Subaru Impreza 2003, Service Manual

Page: 1011 / 3610

ME(H4DOTC)-37
MECHANICAL
INTAKE MANIFOLD VACUUM
5. Intake Manifold Vacuum
A: INSPECTION
1) Warm-up the engine.
2) Disconnect the brake vacuum hose, and then in-
stall the vacuum gauge to hose fitting on manifold.
3) Keep the engine at the idle speed, and then read
the vacuum gauge indication.
By observing the gauge needle movement, the in-
ternal condition of engine can be diagnosed as de-
scribed below.
Vacuum pressure (at idling, A/C “OFF”):
Less than
−−−−
60.0 kPa (
−−−−
450 mmHg,
−−−−
17.72
inHg)
ME-00008
Diagnosis of engine condition by measurement of manifold vacuum
Vacuum gauge indication Possible engine condition
1. Needle is steady but lower than normal position. This ten-
dency becomes more evident as engine temperature rises. Leakage around intake manifold gasket or disconnection or
damaged vacuum hose
2. When engine speed is reduced slowly from higher speed,
needle stops temporarily when it is lowering or becomes steady
above normal position. Back pressure too high, or exhaust system clogged
3. Needle intermittently drops to position lower than normal
position. Leakage around cylinder
4. Needle drops suddenly and intermittently from normal posi-
tion. Sticky valves
5. When engine speed is gradually increased, needle begins to
vibrate rapidly at certain speed, and then vibration increases as
engine speed increases. Weak or broken valve springs
6. Needle vibrates above and below normal position in narrow
range. Defective ignition system or throttle chamber idle adjustment