An adequate amount of oxygen must be present in
the exhaust system for the catalytic converter to
operate; therefore, a supporting system, such as an air
injection system, usually is placed on catalytic
converter equipped engines to dilute the exhaust stream
with fresh air.
AIR INJECTION SYSTEM
An air injection system (fig. 4-51) forces fresh air
into the exhaust ports of the engine to reduce HC and
CO emissions. The exhaust gases leaving an engine can
contain unburned and partially burned fuel. Oxygen
from the air injection system causes this fuel to continue
to burn. The major parts of the system are the air pump,
the diverter valve, the air distribution manifold, and the
air check valve.
The AIR PUMP is belt-driven and forces air at
low pressure into the system. A hose is connected to the
output of the diverter valve.
The DIVERTER VALVE keeps air from entering
the exhaust system during deceleration. This prevents
backfiring in the exhaust system. Also, the diverter
valve limits maximum system air pressure when
needed, releasing excessive pressure through a silencer
or a muffler.
AIR DISTRIBUTION MANIFOLD directs a
stream of fresh air toward each engine exhaust valve.
Fittings on the air distribution manifold screw into a
threaded hole in the exhaust manifold or cylinder
AIR CHECK VALVE is usually located in the
line between the diverter valve and the air distribution
manifold. It keeps exhaust gases from entering the air
Basic operation of the air injection system is as
When the engine is running, the spinning vanes
of the air pump force air into the diverter valve. If not
decelerating, the air is forced through the diverter valve,
the check valve, the air injection manifold, and into the
engine. The fresh air blows on the exhaust valves.
During periods of deceleration, the diverter
valve blocks air flow into the engine exhaust manifold.
This prevents a possible backfire that could damage the
exhaust system of the vehicle. When needed, the
diverter valve will release excess pressure in the
POSITIVE CRANKCASE VENTILATION
The positive crankcase ventilation system uses
manifold vacuum to purge the crankcase blow-by
fumes. The fumes are then aspirated back into the
engine where they are reburned.
A hose is tapped into the crankcase at a point that is
well above the engine oil level. The other end of the
hose is tapped into the intake manifold or the base of the
If the hose is tapped into the carburetor
base, it will be in a location that is between the
throttle valves and the intake manifold so that it
will receive manifold vacuum.
An inlet breather is installed on the crankcase in a
location that is well above the level of the engine oil.
The inlet breather also is located strategically to ensure
complete purging of the crankcase fresh air. The areas
of the crankcase where the vacuum hose and inlet
breather are tapped have baffles to keep motor oil from
leaving the crankcase.
A flow control valve is installed in the line that
connects the crankcase to the manifold. It is called a
positive crankcase ventilation (PCV) valve (fig. 4-52)
and serves to avoid the air-fuel mixture by doing the
Any periods of large throttle opening will be
accompanied by heavy engine loads. Crankcase blow-
by will be at its maximum during heavy engine loads.
The PCV valve will react to the small amount of
manifold vacuum that also is present during heavy
engine loading by opening fully through the force of its
control valve spring. In this way, the system provides
maximum effectiveness during maximum blow-by
Any period of small throttle opening will be
accompanied by small engine loads, high manifold
vacuum, and a minimum amount of crankcase blow-by.
During these periods, the high manifold vacuum will
pull the PCV valve to its position of minimum opening.
This is important to prevent an excessively lean air-fuel
In the event of engine backfire (flame traveling
back through the intake manifold), the reverse pressure
will push the rear shoulder of the control valve against