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PCV 101

PCV system

The positive crankcase ventilation (PCV) system reduces emissions from the engine by capturing "blowby" emissions so they can be recirculated to the cylinders. Blowby emissions are the gases that get past the piston rings during compression and enter the crankcase rather than being burned in ignition. In general, the higher the mileage on the engine, the looser those piston rings are and the more blowby there is.

The PCV valve is part of a system that captures those blowby emissions and recirculates them into the intake. Up until the early 1960s, cars didn't have a PVC and instead just vented blowby emissions to the atmosphere, usually through a "road draft tube" that extended below the engine.

By the mid-1960s, though, PCV systems were on nearly every car made. Those early systems were called "open" PCV systems and utilized engine vacuum to siphon blowby into the intake manifold. The trouble was, these simple systems couldn't draw blowby when the engine was operating at high speeds or heavy loads, making them useless on highway cars and heavy vehicles.

A closed PCV system began to enter the automotive market in the late 1960s. The simple relocation of the inlet tube sending blowby emissions into the engine solved the problems of the open system. The inlet tube, now inside the air cleaner housing (which itself was sealed) meant that the blowby emissions were sucked into the carburetor and thus into the combustion chamber.

The PCV Valve

The main component of the PCV system is the valve. This is a spring-loaded valve with a sliding pintle inside that increases or decreases airflow according to its position within the housing. Designs vary somewhat, but in general, the pintle has an oval or plunger that moves within a chamber. A spring gives the pintle resistance and as the spring compresses, the head extends towards the valve's end.

The movement has the opposite effect from what might be expected, however. As the plunger nears the opening of the valve, the angular or ovular shape of the pintle means that the rear opening becomes larger. This creates more vacuum, pulling more bypass into the valve and to the intake.

The trick of the PCV valve is to separate oil from bypass gases. This is done through a combination of the valve's location on the engine, which varies depending on configuration, and its angle. Baffles inside the valve cover further act to capture oil droplets so they do not continue into the valve itself.

A small hose connects the PCV valve to a vacuum port on the throttle body or intake manifold, finishing the circuit. The PCV system pulls air into the intake and adds nothing more than a few ounces of weight and a small vacuum leak to the system. The leak is compensated for with engine tuning and thus no net loss of fuel economy or performance is caused.

PCV valve flow rates vary by engine displacement and pressures,so the PCV on a four-cylinder naturally aspirated engine will be very different from that of a supercharged V8.

When the engine is off, the PCV valve is closed, preventing vapors escaping into the atmosphere. When the engine is running, vacuum pulls the valve open and controls its intake. Backfire causes the valve to shut as the vacuum pressure reverses, preventing flames from entering the crankcase.

The PCV system requires little maintenance. It's so simple that most mechanics are generally unaware of its existence beyond a few mentions in technical manuals. It's rare that the PCV system has a problem. Most engines have a PCV valve replacement interval of about 50,000 miles, but some modern engines can have double that mileage before requiring replacement. The interval often coincides with timing belt changes.

Starting in 2002, though, the OBD II system of diagnostics now monitors the PCV system for flow rate during various parts of the drive cycle. Problems will trip a code on the computer and trigger the engine light. So in newer vehicles, the PCV system is becoming more prevalent in owner's minds.