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High Altitude Control (HAC) Valve (Precis)

  1. Remove HAC valve (located at rear of engine) and inspect for cracks and other damage. Clean HAC valve air filter.
  2. At altitude below 3900 feet. Disconnect vacuum hose from the valve and connect a vacuum pump to the valve lower nipple. Apply vacuum and ensure it bleeds off. Disconnnect vacuum hose from the valve lower nipple and connect vacuum pump to upper nipple. Ensure vacuum holds when applied.
  3. At altitude above 3900 feet, disconnect vacuum hose from HAC valve and connect a hand vacuum pump to the valve lower or upper nipple, ensure vacuum holds when applied. Replace valve if necessary.
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