How to replace dfi air injectors in a mercury optimax outboard

Written by will charpentier
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When the oil, air and fuel mixture in the direct fuel injection system of your Mercury Marine Optimax outboard motor burns, carbon is left behind on the tips of the air injectors. An improper mix of fuel and oil can cause combustion blowby that leaves streaks of oil on the seals around the air injectors. Both of these conditions merit the boat owner's attention, including replacement of the injectors.

Skill level:

Things you need

  • 5/16-inch open-end, box-end or combination wrench
  • Socket wrench
  • 3/8-inch shallow socket
  • Brass wire brush
  • Teflon O-rings
  • Mercury Marine Teflon ring installation tool
  • Mercury Marine Teflon ring sizing tool
  • Torque wrench

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  1. 1

    Look for the green-coloured fuel vapour purge cap on the fuel line behind the starter's motor. Remove the cap, lay a heavy rag over the valve inside and press the centre of the valve to relieve fuel pressure in the system. Remove the 5/16-inch nut from the negative post of the battery with a wrench and remove the battery cable.

  2. 2

    Remove the two 3/8-inch nuts that hold the fuel rail and injector assembly in place with a socket wrench. Slide the rail off the mounting studs. Take the harness connectors off the direct injectors with a Mercury Marine direct injector removal tool.

  3. 3

    Remove any carbon build-up on the tip of the replacement injector with a brass wire brush. Replace any worn or damaged O-rings on the injector if necessary. Inspect the white Teflon O-ring for dark brown or black streaks, evidence of combustion blowby; replace it, if necessary, with a Mercury Marine Teflon ring installation tool and Teflon ring sizing tool.

  4. 4

    Slide the fuel rail onto the direct injectors and over the mounting studs. Thread the two nuts onto the mounting studs and tighten them to 33 foot-pounds with a torque wrench. Connect the direct injector harness connectors.

  5. 5

    Replace the black ground cable onto the negative post of the battery. Thread the nut onto the battery post and tighten it with the 5/16-inch wrench.

Tips and warnings

  • After completing the repair, pressurise the fuel system and look for leaks.

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