In its simplest form, a PCV valve consists of a body that encloses a moveable valve and a small spring. When the engine is not running the springs holds the valve in the closed position, but when the engine starts and creates a vacuum in the intake system, the vacuum overcomes the spring tension, thereby opening the valve, which allows the engine vacuum to “suck” waste gases from the engine.
However, the principal function of the spring is to close the PCV valve when the negative pressure (vacuum) in the intake manifold suddenly decreases, such as when the throttle is snapped shut at high engine speeds. If this happens, some of the fast-moving air in the intake system could pass into the engine through the PCV valve if the spring cannot close the valve in time to prevent the inrush of intake air into the engine.
Moreover, it should be noted that, since the vacuum in the intake manifold is determined by things like the engine’s displacement, the design of the intake manifold, the position and rate of movement of the throttle blade, as well as the engine speed at any given moment, PCV valves are calibrated to be engine-specific. Put differently, this means that all PCV valves are designed so that any given PCV valve will only work as intended on the engine it was designed for.
So, while an unsuitable PCV valve may fit on a particular engine, the unsuitable valve may cause that engine to have poor idling quality, or poor performance accompanied by misfire-like symptoms because the unsuitable PCV valve allows too much air and waste gases to enter into the engine. Therefore, even though many PCV valves may appear to be identical in all respects and will, therefore, fit on several, or even many engines, PCV valves are in fact, NOT interchangeable between engines with different displacements and/or intake manifold designs.
Note also that since unsuitable PCV valves have a direct bearing on engine operation, it is best to replace a PCV valve only with an OEM or OEM-equivalent part to ensure that the replacement PCV valve provides reliable, consistent, and predictable service.