PCV Valve System, where partially burned crankcase gases get recycled.

PCV Valve System, where partially burned crankcase gases get recycled.
PCV Valve System, where partially burned crankcase gases get recycled.

So, the PCV Valve System (PCV) (Positive Crankcase Ventilation); is a controlled device, used to vent the crankcase gases.

Consequently, it sends partially burned gases, that come from the engine’s crankcase, back to the combustion chamber.

Therefore, burning these gases for a second time is its main objective.

In addition, the PCV Valve System, is also known as, the (Positive Crankcase Ventilation System).

One of the oldest and most used emission devices in today’s engines, is the PCV valve system. Although its name might sound very complicated, it is a very simple device. However, overlooking this system is a common mistake, as it is relatively simple and requires minimal maintenance. But, when it is not working properly, we should see the signs.

Sludge buildup, is a growing concern and the PCV valve system, plays a big role in controlling it.

The (PCV) Valve Has Important Benefits:

  • Eliminates crankcase emissions.
  • Keeps the system free of moisture, due to the constant air circulation that it implies.
  • Helps keep engine-damaging sludge to a minimum, extending the life of your oil.
  • The (PCV) valve will also, protect the engine in case of a backfire. A backfire causes a sudden high-pressure pulse in the intake manifold. This forces the (PCV) valve closed, so that the backfire flame, can’t reach the crankcase.

So, a clean PCV valve system is very important. Otherwise airflow will be insufficient. A plugged or malfunctioning (PCV) system, will eventually damage an engine. Because, contaminants accumulate in the engine oil, and unrelieved crankcase pressure; can blow out gaskets and seals, creating oil leaks. As a result, a poorly maintained engine (PCV) system, will eventually become contaminated with oil sludge.

How Does The (PCV) System Work

PCV Valve System Operation
PCV Valve System Operation

The major component in the (PCV) system, is the PCV valve. It consists of a simple spring-loaded valve, with a sliding pintle inside. The movement of the pintle up and down, changes the orifice opening. As a result, regulating the volume of air, passing through the (PCV) valve.

The location of the valve, allows it to pull vapors from inside the engine; without sucking oil from the crankcase. Also, there are baffles inside the valve cover or valley cover; that deflect and help separate, droplets of oil from the blow-by vapors.

A hose connects the top of the (PCV) valve, to a vacuum port on the throttle body; carburetor or intake manifold. Allowing the vapors, to be siphoned directly into the engine.

Because the PCV valve system pulls air and blow by gases into the intake manifold; it has the same effect on the air/fuel mixture, as a vacuum leak.

This is compensated for by, the calibration of the carburetor or fuel injection system. The (PCV) system, has no net effect on fuel economy, emissions or engine performance; provided everything is working correctly.

How Does The (PCV) Flow Change, With Engine Speed & Load

The flow rate of a (PCV) valve, is calibrated for a specific engine application. For the PCV valve system to function normally, therefore; the (PCV) valve must adjust the flow rate as operating conditions change. When the engine is off, the spring inside the valve pushes the pintle shut. This seals the crankcase and prevents the escape, of any residual vapors into the atmosphere.

When the engine starts, vacuum in the intake manifold, pulls on the pintle and sucks the (PCV) valve open. The pintle, is pulled up against the spring and moves to its highest position. But, the tapered shape of the pintle, does not allow maximum flow in this position. Instead, it restricts flow, so the engine will idle smoothly.

PCV Valve Operation
PCV Valve Operation

The same thing happens during deceleration, when intake vacuum is high. The pintle is pulled all the way up to reduce flow. This will minimize the effect of blow-by. When the engine is cruising under light load and at part throttle; there is less intake vacuum and less pull on the pintle. This allows the pintle to, slide down to a mid-range position and allow more airflow.

Under high load or hard acceleration conditions, intake vacuum drops even more. Consequently, allowing the spring inside the (PCV) valve; to push the pintle valve even lower to its maximum flow position. But, what if blow-by pressure, builds up faster than the PCV valve system can handle it? The excess pressure, flows back through the breather hose, to the air cleaner.

There Are Some (PCV) Valve Issues

If the pintle inside the (PCV) valve sticks open, or the spring breaks; the (PCV) valve may flow, too much air and lean out the idle mixture. This may cause a rough idle, hard starting and/or lean misfire; (which increases emissions and wastes fuel). The same thing can happen if, the hose that connects the valve to the throttle body; carburetor or intake manifold pulls loose, cracks, or leaks. A loose or leaky hose allows “unmetered” air to enter the engine and upset the fuel mixture. Especially at idle, where the idle mixture is, most sensitive to vacuum leaks.

The Most Common Problem, Is A Clogged (PCV) Valve

An accumulation of fuel and oil varnish deposits and/or sludge inside the valve; can restrict or even block the flow of vapors through the valve. A restricted or clogged (PCV) valve, cannot pull moisture and blow-by vapors out of the crankcase. This can cause engine-damaging sludge to form, and a backup of pressure; that may force oil to leak past gaskets and seals. The loss of airflow, through the valve, can also cause the air/fuel mixture, to run richer than normal. As a result, increasing fuel consumption and emissions. The same thing can happen if, the pintle inside the (PCV) valve sticks shut.

Clogged (PCV) Valve
Clogged (PCV) Valve

On late model vehicles, with computer engine controls; the engine management system will detect any changes in the air/fuel mixture and compensate; by increasing or decreasing short term and long term fuel trim (STFT and LTFT). Small corrections cause no problems, but large corrections (more than 10 to 15 points negative or positive); will typically set a lean or rich DTC and turn on the MIL.

Problems can also occur if, someone installs the wrong (PCV) valve for the application. Two valves that appear to be identical on the outside (same diameter and hose fittings); may have different pintle valves and springs inside; giving them very different flow rates. A (PCV) valve, that flows too much air, will lean the air/fuel mixture. And, one that flows too little, will richen the mixture; increasing the risk of sludge buildup in the crankcase.

So, watch out for cheap replacement (PCV) valves. They may not flow the same, as the OEM (PCV) valve.

How Do You Check Your (PCV) Valve

1. Remove the valve and shake it. If it rattles, it means the pintle inside, is not stuck and the valve should flow air.

Check For Vacuum With Finger
Check For Vacuum With Finger

2. Check for vacuum, by holding your finger over the end of the valve, while the engine is idling. This test tells you if vacuum is reaching the valve, but not if the valve, is flowing properly. If you don’t feel vacuum, it means the valve or hose is clogged and needs to be replaced.

3. Use a flow tester, to check the performance of the valve. This method is the best, because it tests both vacuum and air flow.

The volume of air that is pulled from the crankcase, by the PCV valve system is very important. This is because, it takes a certain amount of airflow, to remove the blow-by vapors and moisture. But, too much airflow, can upset the air/fuel mixture in the engine.

How Do You Check The Airflow:

  • Pinch or block off, the vacuum hose to the (PCV) valve, with the engine idling at operating temperature.
  • The engine idle (rpm) should typically drop, about 50 to 80 rpm, before the idle speed corrects itself.
  • If there is no change in idle speed, check the (PCV) valve hose and breather tube for a restriction.
  • A greater change, would indicate too much airflow, through the (PCV) valve.
  • Check the part number on the (PCV) valve, to see if it is the correct one for the engine.
  • The wrong valve, may flow too much air.
  • If there is no part number, replace the valve with a new one; (which meets OEM specifications) and test again.

How To Measure Vacuum And Check For Leaks, In The Crankcase

Vacuum-Pressure Pump
Vacuum-Pressure Pump

With the engine at, normal operating temperature, block off the (PCV) breather tube or vent to the engine. Pull out the dipstick and connect a vacuum-pressure gauge, to the dipstick tube. A typical (PCV) system, should be pulling about 1 to 3 inches of vacuum, in the crankcase at idle.

If you see a significantly higher vacuum reading; the intake manifold gasket is probably leaking and pulling vacuum in the crankcase. If you see no vacuum, or find a buildup of pressure in the crankcase, the PCV valve system is plugged. Or it may not be pulling, enough air through the crankcase, to get rid of the blow-by vapors.

If the engine has a leaky oil pan, valve cover or intake manifold gasket leak, or leaky crankshaft seals; it will not be able to develop much vacuum in the crankcase. Because, it is pulling in outside air; (which is also unfiltered and can further contaminate the oil).

So, to find a crankcase air leak; you can lightly pressurize (no more than 1 to 3 psi) the crankcase with shop air; via the dipstick tube or oil filler cap or breather, after blocking all the other vents. Do not use any more air pressure than this or you may create leaks; where there were no leaks before. Then use a spray bottle to squirt soapy water, around the gasket seams and seals. If you see bubbles, you have found an air leak (replace the gasket or seal as needed).

Vacuum Leaks - Allow Unmetered Air To Enter The Engine
Vacuum Leaks – Allow Unmetered Air To Enter The Engine

A smoke machine, also works great, for finding crankcase leaks as well as vacuum leaks. A smoke machine generates a smoke-like vapor, by heating mineral oil. The mist is then fed into the intake manifold, to check for intake manifold vacuum leaks; or into the crankcase, to check for internal engine air leaks. As a result, any leaks will allow the smoke to escape and you will; see the smoke on the outside of the engine.

Early Signs Of (PCV) Valve Failure

We see varied symptoms, from a bad (PCV) valve, depending on the way it fails. For instance, a (PCV) valve that sticks open, may cause a check engine light. The diagnostic trouble codes or (DTC) will normally, not mention the (PCV) valve. But, diagnostic trouble codes, P0171 and P0174 are common. These codes suggest, a lean-condition in the engine. Finally, other engines, could set a mass-air-flow meter code or even an oxygen sensor code.


So, a failed (PCV) valve, may also cause noise. Some will produce, a whistle or whine and others, can produce a low moaning noise. So, the easiest way to verify the problem, is to temporarily block the vacuum source to the (PCV) valve; and see if the noise changes or goes away. On some vehicles, a bad (PCV) valve, can cause oil to blow onto the air filter element. Finally, an oily or dirty spot, near the (PCV) inlet hose, is a symptom.

Thank You !