By-Pass Air Control Valve - Subaru XT 1988 Service Manual

FUEL INJECTION SYSTEM
2-7
By-pass Air Control Valve
(2700 cc model only)
The by·pass air control valve is regulated by a signal transmit·
ted from the MPFI control unit. It finely controls the flowrate
of air passing through the by·pass passage, allowing the engine
to run at optimum speed under various conditions. This in
turn results in stabilized exhaust gas emission, improved fuel
economy and better performance.
1) The by·pass passage opens partially while the engine idles.
When the engine is loaded (the air conditioner is turned ON,
for example), the by-pass passage opens wider to let more air
pass through. This maintains specified engine idling rpm.
Air flow sensor
By-pass passage
2) When the engine is cold, the by·pass passage opens widely
to allow more air to pass through. This speeds engine warm-up.
3) Variations in engine idling rpm over time and/or among
products (= engines) are compensated for.
4) When the throttle valve closes (= dash pot function), a
sudden drop in engine rpm is prevented to improve driving
performance.
a. When battery voltage drops momentarily, the by-pass
valve sometimes activates to increase idling speed to 9DO rpm.
This is not a problem.
b. Before the engine is warmed up (coolant temperature:
above BDoe (176°F)), idling speed between the N or Prange
and the 0 range sometimes differs. However, this is not a
problem because it assists in rapidly warming up the engine.
Ignition switch
rr----~------~~-,
MPF I control
unit
-=-
Battery
1
Intake manifold
Various sensor signals
1
Air cleaner
Throttle body
Low air flow rate
lin low-load, idling operation)
High air flow tate
(in high·load, idling operation)
Control valve operation
Fig. 5
Idling rpm control system
5
Engine rpm
U-ring washer
Commutator
Coil
Magnet
Shaft
.-lIJT-J+--
Core
.. Housing
::~
O·ring
V
R
.
, eturn spring
Slider
Slider housing