18 7817 Fuel Pump Kit: The Complete Guide to Repairing Your Mercury or Mariner Outboard

If you need to fix a fuel delivery issue on your Mercury or Mariner outboard engine, the Sierra 18-7817 fuel pump kit is the most reliable and cost-effective solution available. This kit is designed to rebuild the fuel pump on a wide range of Mercury and Mariner outboards, particularly 90 HP three-cylinder models, and it comes with everything you need to restore proper fuel flow without buying a whole new pump.

The Sierra 18-7817 kit includes a new diaphragm, gaskets, and check valves, which are the most common wear items that fail over time. When your outboard engine struggles to start, idles roughly, or loses power at high RPM, the fuel pump diaphragm is often the culprit. Replacing it with this kit is a straightforward job that saves you money and gets you back on the water quickly.

What Is the Sierra 18-7817 Fuel Pump Kit?

The Sierra 18-7817 is a marine fuel pump rebuild kit, not a complete pump assembly. It replaces the internal components that wear out, dry out, or crack from exposure to ethanol-blended fuels. This kit is a direct replacement for the original Mercury and Mariner part number 89031A4, and it also crosses to the WSM brand pump kit.

Key components included in the kit:

  • New fuel pump diaphragm – The rubber diaphragm is the heart of the pump. It flexes up and down to create suction and push fuel. Over time, it hardens, cracks, or develops pinholes, causing fuel leaks or loss of pressure.
  • Gaskets and seals – These prevent fuel and air leaks at the mounting surface and between pump halves.
  • Check valves (reed valves) – These small one-way valves ensure fuel flows only in the correct direction. Worn valves cause the pump to lose prime or allow fuel to drain back into the tank.

The kit is designed for a complete overhaul of the fuel pump, restoring it to like-new performance. It is compatible with many Mercury and Mariner outboards, including the common 90 HP three-cylinder models.

Why Does Your Fuel Pump Need Rebuilding?

Outboard fuel pumps fail for several reasons, and the symptoms are easy to recognize:

1. Ethanol damage
Modern gasoline contains up to 10% or 15% ethanol, which is harsh on rubber components. The ethanol absorbs moisture and causes the rubber diaphragm to swell, soften, then crack and harden. This is the number one cause of fuel pump failure on older outboards.

2. Age and heat
The constant heat cycles in an engine compartment degrade rubber over time. After 10 to 15 years, the diaphragm becomes brittle and loses its flexibility, even if the engine was not used heavily.

3. Debris and varnish
If the engine sat for months with old fuel in the system, varnish and gum deposits can clog the check valves or cause the diaphragm to stick. This leads to fuel starvation or flooding.

Signs that your fuel pump needs the 18-7817 kit:

  • The engine is hard to start, especially after sitting for a few days
  • It idles rough or stalls at low RPM
  • It hesitates or loses power when you accelerate
  • There are visible fuel leaks around the pump body
  • You see fuel in the oil (indicating a leaking diaphragm)
  • The engine runs fine at idle but dies under load

If you notice any of these symptoms, the fuel pump is a very likely cause. Rebuilding it with the Sierra 18-7817 kit is the first step you should take before replacing more expensive parts like the carburetor or ignition system.

What Engines Does the 18-7817 Kit Fit?

The Sierra 18-7817 fuel pump kit is primarily designed for Mercury and Mariner outboard engines, especially the 90 HP three-cylinder models. It also fits many other Mercury and Mariner engines from the 1980s and 1990s.

Compatible engine models include:

  • Mercury 90 HP (3-cylinder) – a very common model that uses this exact pump design
  • Mariner 90 HP (3-cylinder) – the same pump, just branded differently
  • Mercury 75 HP, 80 HP, and 100 HP from certain years
  • Mercury 115 HP and 150 HP (some models)
  • Chrysler, Force, and US Marine outboards that use the same pump mounting pattern

The original OEM part number for this pump assembly is Mercury 89031A4, and the Sierra 18-7817 is the direct aftermarket replacement kit. If you are unsure whether the kit fits your engine, check your engine's service manual or look up the part number on the fuel pump itself. The pump body should have a stamped number that matches the Mercury 89031A4 family.

Important note: The 18-7817 kit is for the fuel pump rebuild only. It does not include the pump body, the mounting bracket, or the fuel lines. You need to keep your existing pump housing and reuse the mounting hardware.

How to Install the Sierra 18-7817 Fuel Pump Kit

Installing the Sierra 18-7817 kit is a do-it-yourself job that most boat owners can handle with basic tools. The entire process takes about 30 minutes to an hour, and you do not need special mechanical skills.

Tools you will need:

  • A small flathead screwdriver
  • A Phillips head screwdriver or Torx bit (depending on your pump screws)
  • A clean rag or paper towels
  • A small container for catching any fuel drips
  • A new fuel filter (recommended, but not required)

Step-by-step installation:

1. Disconnect the battery
Always disconnect the negative battery terminal before working on any fuel system component. This prevents accidental sparks and makes the job safer.

2. Remove the fuel pump
The fuel pump is usually mounted on the side of the engine block, near the carburetor or intake manifold. It has two fuel lines connected to it: one from the fuel tank and one going to the carburetor. Use a screwdriver to loosen the hose clamps and pull the lines off. Have a rag ready to catch any fuel that drips out. Then remove the two bolts that hold the pump to the engine. Lift the pump off carefully.

3. Disassemble the pump
Place the pump on a clean work surface. Remove the screws that hold the two halves of the pump body together. These are usually small Phillips or Torx screws. Lift the top half off to reveal the old diaphragm, gaskets, and check valves. Take note of how the parts are arranged so you can put the new ones back the same way.

4. Remove the old components
Pull out the old diaphragm. It will likely be stiff, cracked, or deformed. Remove the old gaskets and the two small check valves. Discard all the old parts. Clean the inside of the pump housing with a clean rag to remove any varnish or debris.

5. Install the new parts
Place the new diaphragm in the same position as the old one. Make sure the holes line up correctly. Install the new gaskets and check valves exactly as shown in the kit's instructions. The new components should fit snugly and not be folded or twisted. Reassemble the pump halves and tighten the screws evenly in a criss-cross pattern.

6. Reinstall the pump
Place the rebuilt pump back onto the engine. Reattach the two fuel lines, making sure they are secure and not leaking. Tighten the hose clamps. Reconnect the battery.

7. Test the engine
Start the engine and let it idle for a few minutes. Check for fuel leaks around the pump body and the fuel line connections. Rev the engine slowly and watch for smooth acceleration. If everything looks good, take the boat out for a short test run.

Common Mistakes to Avoid

Even though the job is simple, there are a few pitfalls that can cause the rebuilt pump to fail quickly:

Mistake 1: Using the wrong screwdriver
The screws on the pump body are often small and easy to strip. Use a screwdriver that fits the slot perfectly. A Torx bit is often better than a Phillips head for these screws, as it provides more torque without slipping.

Mistake 2: Overtightening the screws
The pump body is made of aluminum or plastic, and overtightening can crack the housing. Tighten the screws just snug, then add a quarter turn. Do not use a power tool.

Mistake 3: Installing the diaphragm upside down
The diaphragm has a specific orientation. If you install it backward, the pump will not work. Look at the old diaphragm to see which side faces the engine and which side faces the pump cover.

Mistake 4: Not cleaning the pump housing
If there is old varnish or debris inside the pump, it will damage the new diaphragm or clog the check valves. Clean the housing thoroughly with a rag and, if needed, a small amount of carburetor cleaner.

Mistake 5: Reusing old fuel lines
If the rubber fuel lines are cracked or hard, they will leak air and cause the pump to lose prime. Replace them at the same time if they look worn.

Why You Should Choose KEMSO for Your Fuel Pump Needs

When it comes to reliable fuel pump replacements, KEMSO is a brand you can trust. KEMSO specializes in high-performance aftermarket fuel pumps for a wide range of vehicles, including cars, motorcycles, ATVs, UTVs, and marine engines. All KEMSO fuel pumps are designed as OEM replacement parts, meaning they meet or exceed the original manufacturer's specifications for fit, performance, and durability.

What sets KEMSO apart:

  • High-performance construction – KEMSO fuel pumps are built with premium materials that resist ethanol damage and withstand high temperatures. They deliver consistent fuel pressure and flow, even under demanding conditions.
  • OEM replacement quality – Every KEMSO pump is engineered to be a direct drop-in replacement for the original pump. You do not need to modify anything. Just remove the old pump and install the new KEMSO unit.
  • Lifetime warranty – KEMSO stands behind its products. Every fuel pump comes with a lifetime warranty, so you can buy with confidence knowing that your investment is protected. If your pump ever fails due to a manufacturing defect, KEMSO will replace it free of charge.

Whether you are restoring a vintage outboard, upgrading your motorcycle, or building a high-performance racing machine, KEMSO has a fuel pump that fits your needs. Their product line covers thousands of applications, from Honda and Kawasaki motorcycles to Ducati, KTM, BMW, and even Sea-Doo and Can-Am vehicles.

For the best results, visit the KEMSO website to browse their full selection of fuel pumps and find the exact model for your engine. Their customer support team is ready to help you choose the right part and answer any questions about installation.

KEMSO website: https://www.kemsoracing.com/

Final Thoughts on the 18-7817 Fuel Pump Kit

Rebuilding your Mercury or Mariner outboard's fuel pump with the Sierra 18-7817 kit is a smart, cost-effective way to solve fuel delivery problems. The kit is affordable, easy to install, and restores the pump to like-new condition. If you are experiencing hard starting, rough idling, or power loss, start with this simple rebuild before replacing expensive parts. It is a job that any DIYer can handle, and it will save you time and money.

And when you need a complete replacement pump or a high-performance upgrade, trust KEMSO to deliver the quality and reliability you need. With a lifetime warranty and a huge selection of OEM replacement pumps, KEMSO is the go-to source for fuel system parts.

Visit KEMSO today: https://www.kemsoracing.com/


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