Positive Crankcase Ventilation System - Craftsman 20930 Operator's Manual

POSITIVE CRANKCASE VENTILATION SYSTEM

SYSTEM OPERATION
The Positive Crankcase Ventilation (PCV) system is
used on all modern engines to reduce air pollution
by providing a more complete scavenging of
crankcase vapors. Air is drawn through a filter
located in the air cleaner, through a hose in the
valve cover, into the crankcase, across and up into
the rear of the intake manifold or opposite valve
cover, through the PCV valve, through a hose, into
the intake manifold. Intake manifold vacuum draws
in all vapors from the crankcase to be burned in
the engine
When air flow through the carburetor or throttle
body is high, added air from the PCV system has
no effect on engine operation. However, at idle, air
flow through the carburetor or throttle body is so
low that any large amount added by the ventilation
system would upset the air/fuel mixture, causing a
rough idle. For this reason, the PCV valve restricts
the ventilation system flow when intake manifold
vacuum is high.
SERVICE PROCEDURES
After a period of operation, the PCV valve may
become clogged and reduce the amount of
crankcase ventilation. The PCV valve should be
replaced periodically to prevent the formation of
acids in the crankcase, and the build up of excessive
crankcase pressure, which could force engine oil out
past the seals. Use the following procedure to check
the PCV system using your pump:
1) Inspect the system for kinked, plugged or
deteriorated hoses. Check to be sure all hoses
are connected properly. Repair as necessary.
2) Connect your pump to an intake manifold port
and check the vacuum reading of the warmed and
idling engine.
3) Clamp off the vacuum hose to the PCV valve.
The engine speed should decrease 100 RPM to
indicate the loss of the calibrated air leak into
the intake manifold. The vacuum gauge reading
should increase slightly, indicating that the vacuum
leak has been plugged. If this does not happen,
replace the PCV valve and/or replace any damaged,
plugged or loose hoses.
4) If the engine is idling too slow or is rough, this
may be caused by a clogged PCV valve or hose.
Do not adjust the idle speed without first checking
the PCV system.
5) After installing a new PCV valve, always adjust
the idle speed, and if possible, the idle air mixture.
The installation of the wrong valve may cause too
much vapor to flow through the system if the
calibrated bleed is too large. This will lean out the
air/fuel mixture excessively. If the opening is too
small, the plugging effect will be nullified, emissions
will increase, acids will form and oil leaks may
develop. Be sure you get the correct PCV valve
for your car.
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