A closed positive crankcase ventilation system is used on all Mitsubishi models. This system cycles incompletely burned fuel (which works its way past the piston rings into the crankcase) back into the intake manifold for reburning with the fuel/air mixture. The oil filler cap is sealed and the air is drawn from the top of the crankcase into the intake manifold through a valve with a variable orifice.
This valve (commonly known as the PCV valve) regulates the flow of air into the manifold according to the amount of manifold vacuum. When the throttle plates are open fairly wide, the valve opens fully. However, at idle speed, when the manifold vacuum is at maximum, the PCV valve reduces the flow in order not to unnecessarily affect the small volume of mixture passing into the engine.
During most driving conditions, manifold vacuum is high and all of the vapor from the crankcase, plus a small amount of fresh air, is drawn into the manifold via the PCV valve. At full throttle, the increase in the volume of blow-by and the decrease in manifold vacuum make the flow via the PCV valve inadequate. Under these conditions, excess vapors are drawn into the air cleaner and pass into the engine along with the fresh air.
A plugged valve or hose may cause a rough idle, stalling or low idle speed, oil leaks in the engine and/or sludging and oil deposits within the engine and air cleaner. A leaking valve or hose could cause an erratic idle or stalling.