Porsche 928 Workshop Manual page 602

928 S4
928 GT
Fuel System, Electronic Injection
24
Test point 18: Intake system leaks
Check all connections after the air mass sen-
sor for tightness and freedom from leaks.
Attach special tool 9264/l to the air mass sen-
sor with M 4 x 45 mm bolts and build up a
pressure of 0.3 bar. If there are any leaks, pres-
sure will drop in the intake system or air will be
heard escaping
L
551.24
Note
On cars with oxygen regulator, flashing fault
code l-23 (oxygen regulator at rich stop) ap-
pears if there are severe leaks in the intake sys-
a
tern (for example the intake pipe gasket).
Test point 19: Resonance flap
In certain engine speed ranges and at not less
than one-third of full engine load, the reso-
nance flap in the intake system is opened by
vacuum. It is also opened whenever the engine
is started (provided that vacuum is present in
the reservoir).
a) Checking function of flap
Remove the resonance flap shaft cover (rubber
cover) attached above the intake pipe cluster.
Using a felt marker or similar, mark a central
line on the surface of the flap shaft (this makes
it easier to detect opening movement).
Start the engine: the flap will open and close
again as the engine is started.
When the engine is running, depress the accel-
erator pedal to the full throttle position sud-
denly: the flap will open and close.
(However, this does not happen unless the en-
gine is running at more than 3500 rpm and at
not less than one-third of its full load.)
b) Checking operation of resonance flap
without running the engine: vacuum side
Detach the vacuum supply pipe from the
vacuum reservoir at the solenoid valve. Check
vacuum with a vacuum tester
- 0.8 bar.
After this, build up a vacuum of approx. 0.4
bar at the pipe to the diaphragm valve (below
the intake air pipe cluster = the resonance
flap will open.
If the flap does not open:
check diaphragm valve and supply pipe.
c) Power supply (V)
Use the test leads to bridge terminals 17 and
21 at the LH control unit plug. Use other test
leads to connect temrinal 34 of the control unit
plug to ground = the resonance flap will
open.
LH-Diagnosis/Troubleshooting
Printed In Germany - XX", ,390
D 24 - 25