Service Positive Crankcase ventilation system

Service Positive Crankcase ventilation system

Filtered Air from the air cleaner is drawn through the crankcase by the intake manifold vacuum. The air picks up the blow-by gas or vapor and carries them to the intake manifold. The vapors then flow through the intake manifold to the combustion chambers. There the unburned fuel burns during normal combustion.

Too much air flowing through the intake manifold can cause disturbance in the air-fuel ratio causing engine to stall or poor idling. A fixed orifice or variable valve prevents this to cause.The Blow by gases can cause the Engine oil to sludge or cause the crank case to rust severely.mTo prevent this, vehicles are equipped with an air tube leading the gases out into the atmosphere.

The amount of blow-by gas produced is generally effected more by the intake manifold vacuum than by the engine space, for this reason if the cylinder head cover and intake manifold vacuum is stronger at the loads at that time the amount of the blow-by gases is small and the intake manifold vacuum is weaker at heavy loads, when the blow-by gases is great.

Operation of Positive Crank case Ventilation (PCV): -

1. Engine stopped or Backfiring:

The valve is closed due to its own weight and the weight of the spring (refer figure).

2. Cruising and light load:

The Vacuum is strong, so the valve moves upwards (Opens).However since the vacuum passage is still narrow the volume of the blow-by gas is low (Refer Figure).

3. Normal Operation:

The vacuum is normal, so the vacuum passage is wide open.

4. Acceleration or High Loads:

The PCV valve opens completely and the vacuum passage is fully open (Refer Figure). 

Positive Crank Case Ventilation (PCV) Trouble Diagnosis: -

With Time, mileage, deposits can accumulate in the valve and passages. These can cause engine troubles.On Fuel injected engines, the filter may be in the valve cover. Replace a malfunctioning PCV valve or PCV filter.Vapor flow into the air cleaner and oil in the air cleaner results from the backflow. Instead of the filtered air flowing into the case, crankcase vapor flow into the air cleaner. This causes a plugged PCV valve or a plugged/ leaking condition somewhere in the system.Worn Piston rings and cylinder walls also cause backflow.

Positive Crankcase Ventilation (PCV) Service: -

Replace the PCV valve at regular interval or whenever it clogs or sticks.The crankcase vapors flow up to the PCV valve through the internal passage in the engine. These includes the passage that allows the valve train lubricating oil to drain back to the crankcase.Quick Check of PCV: Pull PCV from its mounting while the engine is idling and warm. You should hear a hissing sound. This indicates that the valve and line are unplugged.

Shake the valve. You should hear a rattle that indicates the valve is not stuck.Put thumb over end of valve and feel for vacuum. If no vacuum than Valve is not working properly Check hoses and connections.On a non-computer controlled engine, the rpm-drop teat can be used for PCV system.With Engine warm and idling, connect a tachometer and note idling speed. Then pinch the hose closed, Or remove valve and place thumb over one end of valve.Idling speed rpm should drop at least 50 rpm. If not PCV is plugged and remove and clean the valve.