Intake Manifold Vacuum - Subaru Legacy 2003 Service Manual

MECHANICAL

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 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)
Vacuum gauge indication
1. Needle is steady but lower than normal position. This ten-
dency becomes more evident as engine temperature rises.
2. When engine speed is reduced slowly from higher speed,
needle stops temporarily when it is lowering or becomes steady
above normal position.
3. Needle intermittently drops to position lower than normal
position.
4. Needle drops suddenly and intermittently from normal posi-
tion.
5. When engine speed is gradually increased, needle begins to
vibrate rapidly at certain speed, and then vibration increases as
engine speed increases.
6. Needle vibrates above and below normal position in narrow
range.
INTAKE MANIFOLD VACUUM
ME-00195
Diagnosis of engine condition by measurement of manifold vacuum
ME(H4SO)-26
Possible engine condition
Leakage around intake manifold gasket or disconnection or
damaged vacuum hose
Back pressure too high, or exhaust system clogged
Leakage around cylinder
Sticky valves
Weak or broken valve springs
Defective ignition system.