Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.
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.
In this article, we will discuss how car owners can test their own PCV valves. Before we get to that, here's this article in a nutshell:
- What happens when the PCV valve fails,
- what the PCV valve does, 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.
What Happens When a PCV Valve Fails?
Symptoms of a Stuck Closed PCV Valve or System
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 of these symptoms.
- Increase in internal engine pressure
- Failure of one or more oil seals or gaskets
- Engine oil leaks
- Low whistling or moaning noise
- Moisture and sludge buildup inside the engine
- Engine surges
- MAF sensor trouble code
- P0171 or P0174 trouble codes
- 02 sensor trouble codes
Symptoms of a Stuck Open PCV Valve
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
- Possibly black smoke
- Oil fouled spark plugs
A PCV valve stuck open can trigger the "check engine" light due to increased airflow. A diagnostic computer may erroneously blame this light on a mass airflow sensor or oxygen sensor instead, making it harder for you to detect the real source of the problem.
What Does a PCV Valve Do?
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.
The History and Function of the PCV Valve
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. Really badly.
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.
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.
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How to Test 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.
1. Locate the PCV Valve
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, 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.
2. Check the PCV Valve System
Luckily, once you locate the valve, it doesn't take much to check the system.
- 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.
- 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.
- 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.
- 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.
- 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.
- If your PCV valve is electrically heated, check the heater coil using an ohmmeter. The coil should have some resistance. If resistance is infinite, the coil is open and the valve needs to be replaced. Consult your vehicle repair manual, if necessary.
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.
3. Alternative Testing a PCV Valve During Engine Operation
Besides visually inspecting the condition of the different PCV valve-related components, test the system during engine operation. These tests can be especially helpful if your PCV valve is difficult to access or you are looking for further evidence that the PCV valve is the cause of your issue.
Ways to Test for Vacuum Using RPMs
- 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 a vacuum from the system sucking at your fingertip and notice a momentary idle speed drop of about 40 to 80 RPMs.
- 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 RPMs and then rise back to normal. If not, look for a blocked or restricted vacuum hose or valve.
- 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 airflow. Clean the PCV valve and hoses with lacquer thinner and a thin hose brush, if necessary.
Alternative Test (Without Using the PCV 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.
4. Finding Moisture in the PCV Valve or System
Once you remove the PCV valve, you may find traces of a milky brown fluid in the PCV valve or hoses when examining the system. This doesn't necessarily mean there's a problem with the valve or PCV system. A milky substance usually means moisture in the oil.
The accumulation of moisture in the oil is usually caused by:
- frequent short trips (usually under 10 miles)
- a restricted fresh air flow in the PCV system
- coolant leaking into the engine
In the first case, you can solve the problem by replacing the engine oil more frequently; since short trips do not allow the engine and oil to reach operating temperature, moisture doesn't have a chance to vaporize and clear out.
If you drive your vehicle long distances frequently enough, usually over 20 minutes or more and at highway speeds, and find moisture in the system, then you may have a coolant leak issue. If you suspect a coolant issue, you may want to bring your car to the shop for a diagnostic.
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. This is especially true if you own a 2001 or older model, where the PCV system is not monitored by the OBD-II system, and won't illuminate the check engine light if something goes wrong.
It just takes a few minutes to check the system. 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 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.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
Questions & Answers
Question: Could a bad PCV affect engine start?
Answer: It can certainly make it hard to start; if it sticks open it may lead to a no start.
Question: Will a bad PCV cause my truck to stall when running and accelerating?
Answer: If it sticks open it's possible.
Question: Is it true that if you suck on PCV valve and air comes in, it's defective?
Answer: Most are designed as a check valve to prevent the valve from flowing back, for example, during a back-fire.
Question: Can a bad PVC valve cause blue smoke to come out of my exhaust?
Answer: Yes it can.
Question: Should you be able to pass air flow in both directions in a PCV valve for testing purposes?
Answer: Usually, a PCV valve is normally closed when the engine is not running; as the engine accelerates, the valve opens to allow blowby gases. So you shouldn't be able to pass air flow in both directions. However, some models like the ‘87 Escort, use a fully open valve design.
Question: Could a bad PCV valve cause my car to use a lot of oil?
Answer: Check if the PCV valve is stuck and check for bad hoses.
Question: Can an intake manifold be damaged by a faulty PCV valve?
Answer: If the valve fails, pressure can cause acid vapor through the breather element. It'll combine with the air-fuel in the intake; after some time you may see buildup.
Question: I replaced a new crankcase on my vehicle but now there is a noise. What is it?
Answer: Carefully look under the car with the engine idling an take a look. Check what side of the engine the noise is coming from. You'll get an idea what's going on.
Question: Will a disconnected PCV valve cause my truck to misfire?
Answer: Yes, it's possible.
Question: I just got a new starter and intake manifold gasket repaired. After a month. service engine light came on with a brake light, car on top of lift, and start is now slow and shaky. Should the PVC value been looked at during this repair?
Answer: I think this has to do with a brake pad sensor. You need to have it looked at, and have the warning light reset, after service. Some shops would include a quick inspection to related items when doing this work because of wear or poor maintenance of systems like this (are not in most car owners' minds as other systems are). Then they would advise the customer to the need for service.
Question: I have a 2012 Beetle with that amount of oil and Volkswagen says I need a new PVC valve at a crazy price. Is it hard to replace a PVC valve?
Answer: It shouldn't be that difficult. This video gives you an idea about the replacement.
Question: Will a bad PCV valve cause a miss? Number 5 cylinder is not firing.
Answer: Yes, it is possible. Misfires are usually tricky to diagnose. If you got a trouble code pointing to cylinder #5, the job could be easier. Check for ignition coil, spark plug, wire, fuel injector as well.
Question: My 2007 Taurus has a heated PCV. What is the wire connection called?
Answer: That's a heated PCV valve. Some Vulcans came with it to prevent cold weather from freezing the valve.
Question: I am currently in the process of having blower motor wiring and PCV valve heater recall fixes done. It’s on a 2007 BMW 328xi Wagon. My valve cover gaskets are also leaking slightly? Are these related?
Answer: If the PCV system was clogged, it may have caused pressure to increase and the gaskets began to leak.
Question: How often must I change my PCV valve?
Answer: You can check your PCV valve every few months. Change it if it's filled with sludge or if it's stuck.
Question: Will a check engine light come on for a misfire?
Answer: The P0300 (random misfire) and the P030x series codes (specific-cylinder misfire codes) point to this type of issues.
Question: I have a 2017 Subaru Impreza, which has just bunny hopped violently, then blown a lot of smoke out the rear. My Subaru dealer has found the center is missing from the PCV Valve. A replacement part is on order. If that fixes the problem, what other issues could this have caused? Where could the missing center part have gone? And what further repairs/checks should I make sure are done if replacing the PCV valve fixes the issue? It is still under warranty.
Answer: Probably what you heard is the releasing of pressure building up in the crankcase, the lower part of the engine. This happens when the PCV valve or part of the system clogs. This type of fault is likely to blow oil seals. So make sure to keep an eye on the oil level and check the front and back of the engine for potential leaks if you see a drop in oil level. If you've never serviced the PCV system, it'll be a good idea to check the engine for sludge buildup. Internally, this is a very corrosive substance that can damage internal components.
Question: Would a bad PCV Valve be a reason for errors when I fill up my tank and attempt to start my car? It does not start, but on the second attempt, if I pump my accelerator while turning the ignition it will start. But there would be a large white-grayish cloud from the exhaust.
Answer: There could be a problem in the evaporative control (EVAP) system. If there’s a leak in the system, fuel vapors may be pushed into the intake. Sometimes, removing the gas cap when trying to start the engine with a full or almost full tank may help. But have the system checked, if necessary.
Question: I have 2009 Honda Civic LX-S and I see oil around my PCV valve. What should I do?
Answer: First make sure there's a good connection; there could be too much internal pressure, or a problem with the seals or leaks. Go over the post, you'll get an idea about checking the valve, if necessary.
Question: Could faulty pcv valve cause blue smoke from exhaust only at idle ?
Answer: There could be a restriction in the PCV system creating pressure in the crankcase. This may push oil into the combustion chamber and burn the oil, which you'll see as blue smoke.
Question: Could a disconnected breather hose to a K&N air intake cause a rough idle and misfire with code #P0272?
Answer: It can lead to a misfire and rough idle, but the P0272 in general points to a cylinder number 4 misfire and cause a rough idle. You can check for the cylinder misfiring by unplugging one cylinder spark plug wire at a time, if possible.
Question: will a bad PCV valve cause my car to vibrate?
Answer: I don't think a bad PCV valve itself will cause the vibration, but it can lead to other issues that may cause the engine to vibrate. You may want to check this post:
Question: Would a PCV valve not working properly cause something like a high-pitched vacuum sound? Would it also be why I get "check gauges" warning in the dash?
Answer: Usually when you see the "Check Gauges" light, one or more of the gauges is indicating a system has gone out of its normal operating parameters.
This may be related to the sound you hear, maybe not. When this high-pitch (whistle) sounds comes with the engine cold, often on deceleration, it often points to an intake vacuum leak. It may disappear after the engine has warmed up. This could be a faulty gasket. The sound may come from the dipstick not fully seated, and the whistling sound will travel through the PCV system.
Check other vacuum hoses and gaskets, if necessary.
Question: Where is PCV valve located in a Chevrolet Beat 2013?
Answer: I don't have the manual, but I hope this video helps:
Question: Would a bad PCV cause a dipstick to blow completely out of the oil tube?
Answer: If the PCV system is plugged, pressure in the crankcase will build and may cause a dipstick to blow.
Question: Does Toyota have a PCV valve?
Answer: Yes it does. If you need to find the PCV valve in your engine, you can search for it in Youtube. But buying the manual for your particular model is better. Check your local auto parts store or Amazon, online. Most likely you’ll be able to replace it yourself, if you need to change it.
Question: Can i clean a heated pcv valve?
Answer: You may be able to clean the valve if you don't damage the electrical circuit or heater. You may want to try electrical contact cleaner, if necessary.
Question: I have a 2008 Mini One 1.4 petrol. The engine idle is rough and there appear to be excessive exhaust fumes through the oil cap when removed, could this be due to a blocked PCV?
Answer: Check the PCV, breather and hoses in the system, make sure they're not clogged.
Question: My Chevy Cruze 2010 1.6 is misfiring when the engine is getting warm and during acceleration. When stooping in traffic lights, it gets worst. It feels like the engine is throwing out. What could be the cause?
Answer: Take a look at this post. This may help you:
Question: What kind of damage can result from a bad PCV valve?
Answer: You'll see sludge buildup that will slowly destroy internal engine components, and oil leaks from damaged seals.
Question: Can a stuck/closed PCV valve cause oil to leak into the intake manifold?
Answer: This is rare, but it may happen. Usually, oil leaks will get stuck in the air filter. As crankcase pressure builds with a stuck closed PCV valve, the oil may leak through seals and gaskets, oil travels up the clean air hose and to the air filter. If the oil passes through the filter, it may find its way to the intake.
Question: Why do I have fuel coming from my PCV valve?
Answer: If you smell gasoline in the oil or fumes going through the valve, possibly the piston rings are too worn and letting fuel pass through. This may happen in a rich mixture condition.
Question: Can a bad PCV valve cause my 03 Ram 1500 5.7 Hemi to keep throwing a p0440 code? I've replaced all the EVAP systems and hoses but still, I keep getting this code. I've also changed the gas cap and nothing changed.
Answer: You have a leak somewhere. The PCV won't throw that code. Check the filler neck and fuel vapor pressure sensor.
Question: Where is the pcv valve of Nissan Sunny 1991?
Answer: Look in one of the hoses that attach to the valve cover
Question: Would a bad valve cause a smell in the car, like an irritant that burns eyes and nose and such?
Answer: You might be getting oil fumes inside the cab. Check the valve and system for a restriction. Some valves connect to a breather filter. Check the filter and change it, if necessary. Hope this helps.
Question: A PCV valve heater, what’s the difference in PCV valve and PCV valve heater?
Answer: Basically, the PCV system is used to draw fresh air into the crankcase through the air filter or a separate PCV breather filter. On some models, a heated PCV system is used to help warm the moisture (icy) contained in the vapors absorbed through the PCV valve in cold weather. It speeds up operation of the valve to prevent excess pressure buildup in the crankcase.
Question: Can a bad PCV valve cause braking problems? There is a vacuum hose on the brake booster.
Answer: The PCV won't cause braking problems. Check the valve and hose to the booster. Make sure they are well connected. This post may help:
Question: Can a bad PCV valve cause bad vacuum at idle?
Answer: This other post may help:
Question: Can a bad PCV cause no vacuum in the crankcase?
Answer: A stuck PCV can reduce vacuum in the crankcase (actually pressure from blowby). Check also for leaking hoses in the PCV system. If you need to check the valve, this other post may help:
Question: Where’s the POV valve on a Camry v6 2008?
Answer: This video might help you out:
Question: I have a 2004 Mazda MPV. The climate control heating doesn’t work in the front, but the rear heating does from the rear controls. Do you have any ideas?
Answer: There could be a problem with the heater control valve or the controls, not letting the valve opening all the way.
Question: Does a PCV valve in a 2008 Jeep Wrangler unlimited with a 3.1 v6 flow in both directions?
Answer: I believe it flows in one direction since it needs to remove harmful blowby gases out of the crankcase.
Question: Should I replace the hose if it is hard and not easy to squeeze?
Answer: Yes. Rubber hoses become hard and brittle from the high temperatures in the engine compartment. Eventually, they break and leak.
Question: I have a 2008 BMW X5 3.0si. It doesn't Start and doesn't crank. What do you think it could be?
Answer: Make sure your battery is properly charged. Then check the starter system circuit and, if necessary, the starter motor itself. This other post can help you check the circuit:
Question: I have an Audi B7 1.8T. It has rough idle, no revs. Engine light and EPC diagnosis results show pedal or throttle body and airflow. Can a PCV valve cause this error?
Answer: The problem is more likely with the throttle body (carbon buildup [airflow obstruction? mechanical biding?), as the diagnostic revealed. The PCV only routes crankcase gases into the intake to reburn. It's possible the vehicle is in "limp mode", hence no revs.
Question: Will a bad PCV valve cause a rear main seal to leak oil?
Answer: If the valve or the system is obstructed, it'll increase pressure in the crankcase and eventually damage one or both seals.
Question: Will I have a faulty PCV valve if I have residue around oil cap? Its a 2009 Holden Commodore VE V6. Valve might be stuck & now has contaminated oil (since replaced). So now could be a seal or gasket problem.
Answer: If the PCV valve or a hose or the system was clogged, it can cause pressure to rise, leading to seal failure. Has the system diagnosed, if necessary, before sludge and moisture cause more damage?
Question: Is it possible to change 2005 Mazda Tribute v6 valve cover gaskets without disconnecting the battery?
Answer: It may be possible, but it’s better to play it safe. You can use a memory saver. Or make one with a 12V dry cell battery. Use a couple of jumper wires and an inline 5 amp fuse in the positive side. Connect ground to the engine and power to the alternator battery terminal. That way you’ll have more room to work if you need it.
Question: Where is the PCV valve on a 2010 Daihatsu Mira manual?
Answer: There could be two possible locations for this model. Look at the valve cover, passenger side, towards the firewall. There should be a hose that connects to the cover (1/2 an inch hose). The valve plugs into a grommet on the valve. Another possible location is under the throttle body. I believe is attached to two rubber hoses.
Question: How can it be the brake pads when the error codes came back Diagnosing Trouble Codes P0171 and P0174. Another code regarding the crankshaft? Also, a warning light that looks like a faucet came on, and the car almost stops on me. Is this trouble for a 2008 BMW 328I? Is it called the valve cover instead of PCV Valve?
Answer: The faucet-like warning light is the check engine light. You may have more than one problem going on. When the P0171-4 codes appear, it means the car's computer is unable to make proper adjustments to control the air-fuel mixture, thus the car almost stopped. Too much fuel is going through the engine. The PCV valve is different than the valve cover. The PCV valve most likely connects to the valve cover on the engine. The cover is on top of the engine and serves as a "lid" for the engine valves. In most engines, the engine oil fill cap is located on a valve cover.