come equipped with a valve, that is nothing more than, a spring-loaded device. Once you remove the valve, shake it with your hand. You’ll hear a rattle. If you don’t, it is time to replace the valve. However, if the valve rattles; and your engine is experiencing, one or more of the bad PCV valve symptoms described above; it’s a good idea to replace the valve.

Besides visually inspecting the condition of the PCV valve and related components; you should also, test the system during engine operation.

Testing Your PCV Valve, For Vacuum

1- Start the engine and let it idle, for about twenty minutes, to warm it up to operating temperature.

2- Then, open the hood and disconnect the valve from the valve cover. Then, block the end of the valve, with your finger.

Block The End Of The Valve With Your Finger
Block The End Of The Valve With Your Finger

3- You’ll feel vacuum from the system, sucking at your fingertip. And, you should notice a momentary idle speed drop, of about 40 to 80 rpm.

4- If you notice a bigger rpm drop and the engine idle smooths out; your (PCV) valve might be stuck open.

5- If you don’t feel vacuum at your fingertip; check the valve and hoses for gunk obstructing air flow.

6- Finally, clean the PCV valve and hoses, with lacquer thinner and a thin hose brush, if necessary.

Alternative, PCV Valve Testing Methods

  • Another way to test for vacuum is, to pinch or block the vacuum hose, connected to the PCV valve. Idle speed will drop, between 40 to 80 rpm, and then rise back to normal. If not, look for a blocked or restricted vacuum hose or valve.
  • On some engines, access to the PCV valve is difficult. In these models, you can remove the engine oil dipstick. And, seal the dipstick tube opening, with a piece of tape.
  • With the engine at idle, remove the cap from the oil filler on the valve cover. Then place a thin piece of cardboard over the opening.
  • Wait for about one minute. You’ll notice vacuum, suctioning and holding the paper against the opening. Otherwise, there’s a leak in the system, or the system is clogged. Check the condition of the hoses, hose connections and grommet.
Diagnostic Chart
Diagnostic Chart


Sometimes, bad PCV valve symptoms, come under the disguise, of a bad sensor. Hence, that’s why it’s important, to check the PCV valve and related components regularly.

Finally, most PCV valves and related components, are inexpensive and will save you money in costly repairs; if you replace them at the suggested interval.

Thank You !