Bad PCV Valve Symptoms and How to Test the PCV Valve Yourself

Updated on July 8, 2017
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Signs of a Bad PCV Valve

You probably don't realize how important the PCV system—the Positive Crankcase Ventilation (PCV) valve and its related components—is to the well-being of your engine.

A bad PCV valve or related component can produce a number of symptoms. For example, if the valve clogs, or gets stuck in the closed position, 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 air flow. And a diagnostic computer may erroneously blame this light on a mass air flow sensor or oxygen sensor instead, making it harder for you to detect the real source of the problem.

Why the PCV Valve Is Important

Bad PCV valves can cause engine oil contamination, sludge build-up, oil leaks, high fuel consumption, and other engine-damaging problems, depending on the type of failure.

Although you can detect some of these problems before they escalate with simple inspections, a failure of the PCV valve or related components often results in expensive repairs. That's because most car owners don'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.

Below in this article, we will discuss how car owners can test their own PCV valves.

But before we get to that, here's this whole article in a nutshell: What the PCV valve does, what happens when it fails, and how to test it.

PCV Valve Function in a Nutshell

What the PCV valve does:
• Uses the engine vacuum to pull blow-by gases out of the crankcase.
• Pushes 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 fail.
• The engine surges.
• The engine may produce black smoke.
• Internal engine pressure increases.
• Moisture and sludge build up inside the engine.
How to test it:
• Inspect rubber parts.
• Replace mesh filter beneath valve.
• Disconnect hoses and carefully inspect them.
• Remove valve and shake. If it does not rattle, it needs to be replaced.
A Toyota PCV valve
A Toyota PCV valve | Source

Understanding the PCV Valve

First, let's discuss the valve's function so that you understand better the reasons behind the symptoms. Understanding this will help you make better sense of the system when you inspect and test it.

Up until the late 1950s, car engines released "blow-by" gases—unburned fuel—to prevent engine damage. Problem was, these gases 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 quantity of the gas in the combustion chambers finds its 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 the engine's 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.

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

Another style of Toyota PCV valve
Another style of Toyota PCV valve | Source

PCV Maintenance

How often do you check the PCV system?

See results

How the PCV Valve Works

Inspecting Your PCV Valve

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.

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

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. 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 your 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 PCV valves contain 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. Even if the valve rattles, if 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 are misrecorded as coming from a bad sensor. That's why it's important to check the PCV valve and related components regularly. It just takes a few minutes. 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 at intervals 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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    • Dan Ferrell profile image

      Dan Ferrell 3 months ago

      Hi Silva

      If the PCV system is clogged, it may build pressure in the crankcase and push seals to release pressure. Check the system.

      Good luck

    • profile image

      Silva 3 months ago

      I recently replaced my 4L ford ranger rear main oil seal as i noticed some leaks coming between the engine block and transmission,and there was some sludge in the oil cap of the engine,but after some mile it started leaking again but a small amount,,,could this be the cause of the PCV valve?

    • Dan Ferrell profile image

      Dan Ferrell 4 months ago

      Hi SK,

      The way you describe the problem sounds like you had an oil leak somewhere, and the pressure made things blew up. On some models, the computer will cut off power if there's a loss of oil pressure to save your engine from self-destructing. You need to check where the leak originated. (was the valve properly installed?). There's where you'll find the problem.

      Good luck

    • profile image

      Scott king 4 months ago

      I have a 2000 Toyota avalon.check engine light came on and it was a bad oil control valve.when heat is on u smell oil.I don't drive it hard ,change oil .well. tonight I was doing about 60,65 and all of a sudden it started smoking,lots of smoke.I went to pull over but now power .never ran hot never made any sounds.I had windows down and radio ticking, nothing sounded like it broke and allot my antifreeze blew out with the oil.I would really appreciate it if you could give me your professional Thoughts of what might have happened and really seem that you know what and y things happen

    • Dan Ferrell profile image

      Dan Ferrell 5 months ago

      Hi Warren,

      If the valve or one of the connectors or hose, or pipe is leaking, most likely you'll get a misfire at idle. It'll be better to replace the valve, but make sure everything else is in good condition and properly connected.

    • profile image

      Warren 5 months ago

      Hi Dan

      I accidently broke the PCV on my car,1989 toyota 4fa, stupidly though. I have glued it just to hold it in one piece, i have put it back. Now the car is misfiring. I also did a test by closing the vacuum pipe. Idling come slightly back to norm. Whistle nose from valve. Is the valve the problem.

    • Dan Ferrell profile image

      Dan Ferrell 7 months ago

      Hi LD,

      Thanks a lot for sharing. Glad to hear it's fixed,

    • Lauwrice Degamo profile image

      Lauwrice Degamo 7 months ago

      Hi Dan,

      just want to share,,

      I finally solved the surging idle problem ( when engine is warm at operating temperature) of my 2008 corolla engine .

      It was a leaking oil pump which cannot deliver the proper oil pressure to activate the VVT-i system. it had a P0016 code Crankshaft-camshaft correlation.

      had to check the oil control valve, filter, timing chain, VVT then finally open the oil pump assembly which had internal damage which caused the leak.

      hope this can help other DIY mechanics.

    • Dan Ferrell profile image

      Dan Ferrell 7 months ago

      Hi MM,

      Have you tried getting the valve loose. Rotate the valve with a pair of pliers, use penetrant oil if necessary. And carefully dislodge the valve with a small screwdriver.

      Hope this helps

      Good luck

    • Dan Ferrell profile image

      Dan Ferrell 8 months ago

      Hi Mike

      Have you checked for coolant leaks? Does it smell like oil burning or an electrical wire burning?

      You can try having an assistant starting the car for you while you watch under the hood where the smell is coming from. You may be dealing with a slipping belt that gains traction a few seconds after the engine starts.

      You can also check the PVC valve yourself. Just remove it and shake it. It should rattle. if filled with gunk, clean the valve and any hoses, if necessary.

      Gook luck

    • profile image

      mike h 8 months ago

      my 2002 sport trac has a burning smell soon after you first crank it then it goes away. oil level is fine so it's not losing oil. friends say it's the pcv valve. not allowing oil to flow properly. any suggestions?

    • Dan Ferrell profile image

      Dan Ferrell 10 months ago

      Hi LD,

      Sometimes you can just blow through the hose to check for a blockage. I use a set of thin brushes to clean hoses and some types of valves. It works great in old Fords. If much of the roughenss is gone, you probably got it all clear. Check for codes once in a while and the proper maintenance. There's a lot of simple maintenance stuff you can do on your own and save you a lot.

      Good luck.

    • Lauwrice Degamo profile image

      Lauwrice Degamo 10 months ago

      yeah it's fine now, though there's a little roughness sometimes.

      by the way my car is a 2008 corolla and has DBW. in the case of the breather hose system, could it be blocked somewhere in the valve cover ? causing an imbalance in the air flow inside the valve cover? pcv valve won't shut off in idle when there is not enough air intake in the breather hose? .

    • profile image

      Greg 10 months ago

      Don't use lacquer thinner it's going to melt anything plastic the rubber diaphragm and probably the hoses

    • Dan Ferrell profile image

      Dan Ferrell 10 months ago

      Hi LD,

      Is the idle fine now? Also trace all the way the blowby gases have to follow from the valve cover back to the intake manifold. Make sure the passages are all clear. Another item you want to check is the idle air controlled (IAC) valve around the throttle body. The passages in this valve usually get filled with buildup and the engine begins to surge. And don't forget to scan for codes. Emission control sensors will usually trigger a code when something doesn't go right.

      Good luck.

    • Lauwrice Degamo profile image

      Lauwrice Degamo 10 months ago

      hi dan,

      i have replaced the pcv valve,(also tried cleaning the valve) but still the engine is surging at idle.

      i tried pulling the breather hose from the valve cover and the idle went fine. so i decided to put a breather filter in and blocked the breather hose form the air duct.

    • Dan Ferrell profile image

      Dan Ferrell 11 months ago

      Hi LD,

      Have you checked the valve? Some you can check; if it rattles, it is still good. Otherwise you may remove all the

      gunk with carburetor cleaner. Most valves are pretty cheap, so you can replace it if you have to. Check the hoses and connections as well.

      Good luck

    • Lauwrice Degamo profile image

      Lauwrice Degamo 11 months ago

      Having problem with surging idle when engine is warm. If i remove the pcv valve from the valve cover and have the engine running at idle speed the idle works fine, but when i reinstall the pcv valve the surging idle comes back.

    • Dan Ferrell profile image

      Dan Ferrell 13 months ago

      Hi donald,

      Yeah, they require some little maintenance once in a while, but it goes a long way. A bit of work or just replacing it as required can save your engine from demise. And it doesn't cost much.

      Thanks for commenting.

    • profile image

      donald 13 months ago

      my pcv valve was stoped up and it was pushing oil out pretty bad. put a new pcv valve in now its ok.203 chrysler town&country 3.8

    • Dan Ferrell profile image

      Dan Ferrell 16 months ago

      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.

    • profile image

      John C 16 months ago

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