The PCV system was developed to remove harmful vapors from the engine.

The PCV system does this by using manifold vacuum to draw vapors from the crankcase into the intake manifold.

Vapor is then carried with the fuel/air mixture into the combustion chambers where it is burned.

The PCV Valve is effective as both a crankcase ventilation system and as a pollution control device.

The most common problem that afflicts PCV systems is a plugged up PCV valve. Also, 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 plugged PCV valve cannot pull moisture and blowby vapors out of the crankcase. Consequently, 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, increasing fuel consumption and emissions.

Symptoms of a bad PCV valve

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. Instead, the diagnostic trouble code P0171 and P0174 are common. These codes suggest a lean-condition in the engine. Other engines could set a mass-air-flow meter code or even an oxygen sensor code.

A failed PCV valve may also cause noise. Some will produce a whistle or whine and others can produce a low moaning noise. 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. An oily or dirty spot, near the PCV inlet hose is a symptom.

PCV System
PCV System

Also, 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 or more of these symptoms:

  • Increase in internal engine pressure
  • Failure of one or more oil seals or gaskets
  • Engine oil leaks
  • Moisture and sludge buildup inside the engine
  • Engine surges and possibly black smoke

So, 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

In addition, a PCV valve stuck open can trigger the “check engine” light due to increased air flow. And a diagnostic computer may erroneously blame this light on a mass air flow sensor or oxygen sensor instead, making it harder for you to detect the real source of the problem.

When replacing a PCV valve, make sure the replacement valve is the same as the original.