Mercury Quicksilver OEM Fuel Pump Diaphragm Kit 851812A01 | 8 HP 9.9 HP 209cc FourStroke Outboard

Mercury Quicksilver OEM Fuel Pump Diaphragm Kit — Part 851812A01

• Part Number: 851812A01
• Full Catalog Number: 21-851812A01
• Brand: Mercury Marine / Quicksilver (Genuine OEM)
• Official Product Type: Fuel System — Mechanical Fuel Pump Diaphragm Repair Kit
• System: Fuel system — mechanical fuel pump diaphragm service for the 209 cc FourStroke outboard
• Application: 8 HP and 9.9 HP Mercury / Mariner 209 cc FourStroke outboard mechanical fuel pump
• Used With: Complete fuel pump assembly 803529T07
• GTIN / UPC: 745061722613
• Weight: approx. 0.05 kg

Outboard application only: Despite the catalog header, 851812A01 is a Mercury / Mariner outboard fuel pump diaphragm kit — it is not a MerCruiser sterndrive part. Confirm fitment against the 209 cc 8 HP / 9.9 HP FourStroke fuel pump diagram and your outboard serial number before ordering.

What It Does

The 851812A01 is the genuine OEM mechanical fuel pump diaphragm repair kit for the 209 cc FourStroke engine-mounted fuel pump used in selected Mercury and Mariner 8 HP and 9.9 HP FourStroke outboards. It services the diaphragm section of the pump used in the intake manifold and fuel-pump assembly, including the diaphragm itself, check valves, and associated gasket components. If the pump body is sound but the internal diaphragm or valves are worn or damaged, 851812A01 is the correct service kit. If the pump body itself is cracked or heavily corroded, the complete fuel pump assembly 803529T07 may be the appropriate repair path instead.

Compatible Applications

Confirm fitment by outboard serial number and the specific Mercury fuel pump diagram before ordering. Horsepower alone is not sufficient to confirm the correct diaphragm kit — Mercury used different fuel pump designs across its small FourStroke families.

Mercury / Mariner 8 HP FourStroke — 209 cc

• Serial 0P325500 and up
• Serial 0R042475 and up

Mercury / Mariner 9.9 HP FourStroke — 209 cc

• Serial 0P325500 and up
• Serial 0R042475 and up

Not sure it fits? Send us your Mercury or Mariner outboard serial number before ordering.

Kit Contents & Related Fuel Pump Components

Mercury fuel-pump and intake-manifold diagrams show 851812A01 used as the diaphragm service kit for fuel pump assembly 803529T07, alongside the following components. These are sold separately and are not included with the diaphragm kit:

• 803529T07 — Complete fuel pump assembly (use if the pump body itself is damaged)
• 853731003 — Fuel pump diaphragm
• 851812003 — Black check valve (must be returned to correct position)
• 851812004 — White check valve (must be returned to correct position)
• 855569003 — Fuel pump gasket
• 895107 / 895107001 — Fuel pump screws
• 16248 — Fuel filter
• 803513 — Fuel system O-ring

Part Numbers

• 851812A01 — Mercury Marine OEM part number
• 21-851812A01 — Full Mercury / Quicksilver catalog number

No separate superseding Mercury part number is confirmed in current parts references. Order 851812A01 / 21-851812A01.

When to Replace

Inspect or replace the diaphragm kit when:

• The mechanical fuel pump is leaking fuel externally
• The diaphragm is hardened, cracked, or distorted on inspection
• Fuel delivery is inconsistent or the engine shows fuel-starvation symptoms
• The fuel pump is being disassembled for any service
• Age or fuel contamination has degraded the internal diaphragm material

Fuel-delivery issues can also be caused by a clogged fuel filter (16248), deteriorated hoses, tank venting problems, carburetor issues, or failed check valves — inspect those components as part of the diagnosis before replacing the diaphragm kit.

Service Interval

No fixed mileage or hour replacement interval for the diaphragm kit — replace on condition. Inspect the diaphragm and pump gasket whenever the fuel pump is removed or the outboard undergoes annual fuel system service. On engines stored with fuel in the system for extended periods, diaphragm degradation from stale fuel or ethanol is a common finding. Replace the fuel filter (16248) at each annual service regardless of diaphragm condition.

Installation Notes

• Confirm 851812A01 is shown as the correct diaphragm kit for your exact outboard serial number in the Mercury 209 cc FourStroke fuel pump diagram before installing
• Note the orientation and position of the black (851812003) and white (851812004) check valves before disassembly — incorrect valve orientation will prevent the pump from supplying fuel correctly
• Inspect the pump body and gasket sealing surfaces for corrosion or damage before fitting the kit — if the pump body is cracked or the sealing surface is pitted, consider the complete fuel pump assembly 803529T07 instead
• Keep dirt and debris out of the pump during assembly — contamination in the diaphragm chamber will cause premature failure
• Inspect all fuel hoses connected to the pump for cracking, swelling, or fuel staining and replace any that show deterioration
• Follow the Mercury FourStroke service manual for the correct diaphragm and valve orientation, screw sequence, and torque specification
• Verify correct fuel delivery after installation before returning the outboard to service

Technical FAQ

Q: Does this kit fit my 9.9 HP Mercury FourStroke?
It fits selected 9.9 HP Mercury and Mariner 209 cc FourStroke outboards in specific serial ranges (0P325500 and up, 0R042475 and up). Mercury produced other 9.9 HP FourStroke models with different engine displacements and fuel pump designs that use different kits. Confirm the 209 cc engine family and your serial number before ordering.

Q: Should I order the diaphragm kit or the complete fuel pump assembly?
If the pump body is undamaged and the sealing surfaces are in good condition, 851812A01 is the correct service kit for the diaphragm section. If the pump body is cracked, corroded, or otherwise damaged, the complete fuel pump assembly 803529T07 is the appropriate repair. Inspect the pump body while it is off the engine before deciding which part to order.

Q: What causes the diaphragm to fail?
The most common causes on small FourStroke outboards are age, prolonged exposure to ethanol-blended fuel, and storage with stale fuel in the system. A hardened or cracked diaphragm will reduce fuel pump output and can result in lean running, misfiring, or engine stoppage. Extended storage without proper fuel stabilization accelerates diaphragm degradation.

Q: Can I replace only the diaphragm without the check valves?
The Mercury kit 851812A01 is designed to service the diaphragm section of the pump. When the pump is apart, always inspect and replace the check valves (851812003 black and 851812004 white) at the same time — a failed check valve will cause the same fuel-delivery symptoms as a failed diaphragm, and replacing only the diaphragm while leaving aged valves in place is a common cause of repeat fuel pump issues.

Parts Desk Insights

The most common issue with 851812A01 orders is customers ordering it based on horsepower alone without confirming the 209 cc engine displacement and serial number. Mercury offered 9.9 HP FourStroke outboards in more than one displacement, and the fuel pump kits are not interchangeable. Always confirm the 209 cc displacement and the serial range before processing the order. The other frequent issue is incorrect check valve orientation during reassembly — the black and white valves are not interchangeable in position, and installing them backwards will result in a pump that cannot deliver fuel. If a customer has just rebuilt their pump and still has fuel-starvation symptoms, check valve orientation is the first thing to verify.