How to Remove a 2013 Mercury 90 HP Lower Unit for Impeller Service

Quick Answer: To remove the lower unit on the 2013 Mercury 90 HP FourStroke configuration covered here, place the control in neutral, support the gearcase, remove four 16 mm side nuts and the 15 mm underside bolt, then lower the gearcase without forcing it. Once it is on a proper padded stand, remove the four 10 mm water-pump fasteners, inspect the housing, impeller, key, wear surfaces, and upper driveshaft seal, and reassemble with the water tube and shift system correctly aligned. Allow about 90–150 minutes for a first careful attempt. Never run the engine without an adequate water supply.

Applies to: The 2013 Mercury 90 HP FourStroke gearcase and water-pump arrangement pictured below. Mercury has used more than one 90 HP FourStroke design, so confirm the serial number, gearcase diagram, and kit contents before ordering parts. The fastener count and socket sizes in this guide apply to the configuration shown; they are not a universal specification for every Mercury 90.

What Should You Confirm Before Removing the Lower Unit?

Start with the engine upright, the ignition disabled, and the remote control in neutral. Neutral matters twice: it establishes the shift position before separation and gives you a known position when the gearcase goes back on. If the gearcase is difficult to reinstall, do not move the control randomly or pull the unit together with the nuts.

Look along both sides of the anti-ventilation plate before turning a wrench. This configuration uses two retaining nuts per side plus one bolt from underneath. Photograph the washer arrangement and note where each fastener came from.

Pro Tip: Place a padded support directly under the skeg before the last fastener comes free. The gearcase is manageable, but the driveshaft makes it tall and awkward; a controlled drop can bend or damage components.

What Tools and Parts Do You Need?

• Serial-number-matched water-pump kit — compare the new impeller, key, seals, gaskets, plate, and housing parts with the removed components before assembly.
• 16 mm ratcheting wrench — for the four side retaining nuts on this gearcase.
• 15 mm socket and ratchet — for the underside retaining bolt.
• 10 mm socket — for the four water-pump housing fasteners shown here.
• Padded gearcase stand — holds the unit upright without loading the skeg or painted housing.
• Rubber-safe assembly lubricant — helps the driveshaft seal pass over the shaft without tearing.
• Clean towels, a parts tray, soft scraper, and a torque wrench covering the values in your serial-specific service literature.

Use the Mercury 90 HP parts collection to narrow the search by engine, then compare your serial number with the listing. For pump components, browse the Mercury water-pump repair kit collection; do not select a kit from horsepower alone.

How Do You Release the Five-Fastener Gearcase?

1. Keep the control in neutral and support the lower unit from below.
2. Loosen the two 16 mm nuts on one side and the two matching nuts on the other side. Capture the washers rather than letting them fall.
3. Remove the single 15 mm bolt accessed from underneath.
4. Hold the gearcase with both hands and pull it straight down. A small gap should open at the mating surface.
5. If it does not release, stop and check for a missed fastener or corrosion. Do not drive a screwdriver into the mating surfaces.

As the gearcase lowers, guide the driveshaft clear and watch the water-tube connection. Do not let the unit hang from any tube, cable, or linkage. Once free, move it immediately to a padded stand.

The complete gearcase layout is easier to understand on the stand: the long driveshaft rises from the pump, the water-tube guide sits beside the pump assembly, and the shift components must remain in their original position.

How Do You Open and Inspect the Water Pump?

Before disassembly, remove loose dirt from the top of the gearcase so it cannot fall into the pump. The housing on this unit is retained by four 10 mm fasteners. Loosen them evenly, lift the upper pieces in order, and lay every component on a clean towel in the same orientation in which it came off.

Inspect more than the impeller. Check the housing and insert for scoring or melted rubber, inspect the plate and gaskets, and look for cracks, hardening, or distortion in the rubber seal above the housing. A flexible impeller can still be worth replacing when its service history is unknown, because the lower-unit labor is already complete.

Pro Tip: Account for the small impeller key as soon as the impeller comes off the shaft. Put it in a parts tray. Without that key, the driveshaft can turn while the impeller does not pump water.

How Do You Install the Impeller Without Damaging It?

Clean the pump surfaces without gouging them, then compare old and new parts. Install the key in its shaft location and align the new impeller hub with it. Ease the impeller into the housing while turning the driveshaft in its normal operating direction so every vane bends consistently.

Do not force the housing down over crossed or reversed vanes. Check that the housing seats flat, the key remains engaged, and no gasket has shifted. If the kit includes an upper driveshaft seal and the old seal is cracked or hardened, replace it now. Apply only a light film of rubber-safe lubricant and protect the sealing lip as it passes over the driveshaft splines.

Start all four housing fasteners by hand. Tighten them evenly in stages so the housing does not cock to one side. Use only the torque value specified for your exact serial number; the 10 mm socket size identifies the bolt head, not the tightening torque.

How Do You Reinstall the Lower Unit?

1. Confirm that the remote control and gearcase are still in neutral.
2. Position the gearcase under the midsection and guide the driveshaft upward.
3. Align the water tube with its guide and keep the other tubes and shift components in their proper paths.
4. Raise the unit squarely. If the driveshaft splines do not engage, slightly rotate the propeller by hand while maintaining the selected gear position.
5. When the mating surfaces meet by hand, start the underside bolt and all four side nuts with their washers.
6. Tighten the fasteners in stages using the serial-specific sequence and torque values.

Never use the fasteners to close a large gap. Resistance usually means the driveshaft, shift connection, or water tube is not aligned. Pull the unit back down, find the obstruction, and try again.

How Do You Test the Repair?

Before starting, reconnect anything disabled for service and supply cooling water to the intake. Mercury warns that insufficient cooling water can damage the engine, pump, and other components. Start in neutral and check promptly for a steady stream from the water-pump indicator.

If no stream appears, stop the engine. Check the water supply and intake first; if those are clear, investigate a missed water-tube connection, pump assembly problem, or cooling-system blockage. Do not keep running the engine to see whether the flow improves.

Common Mistakes to Avoid

• Ordering by “2013 Mercury 90 HP” alone. Year and horsepower do not replace a serial-number check.
• Missing the underside bolt. Forcing the gearcase with a fastener installed can damage the castings.
• Clamping directly on the skeg. Use a purpose-built padded support for the gearcase.
• Losing the impeller key. The pump cannot work correctly without positive engagement between shaft and impeller.
• Mixing vane directions. Fold every vane consistently as the housing goes on.
• Pinching the water tube during installation. The gearcase should seat without being pulled together by its fasteners.
• Guessing torque from socket size. Find the correct value in the manual for your serial range.

Where Can You Find the Correct Mercury 90 HP Parts?

Frequently Asked Questions

How many fasteners hold this 2013 Mercury 90 HP lower unit?

The configuration documented here has five primary retainers: four 16 mm side nuts and one 15 mm underside bolt. Other Mercury 90 HP gearcases may differ. Inspect your unit and consult its serial-specific diagram rather than assuming the same layout.

Does the Mercury 90 HP need to be in neutral for lower-unit removal?

Neutral provides a known shift position for removal and reinstallation on this procedure. Keep both the control and gearcase in that position. If the lower unit will not seat, verify alignment instead of changing the control repeatedly or drawing the joint closed with the nuts.

Should you replace an impeller that still looks good?

Replacement can be sensible when the service history is unknown or the lower unit is already removed. Mercury's later 75–115 HP FourStroke maintenance literature lists impeller replacement at 300 hours or three years, with earlier replacement if overheating or reduced water pressure occurs. Confirm the schedule in the manual for your exact serial number.

Why is the Mercury 90 HP telltale weak after an impeller change?

First stop the engine and verify the intake has an adequate water supply. A weak or absent stream can also result from a blocked indicator passage, a misaligned water tube, an incorrectly assembled pump, or another cooling-system restriction. Do not diagnose by running an overheating engine.

Can you buy a Mercury 90 HP water-pump kit by year?

No. Use the engine serial number and the pump diagram because Mercury 90 HP FourStroke designs and kit applications span different production ranges. Verify the impeller, key, housing, seals, plate, and gasket shapes against the parts listing before installation.