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Home >> Subaru >> 2000 >> Outback Base, Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Diagnostic Procedures >> Preliminary Inspection & Adjustments >> Mechanical Inspection >> Intake Manifold Vacuum
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Intake Manifold Vacuum

  1. The manifold vacuum can be checked with a vacuum gauge. Disconnect brake vacuum hose at intake manifold. Install vacuum gauge to manifold.
  2. Keep engine running at idle speed. Read vacuum gauge. Vacuum at idle, with A/C off, should be 17.72 in./Hg (450 mm/Hg). If reading is steady, but lower than normal and becomes more evident as engine temperature increases, check for leaking manifold gasket or damaged vacuum hose.
  3. If engine speed slowly drops or vacuum gauge reading slowly drops after stabilizing, exhaust backpressure is too high or exhaust system is clogged. If vacuum gauge reading drops lower than normal after stabilizing, check for leaks around cylinders. If vacuum gauge reading suddenly and intermittently drops after stabilizing, check for sticking valves.
  4. If engine speed gradually increases or vacuum gauge begins to vibrate rapidly at certain speeds and increases as engine speed increases, check for defective ignition system or improper idle adjustment.
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