Mercury 40 HP Fuel Pump: Essential Guide to Diagnosis, Repair, and Maintenance

A properly functioning Mercury 40 HP fuel pump is critical for reliable outboard engine performance. If your engine struggles to start, bogs down, loses power under load, or surges erratically, a failing fuel pump is a prime suspect. Understanding how this component works, recognizing the symptoms of failure, knowing how to test it, and mastering replacement and maintenance procedures are essential skills for any owner of a Mercury 40 HP outboard.

This comprehensive guide provides detailed, practical information tailored to the Mercury 40 HP fuel pump. It covers mechanical diaphragm pumps commonly found on older carbureted models and the pulse-driven fuel pumps used in EFI and Optimax models where applicable to this horsepower range. Armed with this knowledge, you can diagnose fuel delivery issues accurately, perform cost-effective repairs or replacements yourself, and implement preventative measures to extend the pump's lifespan, ensuring your outboard runs smoothly season after season.

The Critical Role of the Fuel Pump in Your Mercury 40 HP Outboard

The fuel pump is the heart of the Mercury 40 HP engine's fuel delivery system. Its sole responsibility is to draw gasoline from the fuel tank and consistently deliver it at the correct pressure and volume to the engine's carburetors or fuel injectors. Without this steady flow of fuel, combustion cannot occur efficiently or at all. The pump must overcome the suction required to lift fuel from the tank below deck, counteract the pull of gravity when the boat is on plane, and maintain sufficient pressure to supply the engine's demand across all throttle positions – from idle to wide-open throttle (WOT).

In Mercury 40 HP carbureted models, the pump generates only low pressure – typically between 2 PSI and 6 PSI – sufficient to fill the carburetor float bowls. EFI models require significantly higher pressure, but the core pump assemblies on these 40 HP engines are often still mechanically or pulse-driven. The pump operates continuously whenever the engine is running, responding instantly to changes in throttle position and engine load. Any weakness or malfunction within the pump directly translates into noticeable engine performance problems that worsen over time if left unaddressed. Failure will ultimately lead to an engine that refuses to run.

Decoding Failure: Symptoms of a Bad Mercury 40 HP Fuel Pump

Recognizing the early and advanced signs of a deteriorating fuel pump can prevent inconvenient breakdowns and potential safety hazards on the water. Symptoms often start subtly and progress:

1. Difficulty Starting (Especially Cold): A weak pump may struggle to fill the carburetor bowls or establish sufficient fuel pressure for injection during initial cranking. The engine cranks longer than usual before firing. This is often more pronounced after the engine has sat unused for several days, as fuel pressure may have bled off.
2. Engine Stalling or Hesitation: Intermittent loss of fuel pressure causes momentary starvation, resulting in the engine suddenly dying or hesitating sharply during acceleration. This can feel like a momentary loss of power or a brief "hiccup."
3. Power Loss Under Load: This is a hallmark sign. The engine may run adequately at idle or low RPM in neutral but falters, bogs down, or completely loses power when throttle is advanced and a load is applied (e.g., getting on plane, pulling a skier). The engine feels like it’s starving for fuel because the pump cannot keep up with demand.
4. Engine Surging or Running Rough: Fluctuations in fuel pressure caused by a failing pump diaphragm or worn internal parts lead to unstable engine RPM. The engine may rev up and down erratically without throttle input or run with a consistently rough, uneven idle.
5. Engine Overheating (Rare, but possible): Severely lean fuel mixtures caused by inadequate fuel delivery can cause combustion temperatures to spike, potentially leading to overheating alarms or damage if persistent. This is less common but a serious consequence.
6. Complete Engine Failure: The ultimate stage of pump failure. The engine stops running and cannot be restarted because no fuel is being delivered past the pump. This can happen suddenly or be preceded by worsening versions of the symptoms above. Attempts to restart result only in cranking; no firing.
7. Visual Leaks (Diaphragm Pumps): On mechanical diaphragm pumps, visibly leaking fuel from the pump body, often around the seams or the pulse hose connection, is a definite sign of diaphragm rupture. A strong gasoline smell around the engine compartment is another clue. Caution: Fuel leaks are a fire hazard.

How Your Mercury 40 HP Fuel Pump Actually Works

Understanding the fundamental principles aids diagnosis significantly. Mercury 40 HP outboards predominantly utilize two types of fuel pumps, depending on model year and fuel system type:

1. Mechanical Diaphragm Pump (Common on Carbureted Models):

   • Mounting: Driven directly by the engine, typically mounted on the engine block or intake manifold. It has physical connections: fuel inlet and outlet lines, and a pulse hose.
   • Pulse Drive: The core principle. A small internal hose connects one side of a flexible diaphragm to the engine's crankcase or intake manifold. As the engine runs, alternating pressure and vacuum pulses are generated within the crankcase (due to piston movement) or manifold (due to intake valve action). These pulses act directly on the diaphragm.
   • Pump Action: The crankcase/manifold vacuum pulse pulls the diaphragm inwards, creating suction that draws fuel from the tank through the inlet check valve. The subsequent pressure pulse pushes the diaphragm outwards, forcing the drawn fuel through the outlet check valve towards the carburetor. Two one-way valves (check valves) ensure fuel flows only in the correct direction (Inlet -> Pump Body -> Outlet).
   • Pros: Simple, reliable, no external power required. Output directly relates to engine RPM.
   • Cons: Diaphragms wear or crack over time. Check valves can stick or fail. Pulse passageways can clog.

2. Pulse-Driven Fuel Pump (Common on EFI/Optimax 40 HP Models):

   • Mounting: Often mounted remotely (e.g., near the fuel tank or VST) but still connected to the engine block via a dedicated pulse hose.
   • Pulse Drive: Functions similarly to the mechanical diaphragm pump regarding the pulse source (crankcase or manifold vacuum/pressure). The engine's pulses travel through a dedicated hose to the pump.
   • Internal Mechanism: Instead of the pulse acting directly on the fuel diaphragm, it typically actuates a lever mechanism or a secondary diaphragm that then pumps the fuel. This design allows mounting flexibility and separates the pulse action from the fuel. Most incorporate a vapor separator function. Still self-powered by engine pulses.
   • Output: Generates higher pressure (typically 4-7 PSI for EFI feed pumps feeding a high-pressure pump) or consistent flow. Internal check valves are still critical.

Key Point for 40 HP: While complex multi-stage electric pumps are standard on larger, modern high-pressure EFI outboards, the Mercury 40 HP range typically relies on these mechanically or pulse-driven designs for primary fuel lift and pressure, even on EFI models. Some EFI models might have a small electric primer/lift pump solely for initial prime. Always refer to your specific model’s service manual.

Effective Methods to Test Your Mercury 40 HP Fuel Pump

Diagnosis is critical before replacement. Avoid simply throwing parts at the problem.

1. Visual Inspection & Leak Check: This is always step one.

   • Examine the entire fuel pump body, especially on mechanical/diaphragm types. Look for cracks, significant corrosion on metal bodies, and any visible signs of fuel weeping or drips. Check the pulse hose (if present) for cracks, brittleness, loose connections, or leaks.
   • Check all fuel line connections at the pump (inlet, outlet) for tightness. Ensure all hose clamps (if screw type) are snug. Check the condition of the fuel lines themselves upstream and downstream.
   • Safety First: Perform this inspection with the engine off and well-ventilated. Have a fire extinguisher rated for gasoline and electrical fires (ABC or BC) readily accessible. If a significant leak is found, repair it before proceeding.

2. Fuel Flow/Pressure Test (Most Practical):

   • Disconnect the fuel line after the pump (the line going to the carburetor(s) or VST/injectors). Point the disconnected hose end into a suitable clear container rated for gasoline (capacity at least 1 quart/liter). Ensure the hose end is securely submerged in the gasoline within the container to prevent spraying. Keep the container away from ignition sources!
   • Connect the Mercury Quicksilver Fuel Pump Pressure/Vacuum Tester (or equivalent) to the pump outlet. Alternatively, use a dedicated low-pressure fuel pressure gauge plumbed in. For pulse pumps, ensure the pulse line is connected.
   • Have an assistant crank the engine while you observe. DO NOT run a fuel-injected engine for extended periods without the injectors receiving fuel – consult your manual for safe priming procedures specific to EFI.
   • Observe Flow: A healthy pump should deliver strong, steady spurts of fuel (carbureted) or a continuous stream (EFI pulse pump) into the container. Weak, intermittent, or absent flow indicates pump failure. For pressure, compare measured PSI to factory specifications in your Mercury service manual (common range is 2-6 PSI for carbs, 4-7 PSI for EFI lift pumps).
   • Volume Test (Optional but Recommended): Time how long it takes to fill a known volume (e.g., 1 pint or 0.5 liters) during cranking or brief running. Compare to the spec in your manual (e.g., "Minimum 1 pint in 30 seconds cranking").

3. Vacuum/Pressure Pulse Test (Advanced - Mechanical Pump Specific):

   • Requires: Mercury Quicksilver Fuel Pump Pressure/Vacuum Tester kit.
   • Disconnect the pulse hose from the pump. Connect the tester's "Vacuum/Pulse" port to the pump's pulse port using the correct adapter from the kit.
   • Operate the pump tester according to its instructions. Typically, you pump the tester handle to generate vacuum and pressure. A functional pump diaphragm should hold vacuum and pressure (around 5-7 in. Hg vacuum and 4-7 PSI pressure) without significant drops for at least 10 seconds. A rapid drop indicates a leak in the diaphragm or pump body/gaskets.
   • Check inlet check valve by generating vacuum at the pump inlet – it should hold.
   • Check outlet check valve by generating pressure at the pump outlet – it should hold.

Step-by-Step Guide: Replacing the Fuel Pump on a Mercury 40 HP

Disclaimer: This is a general guide. Always consult the specific Mercury service manual for your engine model and year before starting. Safety is paramount.

Tools/Supplies Needed:

• Mercury Genuine or Quicksilver Fuel Pump Kit specific to your 40 HP model (includes gaskets, diaphragms, check valves). Highly recommended to use OEM.
• Or, Complete Mercury Replacement Fuel Pump Assembly.
• Basic Hand Tools (Screwdrivers, Wrenches, Sockets appropriate for fuel line fittings and mounting fasteners).
• Jabsco or similar Utility Pump for fuel evacuation (optional, but safer).
• Fuel Line Disconnect Tools (If equipped with quick-connect fittings).
• Drain pan suitable for gasoline.
• New Fuel Hose Clamps (if replacing old screw clamps or hose ends look suspect).
• Clean Rags.
• Approved Gasoline Container for waste fuel.
• Safety Glasses & Nitrile Gloves.
• Mercury/Quicksilver 2-4-C Marine Lubricant with PTFE or Permatex Aviation Form-A-Gasket Sealant #3 (non-hardening) for gaskets if recommended in manual.
• Mercury Fuel Line Primer Bulb Kit (recommended to replace if old or during major service).*

Procedure:

1. Preparation & Safety:

   • Work outdoors or in a VERY well-ventilated area away from sparks, flames, heat sources, and electronics that could cause ignition. No smoking!.
   • Disconnect the NEGATIVE (-) cable from the battery terminal.
   • Relieve fuel system pressure: Pinch off or clamp the fuel line between the tank and primer bulb. Pump the fuel primer bulb until it becomes firm (forcing fuel back into the tank). Then, squeeze the primer bulb collar tabs (if quick-connect) and pull the fuel inlet line off the fuel pump OR disconnect the main fuel line at the tank connector. Allow any residual fuel pressure to bleed off slowly into a rag or container.
   • Fuel Removal: Place a drain pan under the work area. If possible, use a jabsco pump to evacuate fuel from the inlet hose and the section between the primer bulb and pump into an approved waste fuel container. Alternatively, be prepared to catch small amounts of fuel leakage when disconnecting lines. Have rags ready.

2. Disconnect Fuel Lines:

   • Clearly label fuel lines: "In" (From Tank/Primer Bulb), "Out" (To Carb/EFI).
   • Use appropriate tools. Disconnect the inlet line from the pump. Immediately plug or cap the open end of this hose to prevent dripping/siphoning. Use small bolts, golf tees, or dedicated fuel line caps.
   • Disconnect the outlet line from the pump. Plug/cap this hose end as well.
   • Disconnect the pulse line (if applicable) from the pump. Plug the open port on the engine block/crankcase (a short bolt of the correct thread pitch works) to prevent debris entry or crankcase pressure loss. Cap the pulse hose end.

3. Remove the Old Pump:

   • Locate and remove the mounting bolts, nuts, or screws securing the pump to the engine block or bracket. Note their positions and any spacers. Keep hardware organized.
   • Carefully lift the pump assembly away. Remove and discard the old gasket(s), O-ring(s), or mounting spacer if used. Thoroughly clean the mounting surface on the engine block using a plastic scraper and brake cleaner or electrical contact cleaner on a rag. Ensure no debris falls into open ports. A small wire brush can help remove stubborn gasket material. Ensure the surface is clean, dry, and smooth.

4. Install the New Pump or Kit:

   • Using a Complete Assembly: Compare the new pump carefully to the old one. Ensure inlet, outlet, and pulse ports align correctly. Install any new gaskets or O-rings supplied with the pump onto the mating surfaces. Lightly lubricate gaskets/O-rings with Mercury 2-4-C or a thin smear of Permatex #3 if specified in your manual. Position the new pump, aligning bolt holes and ports. Hand-start all mounting bolts/fasteners to ensure correct threading. Tighten bolts evenly and gradually in a star pattern to the torque specified in your Mercury service manual (typically in inch-pounds range, e.g., 60-100 in-lbs – avoid over-tightening!). Ensure the pump sits flush without distortion. Never reuse old gaskets.
   • Using a Rebuild Kit: Follow the detailed instructions included in the Mercury/Quicksilver kit meticulously. Lay the old parts out in order of disassembly. Replace diaphragms, valves, gaskets, O-rings exactly as shown. Pay close attention to the orientation of check valve flaps and diaphragms. Ensure valve bodies are clean and undamaged. Reassemble the pump housing carefully. Apply lubricant/sealant to gaskets as directed by kit/manual. Mount the rebuilt pump as described above, torqueing fasteners correctly.

5. Reconnect Lines:

   • Remove the plugs/caps from the inlet line and connect it securely to the pump's inlet port. If using screw clamps, position the clamp over the hose before connecting, push the hose fully onto the barbed nipple (approx. 1-1.5 inches), slide the clamp over the hose barb area, and tighten securely – don't overtighten to cut the hose. For quick-connects, push firmly until you hear/feel a distinct click. Tug firmly to confirm engagement.
   • Repeat for the outlet line. Connect to the pump outlet port securely.
   • Reconnect the pulse line (if applicable) securely to the pump pulse port.

6. Pressure Test the System & Final Steps (CRITICAL):

   • Reconnect the main fuel line to the tank (if disconnected earlier).
   • Double-check ALL fuel connections for tightness. Ensure no lines are kinked or rubbing.
   • Reconnect the battery NEGATIVE (-) cable.
   • Priming: Pump the fuel primer bulb in the fuel line (arrow pointing towards engine) until it becomes firm and solid. This fills the pump and fuel lines.
   • Leak Check: Before starting, carefully inspect EVERY connection you disturbed – inlet, outlet, pulse (if present), mounting surfaces – for ANY signs of fuel weeping. Use a clean rag wiped over connections to detect tiny seeps. Absolutely NO leaks are acceptable.
   • Start the engine (if possible). While running, re-inspect all connections meticulously for leaks. Pay close attention immediately after starting and periodically as the engine warms slightly. Shut the engine down IMMEDIATELY if any leak is detected and rectify the problem.
   • After confirming no leaks, run the engine at varying RPM (in neutral) to check for smooth operation and verify that previous symptoms (rough idle, hesitation, power loss) have resolved. If replacing a failed pump, running characteristics should return to normal.

Selecting the Right Replacement Fuel Pump for Your Mercury 40 HP

Choosing the correct part is crucial for performance and longevity:

1. Use Genuine Mercury or Quicksilver Parts: This is the single most important recommendation. Mercury fuel pump kits (e.g., Mercury Part Number 1395-996K01 for common diaphragm kits) or complete assemblies are engineered specifically for the pressures, pulse characteristics, chemical resistance, and flow rates of your engine. They undergo rigorous testing. Using knock-offs risks premature failure, incorrect pressure, leaks, and potential engine damage. The minor savings are not worth the risk. Purchase from authorized Mercury Marine dealers.
2. Know Your Engine Model & Year: Mercury 40 HP engines span decades, including the classic "Tower of Power" models and newer EFI models like the EFI FourStroke or Optimax. The fuel pump design changes significantly between these types. Serial number lookup on the Mercury Parts Catalog website or consulting with a dealer is essential for finding the exact pump kit (e.g., 8M0105551, 8M0116611) or assembly needed.
3. Kit vs. Assembly:
   • Rebuild Kit (Kit - e.g., 1395-996K01, 1396-9814K01): Contains diaphragms, valves, gaskets, and sometimes springs needed to rebuild the original pump body. Preferred option if the original pump housing is in good condition (no cracks, warping, corroded beyond cleaning). Often the most cost-effective solution. Requires mechanical skill to disassemble/reassemble correctly. Ensure the kit is designed for your specific pump model. Retains the OE pump body.
   • Complete Pump Assembly (Assembly - e.g., 898250T 01, 8M0105551): A brand-new, ready-to-install unit. This is the best option if the original pump body is damaged, heavily corroded, or if you prefer a direct replacement without rebuilding. Ensures all new internals and housing. Faster installation. Ideal for DIYers less comfortable with intricate disassembly. Verify the assembly includes necessary gaskets/mounting hardware or purchase them separately per the assembly instructions/parts diagram.
4. Consider Related Components:
   • Fuel Lines & Primer Bulb: Inspect the entire fuel line from tank to pump and pump to engine. Replace ANY lines showing cracks, brittleness, swelling, or fuel odor permeation. Ethanol fuel degrades old rubber lines. Replace plastic quick-connect fittings if cracked. Install a NEW Mercury/Quicksilver Fuel Primer Bulb Kit during this service – old bulbs crack internally or develop check valve failures, causing problems often misdiagnosed as pump failure.
   • Pulse Hose: If your pump uses a pulse hose, replace it unless it is absolutely new and supple. Age and heat cause rubber hoses to crack internally or leak. Use fuel/oil resistant hose rated for pulse line application. Ensure it’s the correct size.
   • Fuel Filter(s): Replace both the spin-on fuel/water separator filter (if installed) and the small inline filter often found near the fuel pump or carbs/injectors. A clogged filter puts immense strain on the new pump and mimics pump failure symptoms. Use Mercury/Quicksilver branded filters.
   • Fuel Tank, Pickup Tube, Vent: Ensure the fuel tank vent is clear (listen for air rushing in when opening the cap). Verify the anti-siphon valve on the tank outlet (if equipped) isn't sticking closed. Inspect the tank pickup tube screen if accessible. Debris upstream can foul the new pump quickly.

Proactive Maintenance: Extending Your Mercury 40 HP Fuel Pump Life

Fuel pump failures are often preventable with diligent maintenance focused on fuel quality and system integrity:

1. Use High-Quality Fuel & Stabilizer RELIGIOUSLY: This is paramount.

   • Fuel Choice: Use Ethanol-Free Gasoline (89 Octane Minimum) whenever possible. Ethanol (E10) attracts moisture, promotes phase separation, degrades rubber components, and lowers BTU content. If E10 is unavoidable:
     • Use fuel only from high-volume stations with reliable tank turnover.
     • Never use fuel older than 30 days. For boats used infrequently, plan fuel purchases.
   • Fuel Stabilizer: Add a Marine-Specific fuel stabilizer like Mercury Quickstor or Quicksilver Fuel Stabilizer & Conditioner EVERY time you add fuel, year-round. Follow dosing instructions precisely. Stabilizers prevent oxidation and gum/varnish formation inside the pump, carburetors, and injectors, which clogs valves and restricts flow. Stabilizers cannot reverse existing damage; they prevent new deposits.
   • Water Separation: Install a quality Mercury/Quicksilver 10-Micron Fuel/Water Separating Filter (e.g., Quicksilver Part Number 35-8M0118820 or 110-8M0118820) and replace the filter element AT LEAST every 100 hours or annually, regardless of engine hours.

2. Manage Fuel Quantity:

   • Off-Season Storage: Before prolonged storage (winterization), completely fill the fuel tank to its maximum level. A full tank minimizes air space, reducing condensation buildup inside the tank. Adding the correct dose of stabilizer before filling ensures thorough mixing.
   • In-Season: Avoid constantly running with low fuel levels. Keep the tank as full as feasible. Topping off minimizes humid air volume inside the tank that condenses as temperature changes.
   • Fuel Aging: Consume or properly dispose of old fuel. Drain fuel tanks if the engine will be idle for more than 3 months without stabilization treatment.

3. Protect the Fuel System:

   • Regular Filter Changes: Replace your primary fuel/water separator filter (e.g., Mercury 35-8M0089450) annually or every 100 engine hours, whichever comes first. Replace the small inline filter (if equipped) at least every other season or per engine maintenance schedule. Filters are cheap insurance.
   • Clean Connections: Ensure fuel fill caps are tightly sealed to prevent water entry. Inspect fuel vent lines for blockages (insect nests, kinks).
   • Primer Bulb Technique: When priming the system before starting (cold engine), pump the primer bulb firmly until it is completely hard. Avoid excessive or overly aggressive pumping beyond firmness.
   • Fuel Hose Replacement: Replace any degraded fuel lines before they fail internally and contaminate the pump. Use Coast Guard Approved Type A1-15 marine fuel hose designed for ethanol-blended fuels if E10 is used.

4. Avoid Mechanical Stress:

   • Secure all fuel lines and the pump body with appropriate mounts or zip-ties to prevent excessive vibration or movement during engine operation.
   • Ensure fuel lines are routed correctly, avoiding sharp bends or kinks that restrict flow or abrade the line. Maintain clearance from hot engine components.

Troubleshooting Persistent Fuel Delivery Problems

If you've replaced the pump but problems persist, investigate these upstream and downstream culprits:

1. Upstream of the Pump (Obstruction/Supply Issue):

   • Faulty Primer Bulb: Internal valve failure in the primer bulb is extremely common. Replace it.
   • Kinked, Collapsed, or Internally Degraded Fuel Line: Inspect the entire hose from tank to pump inlet. Pinch points occur under deck near clamps or hull chines. Replace aged or suspect hoses. Suction hose collapse is subtle but catastrophic for flow.
   • Clogged Fuel Tank Vent: Listen for a vacuum "whoosh" when removing the fuel fill cap after running. If absent, vent is blocked. Clear the vent fitting or hose. Ensure vent line isn't pinched.
   • Clogged Tank Pickup Tube or Anti-Siphon Valve: Debris blocking the pickup tube screen or a sticking anti-siphon valve restricts fuel draw. Requires accessing the tank outlet fitting.
   • Low Fuel Level: Critically low fuel combined with a pump mounted above the tank level can cause starvation on plane or in turns. Maintain fuel level.

2. Downstream of the Pump (Obstruction/Demand Issue):

   • Clogged Fuel Filters: Did you replace all filters? A clogged primary separator or inline filter downstream of the pump restricts flow.
   • Faulty Fuel Pressure Regulator (EFI models - if equipped): Malfunctioning regulators cause incorrect system pressure.
   • Clogged Carburetor Jets/Float Valve Stuck Closed: Debris or varnish preventing fuel entry into carbs starves the engine regardless of pump output. Requires carburetor disassembly and cleaning.
   • Faulty Injectors or High-Pressure Pump (EFI models): Issues past the low-pressure lift pump (injectors clogged, high-pressure pump weak) cause symptoms mirroring primary pump failure. Requires specialized EFI diagnostics.

3. Air Intrusion: Air leaks in the fuel line or fittings before the fuel pump allow air into the system instead of fuel. Symptoms include erratic operation, vapor lock, and difficult hot restarting.

   • Prime the Bulb: Pump until rock hard. Observe if it gradually softens over minutes/hours while sitting – indicates air leak on suction side. Tighten clamps, replace suspect hoses or O-rings in connectors.
   • Spray Test: With bulb firm, spray soapy water at fittings/hose connections on the suction side (tank to pump inlet). Have an assistant pump the bulb vigorously. Bubbles forming indicate a leak. Replace component/seal.

Conclusion

A failing Mercury 40 HP fuel pump manifests through clear symptoms like hard starting, stalling, power loss under load, and surging. Accurate diagnosis through flow/pressure testing ensures you address the real problem. Opting for Mercury Genuine or Quicksilver fuel pump kits (like 1395-996K01 or model-specific alternatives such as 1396-9814K01) or complete assemblies (e.g., 898250T01) guarantees compatibility and reliability, preventing repeat failures. The replacement process is straightforward with the right tools and safety precautions, primarily involving securing fuel lines, careful mounting, and leak-free reassembly. Prolong your pump's life with consistent use of ethanol-free fuel, marine stabilizers (like Mercury Quickstor), and regular replacement of filters and primer bulbs. Understanding the critical role of the Mercury 40 HP fuel pump and mastering its maintenance is fundamental to achieving dependable performance and maximizing your enjoyment on the water season after season. Always prioritize safety and consult the official Mercury service manual for definitive procedures and specifications related to your specific engine model.