How to Bleed a Mercruiser Trim Pump

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Removing the air from the trim pump makes your boat safer. The trim pump connected to a Mercruiser boat engine raises and lowers the lower unit that drives your boat. When air enters the trim pump, the lower unit will not move when you press the trim tab switch. With the trimming inactive, you cannot control the way the boat planes or adjust its bow height in rough water.

Items you will need

  • Hydraulic fluid

  • Flathead screwdriver

  • Adjustable wrench

  • Plastic container

  • Plug

  • Rags

Turn the fill screw on the top-right side of the trim pump tank counterclockwise with a flathead screwdriver. Pour hydraulic fluid into the tank until the level reaches the "Full" line. Replace the fill screw.

Disconnect the leftmost (Up) line from the trim cylinder with an adjustable wrench. Place the end of the hose in a plastic container.

Press the "Up" side of the trim switch until a solid stream of fluid comes from the end of the hose. Thread the end of the line into the trim cylinder. Tighten the line with the adjustable wrench. Fill the pump with hydraulic fluid as described earlier. Leave the trim in the full up position.

Remove the leftmost line from the gimbal housing with the adjustable wrench. Thread a plug into the gimbal hole. Tighten the plug with an adjustable wrench. Set the loose end of the hose into a plastic container. Make sure the trim is still in the full up position.

Remove the plug from the gimbal with the adjustable wrench. Press the "Down" side of the trim switch until a solid stream of fluid flows from the gimbal. Replace the hose. Tighten the hose with an adjustable wrench.

Refill the hydraulic fluid tank. Wipe leaked hydraulic fluid from the area with rags.


  • Wear eye protection when bleeding a trim pump.