Positive Crankcase Valve

By Reinier (Contact Me)
Last Updated 2022-12-24
Automobile Repair Shop Owner

What Does the Positive Crankcase Valve Do?

The sole function of a positive crankcase ventilation (PCV) valve is to regulate the negative pressure (vacuum) that “sucks” gases and combustion waste products from the engine to prevent excessive positive pressure from developing in the engine.

Why is the Positive Crankcase Valve Needed?

Although cylinder sealing in internal combustion engines has greatly improved in the latest generation of engines, some of the gases that result from combustion fuel still escape into the crankcases of all internal combustion engines past the piston rings.

In a (hypothetical) perfect engine, the seal between the piston rings and the cylinder walls would not allow any combustion pressure to escape into the crankcase, and in such an engine, all of the combustion gases would be used to force the piston down during the power stroke. This would greatly increase the efficiency of the engine, and reduce harmful exhaust emissions by a significant percentage, at the same time.

However, the problem with developing perfect engines is that pistons slide up and down the cylinders during engine operation. Thus, until somebody discovers a way to eliminate friction between the piston rings and the cylinder walls, this interface must necessarily be lubricated with a thin layer of engine oil to prevent the piston rings and cylinder walls from wearing each other away to the point where the engine stops working.

So, to accommodate the formation of a thin layer of lubrication oil on cylinder walls, all engines are designed so that small clearances exist not only between the piston rings and the grooves in the pistons into which the rings fit but also between the outward-facing surfaces of the piston rings and much of the surfaces of the cylinder walls. In practice, the surfaces of cylinder walls might appear to be perfectly smooth, but on a microscopic level, the surfaces of cylinder walls are scored with diagonally arranged “valleys” that act as oil reservoirs. Consider the image below, which is a greatly enlarged view of a section of a cylinder wall-

We need not delve into how this crosshatch pattern of scratches/valleys is produced here, but suffice it to say that these valleys are required to a) facilitate lubrication of the piston ring/cylinder wall interface, and b) assist in improving the seal between the piston rings and the cylinder walls, with the oil stored in the valleys acting as dynamic seals.

However, while the oil-filled valleys improve the seal between the piston rings and the cylinder walls, this seal is far from perfect, and the extremely high combustion pressures in modern engines force some combustion gases past the piston rings into the crankcase via the valleys. Although only a small volume of gas enters the crankcase via this route during a combustion event in a single cylinder, this process is repeated a great many times on all the cylinders when the engine runs for extended periods.

In technical terms, the combustion gases that are forced past the piston rings are known as “blowby” gases, and this phenomenon occurs on all internal combustion engines, albeit to different degrees depending on among other things, the type of engine, the mechanical condition of the engine, the average engine speed and load, and the condition and type of engine oil.

Nonetheless, unless blowby gases and other substances like volatile components of engine oil, and moisture are removed from the engine crankcase on an ongoing basis, the continuous rise in positive pressure in the crankcase will eventually reach a point where oil seals and gaskets in the engine are damaged, which could cause the sudden and catastrophic loss of engine oil.

Therefore, all internal combustion engines divert some of their intake vacuum to remove accumulated blowy gasses and other waste products from the crankcase. In most cases, this vacuum system uses a PCV (Positive Crankcase Ventilation) valve to regulate the intensity of the negative pressure (vacuum) that removes blowby gases from the engine.

Note though that many late-model and new vehicles do not use PCV valves; instead, these engines evacuate their crankcases through a fixed-diameter orifice. In practice, the intensity of the negative pressure that evacuates the crankcase is largely controlled and regulated by the diameter of the fixed orifice, as opposed to being controlled by a calibrated one-way check valve as on older crankcase ventilation systems.

How Does the Positive Crankcase Valve Work?

This graphic shows the basic operating principle of all PCV valves. The yellow arrows indicate the direction of flow through the valve: the hose connection shown at the top connects to the engine’s intake system via a dedicated hose, and the bottom, vertical part of the valve fits into the valve cover or other part of the engine’s crankcase.

As shown here, the engine vacuum pulls the moveable shuttle upwards when the engine is running, which opens the valve against modest spring tension to allow the engine vacuum to “suck” blowby gases from the crankcase. In a fully functional PCV valve, the engine vacuum determines the size of the opening, but it should be noted that the springs in PCV valves are calibrated to prevent the valve from opening fully at low to moderate engine speeds to maintain a proper manifold vacuum.

Note also that when the engine vacuum suddenly decreases, such as happens during deceleration, the spring in the PCV valve will either move the valve shuttle towards the closed position to reduce the flow of gases through the valve, or it will close the valve altogether to preserve proper intake manifold vacuum.

Note that although the design and appearance of PCV valves vary between vehicle makes and models, the example shown here is representative of most PCV valve designs.

Where is the Positive Crankcase Valve Located on the Engine?

This image shows the location (circled) of the PCV valve in the valve cover of a Jeep Wrangler engine. In this design, the actual PCV valve is fully enclosed in the vacuum hose, but in most designs, the vacuum hose connects to a hose barb on one end of the PCV valve, which leaves the actual valve exposed and visible.

Note though that not all PCV valves are located on valve covers. In some cases, the PCV valve might be located directly on the intake manifold, or it might be located on the engine block below the cylinder head, which is a common location on many BMW engines.

Because PCV valve locations vary greatly between vehicle makes and models, we recommend that reliable service information for the affected vehicle be consulted to locate and identify the PCV valve correctly.

What Does the Positive Crankcase Valve Look Like?

This image shows two examples of common PCV valves. The example at the top is made of steel, and screws into place into an intake manifold or elsewhere on the engine. The example at the bottom is made of plastic and fits into a valve cover which is sealed with a rubber grommet.

Note though, that while most PCV valves follow this general pattern, some PCV valve designs are hugely complex and might incorporate an electrical connector, such as on many late-model Audi and other VAG-group vehicles.

NOTE: It is important to bear in mind that although many PCV valves might appear to be identical in all respects, PCV valves are calibrated to be engine-specific to ensure the proper operation of the valves. Therefore, PCV valves are not interchangeable even if one particular PCV valve might fit on multiple engines.

What are the Symptoms that the Positive Crankcase Valve is Bad?

Although the most common symptoms of failed, defective, or malfunctioning PCV valves are largely similar across all vehicle makes and models that use PCV valves, the most noticeable symptoms depend on the nature of the PCV valve’s failure.

Due to their operating principles, PCV valves can fail in one of two ways- they can become stuck in the closed position, or they can become stuck in the open position, and each condition has its own set of symptoms. Below are some common symptoms that occur when-

The PCV valve sticks in the closed position

• On post-2001/2 vehicles, a stored trouble code and an illuminated warning light might be present
• Persistent or recurring oil leaks as the result of excessive positive pressure in the crankcase
• Some engines may show excessive levels of moisture and/or oil sludge as a result of the PCV system’s failure to remove volatile oil components and moisture from the engine. Excessive moisture is usually, but not always, indicated by the presence of a milky substance on the underside of the oil filler cap
• The engine may surge or stall unexpectedly at low engine speeds
• One or more MAF (Mass Airflow) sensor-related trouble codes may be present
• One or more oxygen sensor trouble codes may be present
• One or more catalytic converter-related trouble codes might be present
• One or more fuel trim-related trouble codes may be present

Below are some common symptoms that occur when-

The PCV valve sticks in the open position

• The engine may misfire on one or more cylinders
• One or more misfire-related trouble codes will usually, but not always be present when the engine misfires
• One or more fuel trim-related trouble codes may be present
• Oil consumption may increase noticeably
• The idling quality may be poor, or the engine may not idle at all
• The engine may stall unexpectedly and/or repeatedly at low engine speeds
• Liquid oil might be present in the PCV valve or its dedicated vacuum hose
• Spark plugs may become fouled with engine oil
• A no-start or hard-starting condition may be present, but note that no-start conditions are relatively rare if a PCV valve fails

SPECIAL NOTES: Be aware that many, if not most of the above symptoms can also occur as the result of oxygen sensor failures, MAF sensor failures, engine vacuum leaks, or even faults and defects in one or primary ignition circuits. This could make it difficult to diagnose the root cause(s) of the above symptoms correctly, so be sure to investigate all possible causes of the above symptoms before condemning a PCV valve out of hand. END OF SPECIAL NOTES.

How do you test the Positive Crankcase Valve?

Although the internet abounds with advice on how to test the operation of PCV valves, the fact of the matter is that there are no objective tests that can assess to condition or operation of any PCV valve, because it is impossible to replicate the conditions under which PCV valves work with any degree of accuracy.

Note that this is not the same as saying that all online advice on testing PCV valves is generally wrong, confusing, misleading, or irrelevant- far from it. In fact, test methods such as placing a piece of paper across the oil filler hole in a valve cover while the engine is running might be a valuable screening test for possible issues with the PCV valve. For instance, if the paper is held in place, the PCV valve is working, but this test does not provide definitive information on how well the PCV valve works under all engine operating conditions.

There are other examples. For instance, blowing through a PCV valve to see if it allows gas to flow through it in only one direction might prove that the valve does indeed shut off, but this method does not demonstrate how well (or otherwise) it controls the flow of hot gas through it in the opposite direction under all possible operating conditions.

In practice, testing a PCV valve properly involves taking the observable symptoms into account when performing a visual inspection of the whole PCV system. For example, a ruptured PCV vacuum hose will mimic the symptoms of a stuck-open PCV valve because it creates a vacuum leak. Similarly, a pinched or otherwise restricted PCV vacuum hose will mimic the symptoms of a stuck-closed PCV valve because the restriction will inhibit the flow of gas through the hose.

Thus, as a practical matter, and given that many symptoms that are commonly associated with failed or defective PCV valves may not actually involve the PCV valve, the wisest and most cost-effective course of action is to seek professional assistance with diagnosing and resolving PCV issues when you suspect you might be dealing with a failed or malfunctioning PCV valve.

How do you replace the Positive Crankcase Valve?

If the PCV valve is easy to access, replacing it is usually no more complicated than disconnecting the hose and removing the old valve from the engine by pulling it out of the rubber grommet while taking great care not to damage the grommet in any way. Note that If the grommet is hard, shrunken, cracked, or otherwise less than perfect, you need to replace the grommet along with the PCV valve to prevent oil and vacuum leaks through the damaged grommet.

Be aware that it might be very difficult to access the PCV valve on some engines. In many such cases, it may be necessary to remove major unrelated engine components such as power steering pumps/hoses HVAC lines, intake manifolds, wiring, and/or fuel lines.

In these cases, the possibility of making mistakes or causing damage where there was none before is directly proportional to the difficulty of accessing the PCV valves, so if you cannot access the PCV valve easily, we highly recommend that you seek professional assistance with diagnosing and/or replacing a suspect PCV valve.