POSITIVE CRANKCASE VENTILATION (PCV)
SYSTEMS
Intake manifold vacuum removes crankcase vapors
and piston blow-by from the engine. The vapors pass
through the PCV valve into the intake manifold
where they become part of the calibrated air-fuel
mixture. They are burned and expelled with the
exhaust gases. The air cleaner supplies make up air
when the engine does not have enough vapor or
blow-by gases. In this system, fresh filtered air
enters the crankcase (Fig. 4), (Fig. 5) and (Fig. 6).
PCV VALVE
The PCV valve contains a spring loaded plunger.
The plunger meters the amount of crankcase vapors
routed into the combustion chamber based on intake
manifold vacuum.
When the engine is not operating or during an
engine backfire, the spring forces the plunger back
against the seat. This prevents vapors from flowing
through the valve (Fig. 7).When the engine is at idle or cruising, high mani-
fold vacuum is present. At these times manifold vac-
uum is able to completely compress the spring and
pull the plunger to the top of the valve (Fig. 8). In
this position there is minimal vapor flow through the
valve.
During periods of moderate intake manifold vac-
uum the plunger is only pulled part way back from
Fig. 4 PCV ValveÐ2.4L Engine
Fig. 5 PCV Valve Ð3.0L Engine
Fig. 6 PCV Valve and Fresh Air HoseÐ 3.3/3.8L
Engines
Fig. 7 Engine Off or Engine BackfireÐNo Vapor
Flow
Fig. 8 High Intake Manifold VacuumÐMinimal Vapor
Flow
NSEMISSION CONTROL SYSTEMS 25 - 15
DESCRIPTION AND OPERATION (Continued)
