Mercury 5L8Z-5G232-AA Exhaust Manifold Assembly

Unplug the electrical connector for the oxygen sensor and then remove the oxygen sensor from the exhaust manifold.

Connect a vacuum gauge to an intake manifold vacuum source. Warm the engine to operating temperature, place the transaxle in Park (automatic models) or Neutral (manual models), and apply the parking brake. Note the vacuum reading at idle and jot it down. Quickly open the throttle to near its wide-open position and then quickly get off the throttle and allow it to close. Note the vacuum reading and jot it down. Repeat this test three more times, recording your measurement after each test. If your fourth reading is more than one in-Hg lower than the reading noted at idle, the exhaust system might be restricted, indicating that the catalytic converter could be plugged, or an exhaust pipe or muffler could be restricted.

Although catalytic converters don't break too often, they can become clogged or even plugged up. The easiest way to check for a restricted converter is to use a vacuum gauge to diagnose the effect of a blocked exhaust on intake vacuum.

Use a straightedge and feeler gauge to inspect the exhaust manifold mating surface for warpage. If warpage exceeds limits, replace or resurface the manifold or cylinder head.