How to Replace a MerCruiser Impeller

Written by jared curtis
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How to Replace a MerCruiser Impeller
The impeller keeps the engine cool. (boat image by Byron Moore from

The impeller is located inside the lower unit of your MerCruiser out-drive. It rotates and pulls water into the engine to cool it down. It is an important part of the motor. A faulty impeller will cause your engine to overheat, which can cause severe damage to the internal components of the engine. Replace impellers every season to ensure a well-running boat. Save money by repairing the impeller yourself, using simple tools you already have.

Skill level:

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Things you need

  • Propeller wrench
  • Drain bucket
  • Ratchet
  • Socket set
  • Flathead screwdriver
  • Gear lube

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  1. 1

    Place the throttle into forward gear on the lever near the steering wheel.

  2. 2

    Remove the propeller using a propeller wrench. Set it aside.

  3. 3

    Place a drain bucket under the out-drive. Using a ratchet and socket, remove the gear lube drain plug located directly under the out-drive and allow the oil to drain from the out-drive. Reinstall the drain plug.

  4. 4

    Remove the zinc plate by placing a ratchet into the hole on the upper portion of the out-drive and rotating it counterclockwise to remove it. The zinc plate is located in front of the propeller.

  5. 5

    Remove the four bolts that hold the upper case of the out-drive to the lower case using a ratchet and socket. These are along the edge of the lower case. Remove the lower case and set it onto a work bench.

  6. 6

    Remove the four bolts that secure the impeller housing to the lower case using a ratchet and socket. The housing is located at the end of the drive gear shaft. Remove the housing from the drive gear shaft by sliding it off of the shaft.

  7. 7

    Remove the gasket that is connected to the housing using a flathead screwdriver. Ensure that the mating surfaces are clean and free from debris.

  8. 8

    Remove the old impeller from the housing and set the new one into the housing. Install the newly supplied gasket to the housing.

  9. 9

    Reinstall the impeller housing onto the lower case. Reinstall the lower case to the upper case. Tighten all the bolts.

  10. 10

    Refill the gear lube in the out-drive fill plug located on the side of the out-drive. Reinstall the propeller using the propeller wrench.

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