Intake Manifold Vacuum - Subaru 2009 IMPREZA Service Manual

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5. Intake Manifold Vacuum

A: INSPECTION

1) Warm up the engine.

2) Disconnect the brake booster vacuum hose from

the intake manifold, and install the vacuum gauge.

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

uum gauge indication.

By observing the vacuum gauge needle move-

ment, the internal condition of the engine can be di-

agnosed as described below.

Intake manifold vacuum (at idling, A/C OFF):

–60.0 kPa (–450 mmHg, –17.72 inHg) or more

Diagnosis of engine condition by measurement of intake manifold vacuum

Vacuum gauge indication

1. Needle is steady but lower than normal position. This tendency

becomes more evident as engine temperature rises.

2. Needle intermittently drops to position lower than normal position. Leakage around cylinder

3. Needle drops suddenly and intermittently from normal position.

4. When engine speed is gradually increased, needle begins to

vibrate rapidly at certain speed, and then vibration increases as

engine speed increases.

5. Needle vibrates above and below normal position in narrow range. Defective ignition system

Intake Manifold Vacuum

Air leakage around intake manifold gasket, disconnection or

damage of vacuum hose

Sticky valve

Weak or broken valve springs

Possible engine condition