Mercruiser 5.7 Fuel Pump Location: Your Complete Guide

The fuel pump on a Mercruiser 5.7L engine (also known as the GM 350 V8) is located on the engine itself. However, its exact position and type (mechanical or electric) depend critically on the year of your engine and whether it has a carburetor, throttle body injection (TBI), or multiport fuel injection (MPI). For most common carbureted and early TBI Mercruiser 5.7 engines, the mechanical fuel pump is typically mounted low on the port (left) side of the engine block, driven by the camshaft. Later models with TBI or MPI use an electric fuel pump mounted inside the fuel tank. Knowing your engine's specific year and fuel system is essential to locating the correct fuel pump.

Why Finding the Fuel Pump Matters
A malfunctioning fuel pump is a frequent culprit behind Mercruiser 5.7 engines that crank but won't start, start then stall, struggle at higher RPMs, or lack power. Quickly identifying where to check or replace it saves valuable diagnostic and repair time, especially when you need your boat ready for the water.

Detailed Fuel Pump Locations by Mercruiser 5.7 Type & Year

  1. Carbureted & Early TBI Models (Typically Pre-1991 - Pre-2001 Varies):

    • Pump Type: Mechanical.
    • Precise Location: Mounted directly to the engine block. Look low on the port (left) side of the block (when facing the stern from behind the engine), usually below and slightly behind the distributor or ignition module. It will be near where the mechanical fuel lines run along the block.
    • How it Works: An arm inside the pump is actuated by an eccentric lobe on the engine's camshaft. This creates suction to draw fuel from the tank and pressure to push it to the carburetor or TBI unit.
    • Key Identification: Has two threaded ports for fuel lines (IN from tank, OUT to carb/TBI) and a small vent/drain tube nipple on the bottom. Fuel lines are rigid or flexible rubber lines directly connecting the pump to the fuel system components.

  2. Throttle Body Injection (TBI) Models (Approx. Late 1980s/Early 1990s - Early 2000s):

    • Note: This is a transition period. Location depends significantly on the exact year and model.
    • Possibility 1 (Common): Mechanical Pump: Many early TBI systems retained the mechanical pump mounted on the engine block (location identical to #1 above). This pump supplied pressurized fuel to the TBI unit on the intake manifold.
    • Possibility 2 (Less Common on Base Models): Tank-Mounted Electric Pump: Some Mercruiser TBI systems, particularly in later years or higher-performance variants, switched to an in-tank electric pump. This started becoming more standard as TBI evolved into MPI.
    • Key to Identification: Visually check for the presence of a mechanical pump on the port side engine block first. If absent, the pump is almost certainly inside the fuel tank. You might hear a distinct "whirring" or "buzzing" sound from the tank area for 1-2 seconds when turning the ignition key to the "ON" position before cranking.

  3. Multiport Fuel Injection (MPI) Models (Approx. Early 1990s - Present):

    • Pump Type: Electric Fuel Pump.
    • Location: Mounted INSIDE the boat's main fuel tank. This is the standard for modern EFI systems, including the Mercruiser 5.7L MPI.
    • Why Inside the Tank: Submerging the pump in fuel provides excellent cooling (preventing vapor lock) and lubrication, significantly increasing its lifespan. It also allows for the much higher fuel pressure (typically 30-60 PSI) required for efficient fuel injection compared to carburetion or TBI.
    • Access: Requires removing the fuel tank sending unit/pump access plate (usually a large round cover secured with small screws) located on the top surface of the fuel tank. Often found in the cockpit deck, under a hatch, or occasionally in a compartment accessible from inside the cabin. The pump and sending unit are usually integrated into a single module assembly pulled out through this access hole. There is NO PUMP mounted on the engine block itself for MPI models.

Carbureted Engine Fuel Pump Location Visual

Engine Block Mounted (Mechanical)

  • Side: Port (Left) Side.
  • Height: Low on the block.
  • Relationship to Distro: Below and slightly behind the distributor/ignition module.
  • Fuel Lines: Has two fuel line connections (typically 3/8" IN from tank, 5/16" or 3/8" OUT to carb). Small drain/vapor tube on bottom.
  • Actuator Arm: Visible where it mounts to the block (pushes against the cam).
  • Mounting Bolts: Usually two bolts securing it vertically to the block.

TBI Engine Fuel Pump Location Clarification

  • Check Block First: Physically look for the mechanical pump in the carbureted location (Port side, low). Its presence confirms a mechanical pump system.
  • No Block Pump? If no pump is visible on the engine block, listen intently near the fuel tank when turning the ignition key to "ON" (without cranking). A brief (1-2 sec) buzzing/humming confirms an electric in-tank pump. If silent, further electrical troubleshooting is needed, but the pump location is still inside the tank.
  • Pressure Requirements: TBI often required slightly higher pressure than carbs, making electric pumps more desirable and common later in the TBI era.

MPI Engine Fuel Pump Location Details

  • Access Plate Location: Essential to find this first. Look for:
    • A ~4-6 inch diameter circular plate secured by 6-10 small screws.
    • Located directly on the top of the fuel tank.
    • Common locations: Cockpit deck (center console, near helm, under seats), ski locker floor, cabin sole, engine compartment sole. May be hidden under carpet or a removable hatch.
  • Module Assembly: Removing the access plate reveals electrical connectors and large fuel line quick-connect fittings. Disconnecting these allows the entire pump/sender module to be lifted straight up and out of the tank. The fuel pump is the cylindrical component attached to the bottom of this assembly.
  • No Engine Block Component: Critically, there will be no device on the engine block performing a primary fuel pumping function for MPI models. Any pump-like device near the engine (if present) would be a fuel pressure regulator, not the pump itself.

Important Considerations When Dealing with Your Fuel Pump

  • Safety First! Fuel systems are inherently dangerous. ALWAYS:
    • Work in a well-ventilated area, outdoors is best.
    • Disconnect the negative (-) battery cable BEFORE starting ANY work.
    • Relieve fuel system pressure using the appropriate method for your system (consult manual: often involves pulling a fuse/relay and running the engine dry, or using a pressure relief valve).
    • Have a certified fire extinguisher (Type B) immediately on hand.
    • Avoid sparks, flames, or creating arcs (like disconnecting connectors under load). Ground yourself before touching fuel components.
    • Plug or cap all open fuel lines IMMEDIATELY to minimize spillage and fumes.
  • Ethanol-Blended Fuel (E10): This is common but can wreak havoc over time:
    • Degrades Rubber: Attacks old rubber fuel lines, diaphragms (in mechanical pumps), and seals in electric pumps/modules, causing leaks and premature failure. Always replace fuel lines periodically and use ethanol-compatible components.
    • Water Absorption: Promotes phase separation and corrosion inside the tank, leading to clogged filters and pump damage. Use marine-grade fuel stabilizers religiously, especially during storage, and try to avoid long-term storage with E10 if possible.
  • Check Fuel Filters FIRST: Before assuming pump failure, always check and replace both the primary fuel/water separator filter and any secondary inline filter near the engine. A severely clogged filter mimics many symptoms of a failing pump (lack of power, stalling). Replace filters regularly per your maintenance schedule (often annually or every 50-100 hours).
  • Testing Pressure is Key: Diagnosing pump issues effectively requires a fuel pressure test gauge kit compatible with your engine type (carb/TBI/MPI):
    • Carb: Pressure usually 4-7 PSI. Test at the pump outlet or carb inlet.
    • TBI: Pressure usually 9-13 PSI. Test at the TBI unit test port (if equipped) or tee into the supply line.
    • MPI: Pressure usually 30-60 PSI (specific value is critical - consult manual). Test at the Schrader valve test port on the fuel rail (looks like a tire valve).
    • Compare readings to factory specifications. Low pressure = pump or restriction issue. No pressure = pump likely dead, fuse blown, wiring fault, or clog. High pressure = faulty pressure regulator.
  • Electrical Checks for Electric Pumps (TBI/MPI): If an electric pump is silent and has no pressure:
    1. Verify the 50-amp ENGINE fuse at the main battery switch/battery is good.
    2. Check the specific fuel pump fuse (often 15A or 20A) in the engine compartment or dash fuse panel.
    3. Check the Fuel Pump Relay (location varies: near fuse panels, ECM, or ignition components). Listen/feel for it clicking when key turned to "ON". Swap with a known good relay (like horn relay) to test.
    4. Check wiring connections for corrosion or damage, especially at the tank access plate and relay socket.
    5. Safety Circuits: Engines with EFI often have oil pressure or tachometer (RPM) safety switches that cut power to the pump if no oil pressure is detected after a few seconds of cranking, or if the engine stops running. Confirm oil pressure gauge operation. If the pump runs for the initial 2 seconds with key "ON" but then stops and won't run while cranking, suspect an oil pressure safety switch failure or wiring issue to it/ECM.

Replacement Insights

  • Quality Matters: Use OEM MerCruiser/Quicksilver fuel pumps or highly reputable aftermarket brands (Sierra, Carter, Airtex, etc.). Cheap pumps are notorious for short lifespans and potential safety hazards.
  • Mechanical Pump Replacement (Block Mounted):
    • Identify correct pump for your specific engine serial number. Carb and TBI pumps may differ slightly.
    • Ensure the new pump's actuating arm is correctly lubricated (with engine oil or pump-specific grease) before installation.
    • Be VERY careful inserting the actuating arm into the engine block cavity while guiding it onto the camshaft eccentric. Forcing it can damage the pump or cam lobe.
    • Torque mounting bolts carefully to spec (usually not very tight). Over-torquing cracks the pump body.
  • Electric In-Tank Pump (TBI/MPI) Replacement:
    • Module vs. Pump Only: Often easier and sometimes more cost-effective long-term to replace the entire pump/sending unit module assembly. Replacing just the pump requires disassembling the module carefully underwater/fuel and requires special retaining ring tools. Mistakes are common and lead to leaks or poor function.
    • Cleanliness is Crucial: Absolutely prevent dirt or debris from falling into the open fuel tank during pump access. Cover the hole immediately after module removal. Clean the tank top surface thoroughly around the access hole before opening.
    • Gasket/O-Ring: Always replace the large O-ring or gasket sealing the access plate to the tank. Use a tiny smear of compatible lubricant (e.g., petroleum jelly) to help it seat without pinching. Pinched or reused seals cause dangerous fuel leaks and vapor emission.
    • Float Arm Orientation: When installing the new module, ensure the fuel level sender float arm is correctly oriented to move freely within the tank without hitting baffles or walls.
    • Quick-Connect Fittings: Ensure fuel lines click securely into place. Pull back gently on the line collar to confirm it's locked. Hand-tighten any threaded fittings. Double-check for leaks after reassembly and before engine start.
  • Prime the System: After replacement, turn the key to "ON" several times (waiting a few seconds in between) for electric pumps to prime the system before cranking. Mechanical pumps might require cranking for 10-15 seconds to refill lines. Have starting fluid handy if needed cautiously.

Diagnosing Common Symptoms Related to Location Issues

  • Engine Cranks But Won't Start (Silent Electric Pump): IMMEDIATELY points to in-tank electric pump or its control circuit (fuse, relay, wiring, safety switch). Verify pump activation noise at key "ON".
  • Engine Cranks But Won't Start (Mechanical Suspected): Could be mechanical pump, but also major blockage, empty tank, bad ignition, or stuck anti-siphon valve. Check for fuel delivery at carb/TBI (carefully!) or use pressure gauge.
  • Stalling Under Load / Lack of Power: Often fuel starvation. Check filters FIRST. Could be failing in-tank pump (low pressure), restricted tank vent/anti-siphon valve (causing vacuum lock), collapsing old rubber fuel line (ethanol damage), or a weak mechanical pump (low pressure at higher RPMs). Pressure testing under load is diagnostic.
  • Loud Whining Noise from Tank Area: Usually indicates the electric in-tank pump is failing, cavitating (sucking air, often due to clogged pickup filter or extremely low fuel), or getting clogged with debris.
  • Fuel Leak Under Engine Block (Port Side): Very likely originating from the mechanical fuel pump body, diaphragm, or drain tube. Requires immediate pump replacement. Leaks near the fuel tank could be from access plate seal, sender unit gasket, or supply/return lines.

Understanding the Engine Compartment Layout

Knowing basic terminology helps locate components:

  • Port: Left side of the boat (and engine) when facing the stern (rear).
  • Starboard: Right side of the boat (and engine).
  • Transom: The flat, vertical surface at the very stern of the boat where the outdrive or stern drive attaches.
  • Stern: The rear of the boat.
  • Bow: The front of the boat.
  • Distributor/Ignition Module: A key landmark. On a Mercruiser 5.7L carb/TBI engine, it's typically near the center-top rear of the engine block. The mechanical pump is low on the port side, below and slightly behind this distributor/module.
  • Fuel Rails (MPI): Long metal tubes usually mounted along the top or side of the engine's intake manifold where the fuel injectors attach. The Schrader test valve will be on one of these rails.

When to Call a Professional

While replacing a mechanical pump is often within the capabilities of a competent DIYer with proper tools and safety precautions, working on electric in-tank pumps involves significant risk and technical challenges. Consider professional help if you are not 100% confident in:

  • Adhering to rigorous fuel safety procedures.
  • Safely gaining access to the fuel tank module (cutting decks, moving heavy components).
  • Accurately diagnosing electrical circuits (fuses, relays, safety switches, wiring).
  • Properly handling the sensitive module assembly and critical tank sealing gasket.
  • Performing a leak-free repair.
  • Diagnosing if the pump is actually the cause, versus a wiring, ECM, or sensor problem. Technicians have specialized diagnostic tools beyond a basic pressure gauge.

Key Takeaways

Pinpointing the Mercruiser 5.7 fuel pump location is straightforward only after identifying your engine's fuel system type and approximate year. Block-mounted (port side, low) for most carbureted and some early TBI engines; Electric fuel pump securely tucked inside the fuel tank for most TBI and all MPI engines. Safety is paramount. Filter checks come before pump suspicion. Diagnosing accurately often requires fuel pressure testing. Understanding your specific setup empowers you to troubleshoot effectively, whether you tackle the repair yourself or communicate clearly with a marine technician. Knowing exactly where to look saves significant time and frustration when fuel delivery issues arise.

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