Intake Manifold Vacuum - Subaru Legacy 2002 Service Manual

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5. Intake Manifold Vacuum

A: INSPECTION

1) Warm-up the engine.

2) Disconnect the brake vacuum hose and install

the vacuum gauge to the hose fitting on the mani-

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

4. Needle drops suddenly and intermittently from normal posi-

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

INTAKE MANIFOLD VACUUM

Diagnosis of engine condition by measurement of manifold vacuum

3) Keep the engine at the idle speed and read the

vacuum gauge indication.

By observing the gauge needle movement, the

internal condition of the engine can be diagnosed

as described below.

Vacuum pressure (at idling, A/C "OFF"):

Less than −60.0 kPa (−450 mmHg, −17.72

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 or throttle chamber idle adjustment

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