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Breaking Down Fuel Pump Terminology: EFI, E85, LPH, Small Parts, and More

Breaking Down Fuel Pump Terminology: EFI, E85, LPH, Small Parts, and More

Navigating the world of fuel systems shouldn’t be difficult, especially when you’re simply trying to order quality parts for your machine. When it comes to fuel systems, there is quite a bit of fuel pump terminology, whether you’re talking about automotive, motorcycle, ATV, UTV/Side-By-Side, or marine vehicles.

We’ve put together a helpful list of standard terms and definitions that you’ll come across when diving into a DIY repair for your vehicle. From basic definitions and acronym explanations — being familiar with fuel pump terminology will get you on the right path and keep you informed.

QFS specializes in producing high-quality fuel system components for nearly every EFI-equipped engine on the market. We design our direct replacement parts in-house and will meet or exceed OEM quality. We offer replacement fuel pumps, kits, and components for nearly all makes and models backed by an industry-leading lifetime warranty. Visit our complete catalog to order now.

Fuel Pump Terminology Explained

Electronic Fuel Injection

Electronic Fuel Injection (EFI) is any fuel delivery system that is electronically controlled by an Engine Control Unit (ECU). The name describes its function quite accurately because EFI systems inject fuel directly into an internal combustion engine’s manifold or cylinder. EFI has become the dominant fuel delivery method due to its inherent performance, emissions, and efficiency advantages over antiquated carbureted systems. There are several forms of fuel injection, such as single-point fuel injection, multi-point fuel injection, sequential fuel injection, and direct injection. Yet, they all follow the same fuel injection principles.

Fuel Injector

A fuel injector is one of the most crucial components in your EFI system; its function is where we get the name. Your vehicle’s Engine Control Unity (ECU) electronically control your fuel injectors. A fuel injector is an electronically controlled valve that sprays a precise amount of atomized fuel into the engine’s combustion chamber to facilitate an optimal air-fuel mixture under all operating conditions. QFS is an authorized dealer of fuel injectors from industry leaders such as Bosch, Injector Dynamics, RC Engineering, and Siemens Deka. We offer direct OE replacement and high-performance solutions for automotive, Powersport, and marine applications.


The Engine Control Unit (ECU) or Engine Control Module (ECM) is your vehicle’s brain. It controls one or several electrical systems. Part of its duties is managing the EFI system, performing essential tasks like how long to hold the injectors open during each cycle to create the proper air-fuel ratio for any situation. It achieves this by monitoring data from several sensors and adapting accordingly.    

OE Replacement

Original Equipment Replacement. All QFS products for powersports, from fuel pumps, O-rings, and more, match OE specifications and are guaranteed to be direct drop-in replacements. 


Original Equipment Manufacturer.

Fuel Rail

A fuel rail is a high-pressure fuel line that delivers fuel to the fuel injector.

Fuel Pump Assembly

The fuel pump assembly often houses critical components such as the fuel pump, pressure regulator, fuel sending unit, and strainers or filters. QFS offers entire replacement assemblies for many makes and models. 

Fuel Sending Unit

A fuel sending unit measures the amount of fuel available in your fuel tank and communicates that information to the fuel gauge. It does not send fuel as the name suggests and instead sends an electrical current from the fuel level sensor to the fuel gauge. If your vehicle cannot accurately register fuel levels, a fault with this component can be the cause.

Fuel Pump

Direct OE replacement fuel pumps are what Quantum Fuel Systems (QFS) specializes in, supporting nearly every corner of the automotive, Powersports, and marine market. The fuel pump delivers high-pressure liquid fuel to a carburetor or EFI system within the engine. QFS fuel pumps feature an industry-leading lifetime warranty and are designed to withstand a variety of ethanol-gasoline fuel blends. 

diagnose a bad fuel pump | QFS

Intank Fuel Pump

“Intank” or “in-tank” refers to the physical location of the fuel pump. An intank fuel pump is located inside the fuel tank and explicitly designed for that purpose. Intank fuel pumps typically achieve longer operational lifespans than alternative designs since they are submerged in liquid fuel, helping cool and lubricate the pump.

External/Inline Fuel Pump

External/Inline fuel pumps mount on the exterior and underneath a fuel tank while using gravity to feed fuel into the pump. An external/inline fuel pump is often far easier to service and replace due to its mounting location. 


Small Parts

This is a term that comes up in conversation with gearheads, sales staff, and suppliers within the fuel-system industry. “Small parts” refers to any components that are not the actual fuel pump. This can include but is not limited to; O-rings, gaskets, strainers, filters, pressure regulators, and other parts associated with fuel pumps. QFS has all the small parts you need to get your vehicle running again. 


Filters do the critical job of extracting harmful particulates from fuel to protect sensitive EFI components. QFS filters screen microscopic contaminants that collect in a fuel tank and ensure that your vehicle’s EFI system is running at its best. A car, motorcycle, ATV, UTV/Side-by-Side, or boat may employ one or more filters at various stages of the fuel system. 


Strainers perform the same function as filters and are typically the first line of defense against fuel system contaminants. Common contaminants such as sand, dirt, dust, and gasoline varnish are screened out by QFS strainers, which feature multi-layer strainer media to filter even the smallest particulates. In addition, QFS strainers boast an expanded surface area. This offers improved filtration abilities and extending the life of your fuel system components.

Pressure Regulator

The pressure regulator helps maintain proper PSI within your EFI system. This allows your fuel injectors to function correctly and air-fuel calculations to be as accurate as possible. 

Dual Pump System

Any fuel system that relies on two fuel pumps is a Dual Pump System. In these systems, a low-pressure fuel pump transfers fuel from the fuel tank to the engine, then a high-pressure fuel pump takes over and delivers fuel to the EFI system. These systems are commonly used in marine applications where the fuel tank is not near the engine. 

E85 Fuel (Flex Fuel)

E85 is an ethanol-gasoline blend of fuel that can contain anywhere from 51% to 83% ethanol, depending on the season and location. Flex Fuel or E85, is explicitly formulated for Flexible Fuel Vehicles (FFVs), meaning that the internal combustion engine is designed to support high-volume ethanol fuel blends. While cheaper than gasoline, it has less energy density and often results in reduced fuel economy. This fuel cannot be used in motorcycles and Powersport vehicles.


The fuel tank. Every ICE-powered machine has one (or more).


Performance fuel system equipment means that you need components with superior flow rates than what OE equipment can provide. When building high-horsepower engines, this type of equipment is an absolute necessity to meet the increased performance demands.

Polytetrafluoroethylene (PTFE)

Polytetrafluoroethylene (PTFE) is a durable synthetic fluoropolymer used in all QFS corrugated flex hose lines. This robust plastic is hydrophobic and resistant to the harsh chemicals in today’s fuels, including E85. The corrugated design also prevents kinking and improves fitment in all uses.

AN Fittings

AN is a U.S. military-derived specification standard dating back to World War II, developed as a common standard between the Army and Navy branches. QFS is an authorized dealer of high-quality AN fittings from Phenix Industries. Phenix Industries provides customers with the finest aftermarket high-performance fuel and brake system plumbing components available.


Pounds per Square Inch/Kilopascal are two standard measurement units when describing fuel pressure regulator specifications. PSI is an Imperial measurement, and kPa is metric. Be sure to reference what measurement unit your model-specific service manual cites.


Liters Per Hour/Gallons Per Hour are two frequently cited units of measurement. They are used to reference the amount of fuel a pump can supply over time. 

Pulse Width Modulation (PWM)

Pulse Width Modulation allows an ECU to control EFI fuel pressure by rapidly turning a fuel pump on and off. If more pressure is needed, the length of time the fuel pump is activated extends. Conversely, if less fuel pressure is required, the time the fuel pump is turned on is reduced. This can happen in milliseconds. It extends the useful life of a fuel pump by allowing it to work less, operate at a lower temperature, and introduce less heat to the stored fuel. All QFS fuel pumps are PWM-ready.

Done Studying Fuel Pump Terminology? Reach Out To Our Helpful Staff

Quantum Fuel Systems is open Monday through Friday, 8:00 a.m. to 5:00 p.m. PT. Any of our trained and qualified staff members are available to assist customers over the phone. Be sure to reach out to our support staff with any technical questions, and we’ll get back to you as soon as possible.

How to Diagnose Dual Fuel Pump Problems on Outboard Engines

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.

QFS is a leader in direct-OEM replacement fuel system components, providing high-quality repair parts for major marine brands, such as Evinrude Outboards, Honda, Mercury Marine, Suzuki, Volvo, Yamaha, and more. QFS offers complete replacement kits, parts, and specialty to tools to get you back out on the water.

An unmatched lifetime warranty backs QFS products. All orders are processed on the same business day, with free 2-day shipping within the United States for any order above $60. 

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.


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.

Order Now

Quantum Fuel Systems is open Monday through Friday, 8:00 a.m. to 5:00 p.m. PT. Place orders over the phone or browse our extensive catalog. For technical questions, visit our support page, and we’ll get back to you as soon as possible. 

  • 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
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