How to Install a Mercruiser Impeller

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The impeller in your Mercruiser engine is located inside the lower unit of the outdrive. The lower section must be removed to access the impeller. It attaches to the drive shaft and spins while the engine is running. This action pushes water from the lake into the engine to keep it cool. Replacement impeller kits are found at your local marine shop. Purchase new gear oil to fill the outdrive after replacement.

Remove the boat from the water either on a trailer or a hoist.

Turn the engine off and place the throttle in forward gear.

Lower the outdrive by pressing the button on the throttle. Avoid hitting the ground with the outdrive.

Locate the drain screw on the lower cone of the outdrive near the rear. Locate the vent screw on the top section of the outdrive near the "Mercruiser" sticker. Both screws are flathead.

Place a drain bucket under the drain screw. Loosen and remove the drain screw using a flathead screwdriver. Loosen the vent screw with the flathead screwdriver and allow the gear lube to drain from the outdrive.

Remove the zinc trim tab from the outdrive by inserting a 3/8-inch Allen wrench into the hole on the top side of the outdrive and removing the bolt. Remove the bolts on the back of the outdrive and the two bolts on the front side using a 9/16-inch open end wrench. Remove the bolt that is located under the zinc trim tab with a 9/16-inch open end wrench. Carefully remove the lower unit from the upper. Avoid damaging the spindles on the drive shaft while removing.

Locate the impeller housing on the tops section of the lower unit. The drive shaft extends outward from the impeller housing. Remove the four bolts that secure the housing with a wrench and remove the housing from the lower unit to access the impeller. Remove the impeller and gaskets from the drive shaft by sliding them upward.

Place the new impeller and gaskets in the same direction as the impeller that was removed onto the drive shaft. Reinstall the impeller housing and tighten the bolts with a 10 mm open-end wrench.

Reinstall the lower unit onto the upper unit by lifting it into position. Install and tighten the bolts with a wrench. Reinstall the zinc trim tab with a 3/8-inch Allen wrench.

Attach the gear lube pump onto the bottom fill hole in the outdrive. Screw the gear lube pump onto a new can of gear oil and pump the oil into the outdrive until oil seeps from the vent hole on the upper section of the outdrive.

Tighten the vent hole screw with a flathead screwdriver and remove the gear lube pump end on the lower drain plug. Insert the drain plug screw and tighten it using a flathead screwdriver.

Start the engine and confirm that the engine does run excessively hot.


  • Do not start the engine without gear oil in the outdrive.


  • An assistant will make reinstallation of the lower unit much easier.


About the Author

Living in Utah, Jared Curtis graduated in 2005 with a Bachelor of Science degree from Utah Valley University in Orem, Utah. Curtis is continuing his education in hard sciences to apply to medical school in the future. He began writing professionally in 2010, specializing in cabinet-related articles.

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