PCV Valve: Signs Of A Bad PCV Valve and How to Test It | Autance

The PCV valve, otherwise known as the positive crankshaft ventilation valve, is an integral part of the system that regulates…

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PCV Valve: Signs Of A Bad PCV Valve and How to Test It | Autance © PCV Valve: Signs Of A Bad PCV Valve and How to Test It | Autance

The PCV valve, otherwise known as the positive crankshaft ventilation valve, is an integral part of the system that regulates pressure and airflow in your engine. This small and often overlooked component improves performance and reduces the risk of sludge buildup and oil leaks. However, when the PCV stops working effectively, it can cause a host of different issues. Knowing the early signs of a bad PCV valve and how to test the valve can save you money and stop further damage to your engine.

How Does the PCV Valve System Work?

Pressure is created when the moving parts of the engine churn the oil that lubricates the automotive engine. Valve guides and piston rings also produce pressure in the crankshaft due to slight leakage during their operation. Regulating this pressure is just one of the tasks of the PCV system.

The system comprises of an inlet hose that connects to the filtered air source, providing clean air that is drawn through the engine. The air picks up vapors and fumes as it flows through the engine. It then exits through a second hose that is connected to the manifold vacuum. As the air flows through the system, it draws fumes from the crankshaft, allowing them to be burnt harmlessly in the engine. A slight vacuum is created during this process, which relieves any building pressure. The PCV valve helps to regulate the airflow, preventing oil from being drawn out of the engine.

Additionally, the PCV system removes moisture from the oil. Heat is generated as the engine runs. When it cools, condensation forms, the resulting moisture is absorbed by additives in the engine oil and is held in suspension. Over time the amount of moisture exceeds the additives capacity, at which point metal parts of the engine are attacked by the moisture, leading to potentially extensive damage. Finding a milky or cloudy film in the PCV hose or valve is a clear indication that there is moisture in the system.

Signs of a Bad PCV Valve

The cloudy or milky film mentioned above is one sign that your PCV valve needs replacing. However, this is also an indication of other problems. If there is moisture in the system, then replacing the valve alone won’t fix the problem, you also need to change the oil. Those that use their vehicle for regular short trips, under ten miles, need to change their oil more regularly than those who undertake longer trips. This is due to the engine not reaching full temperature during its use and, therefore, not being able to boil off and be scavenged by the PCV system.

A buildup of sludge in the engine and worsening of oil leaks are also signs of a bad PCV valve. However, there are other early signs that the PC valve is not working effectively. Noise is one symptom to look for. A failed PCV valve may produce a low moaning, whining, or whistling sound. The source of the noise can be tested by blocking the vacuum source to the valve and listening for changes to the noise.

A bad PCV valve, can in some vehicles, cause oil to blow onto the air filter element. If you see a dirty or oily spot near to the PCV inlet hose, then this is an indication of a bad PCV valve. If the PCV valve sticks open, it may cause the check engine light to come on. Diagnostic codes P0171 and P0174 are common when there is a problem with the PCV valve. Other codes to watch for include the oxygen sensor code and the mass-air-flow meter code.

If the PCV valve is failing or has failed, you may also notice an increase in internal engine pressure, black smoke, or surges in the engine. However, of the PCV valve is stuck open, then you may notice different signs. When the PCV is stuck open, or there is a disconnection or rupture in a system hose, a vacuum leak is created. In these cases, you may experience the engine misfiring while it is idling, a lean fuel to air mix or increased oil consumption. You may also see engine oil in the PCV valve or the hose. A stuck open PCV valve can also trigger the check engine warning light. The diagnostic computer may lay the blame at the oxygen sensor or the mass airflow sensor because of the airflow increase caused by the stuck PCV valve.

How to Test Your PCV Valve

If you are experiencing any of the problems outlined above, then there is a chance that your PCV valve has failed or is stuck open. The problem, as we have described, is that you may not get precise readings from your diagnostic computer as to the cause of the problem. If none of the issues highlighted by your engine check system is the problem, then you need to check the PCV valve. There is no one PCV valve test; however, there are a few steps you can take to determine whether it is working correctly.

To do this, you need to first identify the PCV valve and the hoses connected to it. Begin your checks by inspecting the rubber parts of the valve. If these are worn, then the part needs to be replaced. Next, identify the mesh filter beneath the valve and replace it. If this does not solve your problem, then disconnect the hoses and scrutinize them. Deal with any issues you find here or replace the hoses if needed before reconnecting and checking if the problem is solved. One final test is to remove the valve itself and give it a shake. If the PCV valve does not rattle, then it needs to be replaced.

What is the Lifespan of a PCV Valve?

There is no simple answer to the lifespan of a PCV valve. If you undertake regular vehicle maintenance, including changing your oil, then you will increase its lifespan. However, if you do not do this and allow sludge, dirt, and debris to build up in the oil, then you risk significantly shortening the lifespan of your PCV valve.

It is recommended that the valve is changed at every significant vehicle service. However, this doe not mean it cannot or will not fail in between services. Being aware of the main symptoms of a failing PCV valve is the best way to ensure that you can deal with the problem quickly and reduce the risk of any further damage to your engine.


  1. Symptoms of a Bad or Failing Positive Crankcase Ventilation (PCV) Valve – Your Mechanic
  2. How does a positive crankcase ventilation (PCV) system work? – How Stuff Works
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