Bad PCV Valve Symptoms and Tests

PCV Valve Function, Signs of Failure, and Tests

What the PCV valve does:
Use the engine vacuum to pull blow-by gases out of the crankcase.
Push the gases down the intake manifold and back into the combustion chambers where they are re-burned.
Some signs it's failing:
• One or more oil seals or gaskets failed
• Increased internal engine pressure.
• Engine surges and possibly black smoke.
• Moisture and sludge build-up inside the engine.
How to test it:
• Inspect rubber parts.
• Disconnect hoses and carefully inspect them.
• Replace mesh filter beneath valve.
• Remove valve and shake. If it does not rattle, it needs to be replaced.

Bad PCV valve symptoms range from engine oil contamination, sludge build-up, oil leaks, high fuel consumption, and other related conditions depending on the type of failure.

Although you can detect some of these problems before they escalate with simple inspections, a failed Positive Crankcase Ventilation (PCV) valve, or related components, often results in expensive repairs. That's because most car owners won't include the PCV system in their maintenance routine. Even though some car manufacturers suggest replacing the valve at regular intervals, car owners still forget to replace it. To add to the problem, not all manufacturers stress the importance of regular system inspections.

So let's look at the symptoms you would see if your PCV valve goes bad. But first, let's discuss the valve's function so that you understand better the reasons behind the symptoms. Understanding this help you make better sense of the system inspection and testing.

PCV Location

A common automobile PCV valve.
A common automobile PCV valve. | Source

What is the PCV Valve?

Up until the late 1950s, car engines released "blow-by" gases, or unburned fuel, to prevent engine damage. Problem was, they were harming the environment. Real bad.

When your car engine is running, an air-fuel mixture enters each cylinder. Hundreds of powerful explosions take place to release the fuel's energy, producing highly toxic and harmful gases. 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 (engine block) by way of pressure leakage between the piston rings and the cylinder wall.

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

In 1961, the PCV system was introduced to deal with this problem. This simple emission control system uses engine vacuum to pull blowby gases out of the crankcase, pushing them down the intake manifold and back into the combustion chambers where they are reburned.

Yet, the PCV system will fail with poor system or engine maintenance.

PCV System

Many PCV valves are found around the top or one side of the engine.
Many PCV valves are found around the top or one side of the engine. | Source

PCV Maintenance

How often do you check the PCV system?

  • Every other month
  • Every six months
  • Once a year
  • Never
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Signs of a Bad 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.

A bad 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

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 in air flow. 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.

How the PCV Valve Works

PCV Valve Check and Test

Unfortunately, many car manufactures are not strict about PCV system maintenance. 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.

To start checking the PCV system in your vehicle, first locate the PCV valve and its related components. 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.

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.

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. The aftermarket manual costs around $20 dollars, and it contains instructions for many simple maintenance tasks and repairs. Still, if you don't want to buy a copy right now, check the reference section of your local public library for the manual, or your library's website for access to an online shop manual.

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 PCV 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, and many car owners just replace it every service interval. Other valves incorporate heating elements and cost more. Regardless of the type of PCV valve you engine uses, always buy a quality brand valve, since 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.

Some vehicles—including some old Ford Escort models—come equipped with a small, hollow, plastic block with no moving parts. If you have this type of valve, just clean with lacquer thinner, if necessary, and reinstall.

Servicing the PCV Valve

Besides visually inspecting the condition of the different PCV valve and related components, test the system during engine operation.

1. Testing for Vacuum

  • Start the engine and let it idle for about twenty minutes to warm it up to operating temperature.
  • Then, open the hood and disconnect the valve from the valve cover and block the end of the valve with your finger. You'll feel vacuum from the system sucking at your fingertip and notice a momentary idle speed drop of about 40 to 80 rpm.
  • If you notice a bigger rpm drop and the engine idle smooths out, your PCV valve might be stuck open.
  • If you don't feel vacuum at your fingertip, check the valve and hoses for gunk obstructing air flow. Clean the PCV valve and hoses with lacquer thinner and a thin hose brush, if necessary.

2. Alternative Tests

  • 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.

Maintaining the PCV System

Sometimes, bad PCV valve symptoms come under the disguise of a bad sensor. That's why it's important to check the PCV valve and related components regularly. It just takes a few minutes. However, if your engine lacks a PCV valve, or you can't reach it without removing one or more components, consult your workshop manual for the best way to check your particular system. Also, check the service schedule for your PCV system and replace the valve even if it seems to be in good condition. Most PCV valves and related components are inexpensive and will save you money in costly repairs if you replace them at the suggested interval.

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John C 5 weeks ago

Can I route the hoses from each valve cover (one with pcs valve, one without) to a single vacuum port?

Dan Ferrell profile image

Dan Ferrell 4 weeks ago Author

Hi John

It seems to vary (model, engine, and hose size) and possibly wether you are connecting to manifold or ported.

Check to see if you can find those vacuum tees with the correct size for your application.

Good luck.

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