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Mustang Fuel Systems Explained

Written By: Stanley Sadowski

From the tank to the cylinder, we explain everything you need to know about your Mustang's fuel system and related fuel delivery parts. For the most part there are two different styles of set-ups. Today we dive in to discover the pros and cons for return style and returnless style systems and their applications.

American Muscle

How Your Mustang's Fuel System Operates

The fuel pressure regulator, fuel rails and fuel injectors make up a system that delivers the correct amount of fuel to each cylinder for proper combustion. The pressure in the fuel rail and intake manifold should be maintained at a level to suit the optimum air/fuel ratio. Your Mustang, as well as most fuel injected cars, has a one-to-one ratio fuel regulator. It varies the fuel pressure by being connected to manifold pressure via a vacuum connection. It also utilizes an internal diaphragm to control the pressure.

At idle, the engine is in a manifold vacuum situation and the fuel pressure is around -9psi to manifold pressure ratio. At full throttle, the engine is in a no vacuum situation, in theory it would be 0psi to standard manifold pressure ratio. If you add more positive manifold pressure to your system with turbo or supercharger, the added boost pressure will further increase fuel pressure. In factory regulators, for every pound of boost, it adds a pound of fuel psi since it has a one-to-one ratio. Most aftermarket adjustable regulators are also one-to-one or close to that, however you have the ability to adjust the pressure at idle or full throttle for fine tuning.

Aeromotive Mustang Fuel Rail Kit with Pressure Regulator and Gauge
Fuel Rail Kit with Pressure Regulator and Gauge

Return Style Fuel Systems

The return style fuel system, used in the pushrod 5.0L and early 4.6L motors, uses a traditional in-tank pump just like the returnless style. From the tank, there is a feed line that runs through the fuel filter and up to the fuel rails, which supplies the injectors with fuel. The pressure is monitored by a fuel pressure regulator or FPR. Most enthusiasts replaced the regulator with an adjustable aftermarket product so they can tune their fuel system to the desired fuel pressure.

Now with return style, there is a return line that takes the excess, or unused, fuel back to the tank and the cycle is continuous. This style system is constantly flowing, which means your fuel pump(s) are constantly operating at 100% duty cycle. Return systems are more beneficial in big horsepower applications where there is a high demand for fuel. Since the fuel is constantly cycling through the system, it’s always there ready to be used as opposed to a returnless system that takes a couple things into account before sending fuel.

  • Fuel pump(s) is always working at 100%
  • Simpler system when compared to the average returnless style
  • Great for high horsepower applications
  • Requires more fuel lines, more lines, more potential leaks
BBK Mustang In-Tank Fuel Pump
In-Tank Fuel Pump

Returnless Style Fuel Systems

Returnless fuel systems are quite different and can be complicated. The fuel pressure is controlled by the amount of voltage being sent to the pump and other electronics. In a returnless system, the fuel pump is not operating at 100% of its duty cycle. These systems also use an FPDM (Fuel Pump Driver Module), which is an electrical device designed to keep the desired pressure at the injectors and controls the power sent to the fuel pump. A PPRV (Positive Pressure Relief Valve) which is implemented to maintain the pressure in the system when you turn the car off, which allows for easy start-ups and it also relieves excessive pressure in the lines by draining back into the tank. There is no return line from the fuel rails back to the tank.

  • More technological and modern when compared to the return style system
  • No return line (hence returnless), less line, less weight and fuel line cost
  • Easy start ups
  • Fuel is always pressurized even when the engine is off
  • Requires module tuning for fuel pressure change
EV6 High Flow Injectors Installed on a 1987-2014 Mustang Engine
EV6 High Flow Injectors Installed on a 1987-2014 Engine

Fuel Pressure Regulator - Keeping the Pressure On

The FPR is responsible for keeping the desired fuel pressure at the fuel rails. It runs off of a vacuum line from the intake manifold and adjusts the fuel pressure based on the pressure in the intake manifold. If the pressure in the manifold increases, the regulator will up the fuel pressure to compensate. The installation of an aftermarket fuel pressure regulator allows you to adjust the fuel pressure to suit larger aftermarket injectors and other engine modifications—most commonly, forced induction. They are also necessary to regulate the flow of increased volumes of fuel pumped by high flow aftermarket fuel pumps.

Mustang Adjustable Fuel Pressure Regulator
Adjustable Fuel Pressure Regulator

Fuel Pump Driver Module - Monitoring the Pump

Returnless systems use an FPDM (Fuel Pump Driver Module) which is an electrical device designed to keep the desired pressure at the injectors and controls the power sent to the fuel pump(s). Unlike a return fuel system, the fuel pump is controlled by an FPDM and is not constantly operating at 100%. Instead, the FPDM sends pulses of power that tells the fuel pump how to respond. It is located behind the inner lining of the trunk on the driver side. For people using bigger fuel pumps that demand more power, it is common to use a modified FPDM or dual FPDMs.

Positive Pressure Relief Valve - For Easier Starts

A PPRV (Positive Pressure Relief Valve) maintains the pressure in the system when you turn the car off, which allows for easy start-ups and it also relieves excessive pressure in the lines by draining back into the tank. It is located inside of the brownish tube that connects to the fuel hat and then to the fuel pump. Inside of that little white casing is the PPRV. There is actually a way to remove this valve, which would be beneficial with high horsepower or boosted applications because it prevents the loss of fuel pressure between shifts.

High Performance Dual Fuel Pump Kit for 2005-2009 GT Mustangs
High Performance Dual Fuel Pump Kit

Fuel Management Units - Forced Induction Mustangs

There is one other type of regulator that is used with aftermarket forced induction. These are rising rate regulators that commonly go by the term FMU (Fuel Management Unit). This style of regulator increases fuel pressure at a greater than one-to-one ratio. So, instead of adjusting fuel enrichment with complicated computers and injection duty cycles, these systems just increase fuel pressure to add fuel. They go inline, down from the factory regulators, and only start to add pressure under boost. So when you are off the boost, you maintain factory tuning and drive-ability. Only as you get into a boost situation does the FMU begin to increase fuel pressure.

PLX Mustang Air-Fuel Ratio Sensor Module
Air-Fuel Ratio Sensor Module

Fuel Rails - For Style and Reliable Power

Besides upgrading to an aftermarket fuel pressure regulator, you may also need to look at your fuel rails. Many stock rails can have small passages or restrictive bends in the tubing that do not support proper flow of fuel to your injectors. This is especially true when you are requiring more fuel than your stock fuel system was originally designed to deliver. One particular area of concern is the fuel line that connects one fuel rail from one side of the motor to the fuel rail on the other side of the motor. Most of the time you will find an aftermarket fuel rail kit that includes both rails and connecting line and other various fittings to connect it to your fuel lines.

The majority of aftermarket fuel rail manufactures have also designed their fuel rails to look better than a stock OEM part. Since fuel rails sit on top of your motor they have a high visibility factor and many companies make them out of color anodized or polished billet aluminum to improve your car’s under-hood look.

BBK Mustang Fuel Rail Kit
Fuel Rail Pair

Fuel Pumps and Their Operation

The fuel pump is responsible for getting the fuel out of the tank and into the feed line in order to reach the fuel rails and injectors. The flow rate of a fuel pump is determined by its design and size, but they also rely on two variables. The amount of power being supplied to the pump and the desired pressure of the fuel system. In order to find out what size fuel pump to get, you want something that will support the highest demand of fuel that your engine/fuel system needs, which is at wide open throttle in a high range of rpms. Three things will ultimately determine which fuel pump to run:

  • Flow Rate: measured in lbs/hr and pressure, having the flow to push the horsepower you want is essential
  • Price: like any aftermarket part, fuel pumps are not all created equal. Keep your budget and your build in mind while looking for a pump
  • Application: return style and returnless style systems will dictate which pumps your Mustang can use
Fuelab Mustang Fuel Pump

In-Line Fuel Pumps - 1986-1995 Mustangs

These fuel pumps are only used in 86-95 Mustangs with the 5.0L engines which use the return style fuel system. An in-line fuel pump installs after the fuel pump similar to a fuel filter. They help push a little more fuel than the fuel pump would push on its own, but it’s no major increase. If you are switching to a different setup that will be making more power than your previous setup by a good amount, then I would recommend upgrading the fuel system elsewhere like in the in-tank pumps, fuel rails, lines, etc. These pumps are usually good for a gain of around 50lph depending on which model you select. So for example, if you are running a 255lph pump, and due to a couple modifications that have netted you some power and you think you could use some extra fuel, then I would suggest installing an in-line pump which would put you in the neighborhood of 305lph. Again, this depends on the fuel pump you are running and the in-line pump that you chose to install.

Accel Foxbody Mustang V8 Fuel Pump
Foxbody Fuel Pump

What About a Boost-A-Pump?

A boost-a-pump is a fuel pump upgrade that increases the amount of fuel a pump can flow. It works by increasing voltage to the factory or aftermarket pumps already installed. By increasing fuel pump voltage, you can increase the amount of fuel supplied to the engine. A boost-a-pump can add up to 75% capacity to your fuel system, making it ideal for use on supercharged and turbocharged Mustangs still running the stock pump. Boost-a-pumps are wired to the factory wiring harness for the fuel pump, and then they have a manifold pressure line run to them for reference. As the manifold pressure increases, the voltage to the fuel pump is increased. So as your forced induction Mustang builds boost pressure, the boost-a-pump will increase pump voltage.

2010-2014 Boss Mustang with a Centrifugal Supercharger

Can My Stock Fuel Pump Handle a Boost-A-Pump?

Many people may worry about the increased voltage overworking or burning out their stock pump, but there is nothing to worry about. The boost-a-pump is totally safe for your stock fuel pump and can actually extend the life of your pump. The boost-a-pump only activates when the extra fuel is needed, and at any other time the fuel pump is running at a lower voltage. Combining a boost-a-pump with a stock fuel pump is the perfect solution when it comes to performance, ease of install, and OEM reliability.

Roush Mustang Fuel Voltage Regulator

Is a Boost-A-Pump Right for My Car?

To break it down, here's why a boost-a-pump is a good idea:

  • Fuel pressure is dictated by the voltage sent to the pump via the PCM. More voltage = more fuel pressure
  • Boost-a-pumps increase the voltage to the fuel pump, essentially increasing the amount of fuel your Mustang can handle by 75%
  • Boost-a-pump works well with boosted engines by reading manifold pressure and adding fuel accordingly
  • Boost-a-pumps will not fry out stock Mustang fuel pumps
  • Boost-a-pumps can prevent severe engine failures from running too lean
  • Still worried? Use data logging to keep track of the boost-a-pump 

If you are installing any form of forced induction, it would be wise to install a boost-a-pump as well. You want to make sure your engine always has enough fuel, otherwise you can suffer catastrophic engine failure due to a lean air/fuel ratio. You can also monitor your fuel pump duty cycle with data logging software and a tuner. If your duty cycle is nearing 100%, it would be a good idea to install a boost-a-pump to give your Mustang a larger safety margin. The ease of install and cost compared to a larger fuel pump makes the boost-a-pump an excellent choice for someone needing for fuel for their Mustang. It can be used with single and dual fuel pump set ups as well, so it works on all Mustangs.

1993 Foxbody Mustang with a Centrifugal Supercharger
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