Blog Car Care Advice 6 Bad PCV Valve Symptoms to Look Out For (+ How to Test It)
Car Care Advice

6 Bad PCV Valve Symptoms to Look Out For (+ How to Test It)

Looking for a mechanic near you for maintenance or repair? AutoNation Mobile Service brings the shop to you. Get a free instant quote today.
Get a Quote

The positive crankcase ventilation valve (PCV valve) may seem like a small component, but it plays a significant role in maintaining your engine. 

It routes the crankcase vapor, which contains traces of unburned fuel, back into the intake manifold to be burned in the combustion chamber. Simply put, it helps increase fuel economy by recycling unburned fuel back into the combustion chamber. 

So, how can you know when this compact device has stopped doing its job?
We’ll walk you through how to spot a failing PCV valve and how you can check on it!

This Article Contains

Let’s get to it!

6 Bad PCV Valve Symptoms to Watch Out For

Sludge, dirt, and soot from fuel combustion can accumulate inside the PCV valve and eventually block it, causing the bad PCV valve to get stuck open or closed. This prevents the crankcase gases from flowing back into the intake manifold.

Here are some signs of PCV valve failure:

1. Noises from the Engine

The first common sign of a failed PCV valve is a whistling or hissing sound from the engine. This happens because the PCV valve has a slight blockage, leaving it stuck open. Although the noises would eventually disappear as you continue driving, it’s always best to get it checked out. Ignoring it for too long would lead to further engine issues.

2. Blue Exhaust Gas

If the positive crankcase ventilation valve is stuck open, you may notice blue smoke from the exhaust pipe. That’s because a stuck valve forces engine oil vapors to mix along with the fuel and excess crankcase gases during combustion. This causes the engine to burn the air-fuel mixture along with engine oil. Hence, the exhaust pipe releases blue smoke. Sometimes, the exhaust gas can appear white or black instead of blue.

3. Rough Idling or Engine Misfiring

Another common faulty PCV valve symptom is rough idling and engine misfiring. If the bad PCV valve is stuck closed or clogged, it causes less air to enter the intake manifold and the engine to undergo rough idling as the idle RPM would get too high. You may also notice rough acceleration on higher RPMs.

Since the valve controls the flow of blow-by crankcase vapors into the intake manifold, the issues it may present are similar to those of an intake vacuum leak.

4. Illuminated Check Engine Light

A clogged PCV valve can cause the Engine Control Unit (ECU) to trigger the check engine light and produce a DTC code.

Some common trouble codes often associated with a faulty PCV valve include:

5. Oil Leaks

When the PCV valve is stuck closed, the blow-by gases and oil vapor accumulate, and the crankcase pressure builds up. This could lead to oil seeping out of the PCV system seals and valve cover. So, if you see puddles of oil underneath the car and there isn’t any damage to the oil system, you can safely assume that the oil leak is stemming from the PCV system or valve cover. If the oil leak is left unattended, you could end up with fouled spark plugs and excessive oil consumption. 

Note: Sometimes, a bad PCV valve could manifest as excessive suction on the oil cap. You may also notice a film-like residue on the underside of the oil cap.

6. Poor Engine Performance

Poor engine performance can also be a faulty PCV valve symptom. Since your car’s PCV valve is stuck, it can throw off the air-fuel mixture ratio, and your engine would burn lean fuel instead.

That’s because the vacuum region in the intake manifold wouldn’t supply the right amount of air into the combustion chamber. When the air-fuel ratio is off, your engine’s fuel efficiency will reduce, leading to poor fuel economy, and the engine performance will drop. 

Now you know the symptoms of a failing PCV valve. 
But how do you confirm that these bad PCV valve symptoms are indeed caused by one and not some other issue?

Read along to know how.

How to Diagnose a Bad PCV Valve

Here’s how a mechanic will diagnose a failed PCV valve at the workshop:

  1. First, they’ll inspect the PCV valve grommets for cracks or splits. Since they’re made of rubber, these grommets can crack the longer they get exposed to the engine heat and cause dirt and dust to enter the valve.
  1. If the grommets are right, they’ll check the PCV valve hoses and look for any swelling or cracks along them. The vacuum hose can degrade and harden over time and eventually form cracks, giving way to a vacuum leak. They’d also look at the seals and valve cover to ensure they’re in top condition and aren’t oil-soaked.
  1. The mechanic will inspect the PCV valve if the hoses and their seals are perfectly fine. They’ll remove the valve from the cover grommet and disconnect the vacuum hose. Testing it is pretty straightforward — they have to shake the valve and listen for any rattling noises. 
    • A perfectly fine valve would produce a rattling sound when shaken, indicating that it’s functional. 
    • A faulty PCV valve wouldn’t produce any sound, meaning that something is causing the valve to get stuck, and they’ll have to clean it.
  1. While checking the defective PCV valve for blocks, the mechanic will also observe the inner lining for any signs of moisture contamination — a cloudy or milky film inside the valve or hose. If the valve is contaminated, they’ll need to conduct a PCV valve replacement and also an oil change. 

Because moisture would only enter the system if there’s contaminated oil consumption.

Now, let’s look at some frequently asked questions about the PCV valve.

5 FAQs on the PCV Valve

Here are the answers to five common questions on the PCV valve:

1. How Does the PCV System Work?

As the engine runs, the pistons move up and down inside the crankcase. This movement creates a vacuum pressure, which the PCV system uses to pull the crankcase gases into the air intake valve. The oil vapor then gets mixed with the fuel-air mixture to burn in the combustion chamber.

When the engine is idle (or at low RPMs), the vacuum in the intake manifold increases, causing the PCV valve to close. When the engine is at higher RPMs or accelerating, the crankcase pressure drops and causes the spring inside the PCV valve to open the valve. Thus sucking out all the crankcase gases from the cylinder.

If a backfire occurs inside the intake manifold, the PCV valve will close shut to prevent a vacuum leak and potential engine misfires.

2. Can You Drive With a Malfunctioning PCV Valve?

No. Driving with a defective PCV valve is not advisable.

Even though you can drive for a couple of miles, driving longer distances could cause further engine damage (like rough acceleration, blown-out gaskets, and excessive oil consumption due to severe oil leaks).

3. Can You Clean the PCV Valve?

Yes, but it isn’t recommended. 

Though cleaning a clogged PCV valve with WD-40, a brake cleaner, or an electrical contact cleaner will work, it’s always better to get a PCV valve replacement. Sometimes, the valve may appear clean, but the springs inside might be too old and worn out without you noticing.

4. How Long Does the PCV Valve Last?

Most manufacturers suggest replacing the PCV valve every 20,000 miles to 50,000 miles

However, this range may change depending on your driving habits. If you drive short distances every time, there’s a chance that you’ll encounter a PCV valve failure earlier due to excessive moisture buildup compared to constantly driving long distances. 

5. How to Maintain and Prolong PCV Valve Life?

One way to make your PCV valve last longer is to always be on time for your car’s oil change and air filter replacement. This helps remove potential build-up from blocking the valve, as engine oil can accumulate dirt and sludge, while the air filter can get clogged by dust over time.

Final Thoughts

Understanding the symptoms of a malfunctioning PCV valve is crucial for maintaining optimal engine performance. Signs like an active engine light, excessive oil consumption, or reduced fuel efficiency can help you identify and address issues promptly and avoid engine damage.

To play it safe, it’s best to get your PCV valve or any related component inspected and serviced by a professional — like AutoNation Mobile Service!

AutoNation Mobile Service is a mobile auto repair service you can book online. Our professional services come with a 12-month|12,000-mile warranty.

Contact us today to have your PCV valve looked at, and we’ll send our trained technician to your driveway!