PCV - Positive Crankcase Ventilation valve/pipe

1.8) PCV - Positive Crankcase Ventilation valve

What is it? The PCV valve is a thumb sizedplastic elbow that is designed to relieve the build up of pressure in the crankcase. A pipe connects the PCV to the airducting. The function of the PCV is toreturn exhaust gases (blow-by of the piston rings during ignition) from thecrank case to the air intake.

Where is it located? PCV is found on the crankcasebreather pipe or cam cover

How does it work? Some pressure relief systems have a convoluted set of internalpassageways within the crankcase to allow oil mist to settle out and to allow excess pressure tovent in an unrestricted manner through a pipe directly into the air ducting. On other systems thePCV opens when the vacuum in the inlet manifold is low i.e. when the engine isrunning at speed and when the positive pressure of blow-by gases builds up inthe crank case. The PCV is held closefor the most part by the vacuum of the inlet manifold at idle. Often the PCV is unidirectional, allowing gas/airto flow only from the crankcase to the throttle intake. If the valve becomes clogged and stays closedthis can cause pressure buildup in the crankcase and this in turn causes oil toleak through seals etc. Alternatively ifthe valve stays open the ingress of unmetered air can upset thefuel/mix and cause rough idling.

Symptoms of faulty PCV

• Erratic idling - If the PCV fails to close when at idle then the extra flow of unmetered air from the crankcase can compromise the ECU's ability to maintain a balanced fuel to air mixture. The symptoms very much mimic a leak in the air ducting.