How to Diagnose Dual Fuel Pump Problems on Outboard Engines
Modern outboard motors are reliable pieces of equipment capable of propelling our vessels for hundreds of service hours. As robust as they are, fuel pump troubles can still occur. It’s wise for any outboard owner to be able to diagnose dual fuel pump problems in case of emergency or preventative maintenance.
Today’s intricate Electronic Fuel Injection (EFI) marine engines often add complexity with features like dual fuel pump systems, making pinpointing fuel-related problems difficult. We will discuss how to diagnose fuel pump problems on outboard motors, specifically those with dual fuel pump systems.
If your outboard motor is suffering from a loss in performance, unable to run reliably, or won’t start at all. You may be dealing with fuel pumps that have failed.
We strongly suggest referencing the model-specific service manual for your outboard motor to confirm all technical specifications when performing diagnostics. Those figures can change depending on the year, make, and model of your outboard motor.
If you do not feel qualified performing the following diagnostic work, please consult an authorized and licensed service technician.
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What Is a Dual Fuel Pump System?
A dual fuel pump system relies on a low-pressure and a high-pressure fuel pump to perform separate tasks. A low-pressure fuel pump lifts fuel from your fuel tank to your outboard motor’s Vapor Separator Tank, also referred to as a Fuel Supply Module, depending on the manufacturer.
VSTs/FSMs house all sensitive fuel-delivery-related components. Often, you may see the low-pressure pump described as a “lift pump” because of its primary function. It does not pressurize the EFI system or injectors in any capacity.
Once fuel is transferred into the VST/FSM, the high-pressure fuel pump can pressurize the high-pressure fuel rail and allow your EFI system to function as intended.
It is essential to recognize that high and low-pressure fuel pumps have unique technical specifications. Each pump has different purposes in the system. However, the same testing procedures apply to both fuel pumps, and it simply means you will repeat the processes for each pump.
Typically, both fuel pumps in a dual-pump system will not fail simultaneously, meaning that owners can save money by knowing how and when to upgrade with high-quality QFS fuel pumps.
Symptoms of a Bad Outboard Motor Fuel Pump
Before diving into diagnosis methods, we will outline a few common symptoms related to bad fuel pumps. These are general symptoms and are often seen in various automotive, marine, or Powersports applications.
Failing to start or Maintain Idle
An engine that fails to start when electrical, starting, and filter systems appear to be in good condition usually indicates that fuel is not reaching the EFI system. In most cases, that means a fuel pump has ceased working entirely.
Similarly, an engine that starts but cannot maintain a steady idle is another sign that the pumping system has a fault.
Losing Power When Throttling Up
Throttling up and raising engine rpm puts more stress on the fuel system, demanding greater fuel pressure and flow to meet performance requirements. While your outboard engine might behave normally at low rpm, it may stall, sputter, or lose power as the rpm climbs. These symptoms signal that the fuel pumps are operating inconsistently and unable to keep up.
Increased Engine Temperature
Your outboard motor could appear to be working normally and propelling your vessel as it should, yet it seems to be running hotter than average. Often, this situation can signal that the optimal fuel/air mixture isn’t achieved, due to a restricted or underperforming pumping system.
Diagnosing a Bad Dual Fuel Pump System
Resolving fueling issues is done through the process of elimination. This can be tricky since electrical and fuel systems are related. These tips should help you narrow down the possibilities and uncover the cause of your problems.
Most of the diagnostic suggested here will only require you to listen, observe, and use basic hand tools (screwdrivers, sockets, and pliers). Advanced diagnosis methods will require specialized tools such as a pressure testing gauge and a voltage meter or multimeter.
QFS DUAL FUEL PUMPS W/ PRESSURE REGULATORS FOR MERCURY MARINE OUTBOARD ALL 1984-2023, REPLACES 880596T55
We also encourage wearing eye protection and nitrile gloves when dealing with fuel and fuel-related components.
Start With the Basics
- Confirm there is fuel in the tank and that the fuel gauge is registering accurately.
- Confirm that your electrical charging system is in good health; all connections are free of corrosion and buildup, all fuses are intact, and the electrical system is functional.
- Confirm all fuel system components are in working order; check that filters, screens, fuel lines, connections, and seals are adequately maintained.
Listen for Fuel Pump Priming
In most cases, your fuel pump(s) will quit working unexpectedly. When faced with a situation where you go to fire your boat up, it cranks but won’t start; we’ll need to pause and listen if the fuel pumps are priming.
Priming is when a fuel pump engages for a short period. Note that on a dual pump fuel system, both pumps will prime — the low-pressure pump draws fuel into the VST/FSM, and the high-pressure pump primes the EFI system.
Testing this is simple. Cycle the ignition to the “on” position and listen closely for an audible whine or whirring coming from the outboard motor. This whirring sound will last a few seconds. Repeat this process multiple times and confirm whether or not you hear a mechanical whine.
If nothing is heard while cycling the ignition on, the fuel pumps may have failed, or the pumps may not be receiving electrical power.
Inspect the VST/FSM Assemblies
Most outboard motors use VST/FSM assemblies to house fuel-related components such as a limited amount of fuel, a float or float switch, various filters, and a pressure regulator. These designs and specifications will change with the make and model of your particular engine, so please reference your service manual for exact information.
The low-pressure fuel pump draws fuel from the tank into the VST/FSM. Gain access to the VST/FSM and inspect whether or not fuel is being deposited into the VST/FSM container.
Once again, always refer to your authorized service manual for proper removal procedures.
If no fuel is present in the VST/FSM, chances are the low-pressure fuel pump is malfunctioning and not delivering fuel. If an adequate amount of fuel is available, the problem could be with the high-pressure fuel pump.
Check for Electrical Power to Fuel Pumps
Generally, if your fuel pumps are not priming, you will want to confirm that they’re receiving electrical power. These tests will help determine if your issues are tied to the electrical system or the fuel pumps.
Using a multimeter or voltage meter, we can inspect the various electrical connections and determine if the required amount of electrical current is traveling down through the wiring harness to each fuel pump. Since we are dealing with dual pump systems, you’ll need to repeat the steps for both the high and low-pressure fuel pumps.
Consult your service manual and wiring diagram to determine which terminal connections to measure. Remove the relevant terminals from the pump and cycle the key to the “on” position while measuring the voltage.
The voltage should rise to the stated technical specification in your service manual and return to zero after several seconds. If no voltage or insufficient voltage is measured at these terminals, your problem is likely electrical.
If your wiring harness is delivering power, then it’s time to perform a continuity test on the fuel pumps themselves with your multimeter. A continuity test confirms if an electrical circuit can be completed and is impossible when a fuel pump’s electric motor burns out.
Pressure Testing Fuel Pumps
Pressure testing is helpful if your outboard motor is experiencing sputtering or stalling since inconsistent pressure in your fuel system can create these problems. Luckily, most modern outboard motors feature threaded testing valves on the high and low-pressure sides of the fuel system.
A pressure gauge can be acquired from any authorized dealer or marine retailer.
Refer to your service manual for fuel pressure specifications, install the pressure tester, and then cycle the ignition “on.” The pressure should rise and maintain at the cited specification.
Start the engine and increase the throttle slowly while noting if the fuel pump can maintain pressure within its recommended range. Repeat the process for both fuel pumps.
If pressure is abnormally low or nonexistent, then you’ve most likely discovered your problem.
For reference, pressures higher than the recommended range indicate that the pressure regulator is defective. By the same logic, clogged fuel filters and leaking fuel lines can decrease pressure significantly.
Inspect all components thoroughly, and we offer complete replacement kits for all major brands, including fuel pumps, seals, and filters, to get the job done.
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- Identifying problems with dual-pump setups. Many outboard systems consist of high & low pressure fuel pumps in a single system. Users can save money by identifying which pump is causing issues.
As far as the dual pump system goes, its crucial to test them out independently. For the high pressure pump, the standard procedure for testing the fuel pump (electric, pressure test, etc.) will suffice for determining if that fuel pump needs to be replaced. Pressure test the low pressure pump is more difficult due to its nature of it being a low pressure pump. They suggest the electric connection test as the most efficient way to test the low pressure pump.
- Understanding the symptoms and how to diagnose a bad fuel pump on an ATV, motorcycle, or UTV/SxS is handy knowledge for any enthusiast
- When a fuel pump fails to operate within its designed specifications, your EFI-powered vehicle can suffer in a few ways — robbing performance, causing it to run poorly, or failing to start at all
- Pinning-down fuel issues can be complex, as fuel systems and electrical systems are interdependent
- Priming means that the fuel pump engages for a short time, pressurizing the fuel lines and providing adequate fuel flow for the injectors
- Pressure testing is crucial when faced with lean and rich conditions, power loss, or surging