Positive Crankcase Ventilation

[MoffZ28]

http://members.rennlist.com/pbanders/PCV.htm

D-Jetronic PCV (Positive Crankcase Ventilation) Valve
The PCV valve regulates the flow of blow-by gasses from the crankcase into the intake manifold, where they are re-burned in the cylinders. The design of this valve is critical for proper crankcase ventilation and idle performance.

PCV Valve Components


Above is a photograph of a new PCV valve and a disassembled PCV valve. The narrow portion of the new valve at the top connects to manifold vacuum, the bottom is exposed to the crankcase. When the valve is open, gasses from the crankcase flow from the bottom to the top.





Above is a photo of the intake manifold side of the valve, with the spring inserted into the port, and the disk positioned on top of the spring. When the engine isn't running, the spring presses the disk against the valve seat on the crankcase side of the PCV valve, closing the PCV valve completely.



The photograph above shows the inside of the crankcase side of the valve. You can see that the circular valve seat is continuous, so that when the disk is pressed against it, there is a complete seal. In the case of a backfire, where the intake mainfold becomes pressurized, the PCV valve disk is forced against this seat, preventing the backfire from propagating into the crankcase and causing an explosion.



Above is a photograph of the inside of the crankcase side of the valve, with the disk in place. Note that the disk is square, so that crankcase gasses can pass around the disk when it is unseated.



The photograph above shows the disk clamped against the manifold side seat. You can see one of the three metering slots on the seat that permit a regulated flow of gasses through the valve when it is in this position. When the pressure differential across the valve is high (e.g. at idle, where there is 15 in. Hg of vacuum on the manifold port, and only a small pressure on the crankcase side from blow-by gasses), the disk is pulled up against this seat. The metering slots permit a small flow of gasses so that even at idle, crankcase ventilation occurs. If these ports become clogged with oil residues and debris, flow is reduced and there is insufficient crankcase ventilation. Wear and debris on the seat causes problems with inconsistent idle.



Above are photographs of each side of the disk. The photo on left shows the side of the disk that contacts the crankcase side seat, the photo on the right shows the side that contacts the intake manifold side seat. Note the mark from the spring on the intake manifold side.

PCV Valve Operation Modes
High Manifold Vacuum, Low Crankcase Pressure
This mode corresponds to the idle condition. The pressure differential presses the disk against the intake manifold side seat, where the metering slots permit a regulated flow of gasses into the intake manifold. This flow is kept to a small amount so as not to decrease manifold vacuum level and cause the ECU to richen the mixture.

High Manifold Vacuum, Moderate Crankcase Pressure
This mode corresponds to over-run (coasting in gear with the throttle closed). In over-run, manifold vacuum can exceed 20 in. Hg. Under these conditions, the disk is pressed against the intake manifold side seat. A small amount of flow is passed through the metering slots. Blow-by is minimal as combustion is at a low level.

Low Manifold Vacuum, Moderate Crankcase Pressure
This mode corresponds to part-load conditions. When the pressure differential becomes greater than the spring tension against the disk then the disk floats between the intake manifold side seat and the crankcase side seat and a large flow of gasses can be passed through the valve. This is the normal operating position of the valve when cruising under light to moderate part-load conditions.

Very Low Manifold Vacuum, High Crankcase Pressure
This mode corresponds to heavy load to full-load conditions. Here, most of the pressure differential that opens the PCV valve comes from crankcase pressure. Under heavy load, blow-by gasses are at a maximum and the PCV valve is open to permit the gasses to be passed to the intake manifold for re-burning. If the blow-by volume exceeds the ability of the PCV valve to draw in the vapors, the excess blow-by flows back through the crankcase fresh air intake system to the air cleaner box, where it is pulled through the throttle body and into the cylinders.

High Manifold Pressure
This mode corresponds to an intake backfire condition. Here, the high positive pressure in the manifold presses the disk tightly against the crankcase side seat, sealing the PCV valve and preventing flame propagation into the crankcase to prevent an explosion. The crankcase fresh air intake system incorporates a flame trap that prevents the flame front from propagating into the heads.

Comparison With Modern PCV Valves
The D-Jetronic PCV valve has three flow rates: metered (when the disk is on the intake manifold side seat), full (disk not on either seat), and closed (disk on crankcase side seat). The abrupt transition from metered to full flow when the pressure differential changes from idle to part-load conditions can cause potentially some driveability problems such as bogging, due to the abrupt change in intake manifold vacuum that can occur. Most D-Jetronic systems are designed so that when the valve opens fully, the airflow in the intake manifold is quite large, and there is only a minimal effect on the manifold pressure.

Modern PCV valves use a shaped plunger instead of a disk valve. The plunger's shape is so that a smooth transition in flow from metered to full flow occurs when the pressure differential changes. This link has some good diagrams of the internals of a modern PCV valve and a description of its operation, as well as the the entire crankcase ventilation system.

Maintenance
The PCV valve should be removed at every oil service, inspected, and cleaned with contact cleaner spray.