Mercury 115 XL Impeller or Full Water-Pump Kit?

Quick Answer: On the Mercury 115 HP FourStroke XL gearcase covered here, remove the trim-tab anode first because it hides an additional lower-unit bolt. After the gearcase is supported and removed, open the water pump and inspect the stainless cup and wear plate before deciding whether the job needs only an impeller or a complete pump kit. An impeller-only service is reasonable only when the housing, cup, plate, key, seals, and gaskets remain serviceable. Plan on roughly 2–3 hours for a careful first attempt, especially with the longer 25-inch driveshaft.

Applies to: Newer Mercury 115 HP FourStroke outboards using the XL 25-inch gearcase arrangement pictured below. Do not order from horsepower and shaft length alone. Confirm the engine serial number and the water-pump diagram because Mercury 115 HP production ranges use different kits.

Should You Replace Only the Impeller or the Complete Water Pump?

Make that decision after opening the pump, not before. Replacing only the impeller saves parts when every reusable surface is smooth and undamaged. A complete kit is the better choice when the stainless cup or wear plate is grooved, the housing is distorted, a gasket tears during removal, the upper seal is hard or cracked, or corrosion has compromised the fasteners.

Run a clean fingernail across the cup and plate. A visible circular mark is not automatically a failure, but a groove you can feel can reduce pump efficiency. Also look for transferred rubber, heat discoloration, sand abrasion, uneven wear, and corrosion around the sealing surfaces.

Pro Tip: Buy against the serial number and compare every new component with the removed stack before installation. JLM lists a complete pump kit for certain 2.1L Mercury 75–115 HP FourStroke engines beginning at serial 2B094996, but that range must match your engine.

What Tools and Parts Do You Need?

• Serial-matched impeller or complete water-pump repair kit — choose after inspecting the housing, cup, plate, seals, and gaskets.
• Socket and wrench set sized to the fasteners actually fitted to your gearcase.
• Torque wrench covering the serial-specific values in the service manual.
• Padded gearcase stand capable of supporting an XL unit vertically.
• Small parts tray for the impeller key, washers, and trim-tab hardware.
• Soft scraper, clean towels, and a small wire brush for external threads.
• Mercury-approved marine lubricant specified for the pump and shaft components.

Start with the Mercury 115 HP parts collection, then narrow the results by engine type and serial range. You can compare impellers and complete assemblies in the Mercury water-pump kit collection.

How Do You Remove the Mercury 115 XL Lower Unit?

Disable the ignition and place the shift system in the position specified by the service procedure you intend to follow. The configuration documented here is removed and reinstalled in forward gear so the propeller can be turned during spline alignment. Keep that position unchanged until the gearcase is back in place.

1. Mark the trim-tab anode position so its steering correction can be restored.
2. Remove the access plug and loosen the long fastener retaining the trim-tab anode.
3. Lift out the anode and remove the lower-unit bolt hidden beneath it.
4. Remove the two side nuts and washers on each side of the gearcase.
5. Support the lower unit before the final retainer releases.
6. Lower the gearcase squarely. Do not pry against a sealing surface or allow the long driveshaft to strike the midsection.

The commonly missed fastener is the bolt under the trim-tab anode. If the gearcase does not separate after the four side nuts are removed, stop and confirm that both the anode and hidden bolt are out. Hammering a retained gearcase can damage the castings.

Pro Tip: XL gearcases are awkward because of their long driveshaft. Use a second person or a stable lifting support if you cannot control the unit through the full drop. Never catch it by the driveshaft, shift shaft, or water guide.

How Do You Open the Water Pump Without Mixing Up the Parts?

Transfer the gearcase to a padded stand and photograph the pump area before disassembly. The tall driveshaft, smaller shift shaft, pump housing, water guide, and surrounding seals must return to their original relationships.

Lift off the water-tube guide and its protective grommet, then clean loose sand and scale away from the pump. Remove the housing fasteners evenly. Lay the upper housing, cup, impeller, key, plate, gaskets, and seals in removal order rather than dropping them into one pile.

Do not reuse a gasket that has separated or torn. Do not scrape the aluminum gearcase with a hard tool. The sealing surfaces must remain flat and free of raised residue.

What Wear Determines Whether You Need a Full Kit?

Inspect the stainless cup where the impeller vanes sweep. Scoring, a distinct wear track, melted rubber, or deformation calls for replacement. Apply the same inspection to the lower plate. The pump cannot maintain its designed clearance when either working surface is badly worn.

Next, flex every impeller vane and examine the roots for cracks. Check that the hub has not separated from the rubber. Inspect the small drive key and its slots in the shaft and impeller. Finally, check the housing, water guide, grommet, face seal, and fastener threads.

If all reusable surfaces are smooth and the removed seals and gaskets remain within specification, an impeller-only repair may be appropriate. When service history is unknown or more than one component is questionable, a complete kit avoids rebuilding a new impeller around worn supporting parts.

How Do You Install the New Impeller?

1. Clean the shaft and pump surfaces without scratching them.
2. Apply a very small amount of the specified marine lubricant to hold the impeller key in its slot.
3. Align the keyway in the new impeller hub and slide the impeller onto the shaft.
4. Lubricate the cup lightly with the approved product; do not pack the housing with grease.
5. Lower the housing while rotating the driveshaft in its normal operating direction so all vanes fold consistently.

Before installing the pump fasteners, confirm that the key stayed engaged and the housing sits flat. Start every fastener by hand, then tighten evenly in stages to the value specified for the exact serial number. “Snug” is not a repeatable torque specification and should not replace the service manual.

How Do You Align a 25-Inch XL Gearcase During Installation?

Reinstall the water guide and protective grommet, then apply the specified lubricant only where the service literature directs. Clean the external fastener threads. Keep grease away from areas where it could block a water passage or alter a dry torque requirement.

Raise the gearcase while independently aligning the driveshaft, water tube, and shift shaft. With the system still in forward gear, slight propeller rotation can help the driveshaft splines engage. The mating surfaces should close by hand; never use a nut to pull a large gap shut.

Install one side nut finger-tight to hold the unit, then start the remaining side hardware and the hidden bolt. Restore the trim-tab anode to its marked position. Tighten all hardware with the sequence and torque values for your serial range.

How Do You Verify the Shift System and Cooling Flow?

Before completing final tightening, verify the shift connection: forward should lock the propeller in the expected direction, neutral should allow free rotation, and reverse should engage cleanly. If the indications are wrong, lower the gearcase and correct the shift-shaft alignment.

Supply adequate cooling water before starting. Mercury warns that operating without sufficient water can damage the pump, engine, and related components. Once started, check for a steady indicator stream and stop immediately if the flow is absent or intermittent.

Mercury's published maintenance schedule for the applicable 75–115 HP FourStroke family calls for impeller replacement every 300 hours or three years, with earlier service if overheating or reduced water pressure occurs. Use the schedule in the manual matched to your serial number as the final authority.

Common Mistakes to Avoid

• Forgetting the lower-unit bolt hidden beneath the trim-tab anode.
• Failing to mark the trim-tab position before removal.
• Ordering a pump kit from “115 HP” or “25-inch shaft” alone.
• Replacing only the impeller despite grooves in the cup or wear plate.
• Losing the small drive key or failing to engage it with the impeller hub.
• Pulling a misaligned XL gearcase into place with the retaining nuts.
• Starting the engine before supplying cooling water.

Where Can You Find Mercury 115 HP Water-Pump Parts?

Frequently Asked Questions

Can you replace only the impeller on a Mercury 115 FourStroke?

Yes, but only when the cup, wear plate, housing, key, seals, and gaskets pass inspection. If either working surface is grooved or the sealing parts are damaged, install the appropriate complete kit rather than placing a new impeller into a worn pump.

Where is the hidden lower-unit bolt on a Mercury 115?

On the arrangement covered here, one lower-unit bolt sits beneath the trim-tab anode. Remove the access plug and long anode fastener, mark and remove the anode, then remove the bolt below it before attempting to lower the gearcase.

Is an XL Mercury outboard a 25-inch shaft?

XL commonly identifies the 25-inch shaft configuration on the engine covered here. Still verify the model designation and serial number because shaft nomenclature alone does not identify the water-pump kit or every gearcase component.

How often should a Mercury 115 impeller be changed?

The referenced Mercury 75–115 HP FourStroke manual lists replacement at 300 operating hours or three years and recommends earlier replacement after overheating or reduced water pressure. Always follow the maintenance schedule tied to your engine serial number.

Why will a Mercury 115 lower unit not slide back into place?

The driveshaft splines, water tube, or shift shaft may not be aligned. On a correctly positioned forward-gear installation, slight propeller rotation can help the splines engage. Do not use the retaining nuts to force the mating surfaces together.