Modifying Returnless Fuel Systems, Part 1

This article was first published in 2005.

Nearly all current cars use an in-tank fuel pressure regulator and so have only a single fuel line going forward to the fuel injector rail. Yep, that’s right – there’s no return line. So what happens when you want to make some modifications to one of these systems – say, to increase fuel pressure? Basically, it’s a whole new ballgame.

But before we get into the modification, we need to know in detail how fuel systems work.

Conventional Fuel Systems

A conventional EFI fuel system uses the following components:

• High pressure roller cell pump (either internal to the tank or mounted externally)
• Fuel filter
• High pressure fuel line from the filter to the injector rail
• Fuel pressure regulator
• Return line to the tank
• Additionally, there may be one or more pressure dampers

This diagram shows the layout of this type of fuel system. Fuel is pressurised by the pump and flows forward to the fuel rail through the filter. The regulator is positioned on the exit to the fuel rail and its degree of opening therefore regulates how much pressure there is in the fuel rail (and also in the hose between the pump and the rail). Because the pump is sized to flow more fuel than the engine will ever need, the return line carries lots of fuel and so much fuel is circulated from the fuel tank, through the injector rail, and then back to the tank. (This also has the important function of circulating fuel through the filter many times.)

Looking at what occurs in the tank in more detail, the fuel pump draws fuel through a strainer that is attached to its base. The strainer is positioned so that it can pick up the last drops of fuel, eg in a depression in the tank or in an area surrounded by an internal tank baffle. For the same reason, the return line from the regulator normally directs the fuel into the area near the pump pick-up.

In order that the recirculation of fuel through the injector rail is minimized (because the fuel gets heated each pass), some cars use a variable speed pump, where pump speed (and so flow) is matched to the engine’s demands. In other words, in these cars the pump isn’t working flat-out all the time. This speed control is obtained by varying the current fed to the pump.

So that the car is quick to start, fuel pressure is maintained between the pump and the regulator by a one-way valve in the pump. This also has the important side effect that fuel pressure should be lowered before any part of the high pressure system is disconnected. (The easiest way of achieving this pressure reduction is to pull the fuel pump fuse or relay and then run the engine until it has used up this fuel and so stops.)

In conventional systems, the fuel pressure regulator uses a diaphragm that’s backed by a spring and an intake manifold connection. This causes fuel pressure to vary with manifold pressure – in other words, when there is a strong vacuum in the manifold (eg at light load cruise) the fuel pressure is lower than at wide open throttle, when manifold vacuum is less.

However, it’s important to note that in this type of system, the fuel pressure headroom above manifold pressure remains constant – there’s always the same pressure difference across the injectors. Forced aspiration engines use a pressure regulator that’s designed to see positive pressures from the manifold, but again the fuel pressure regulator keeps the fuel pressure a fixed amount above manifold pressure. In most EFI cars, the fuel pressure is maintained at 2.5-3 Bar (36-44 psi) above manifold pressure.

Returnless Fuel Systems

A returnless EFI fuel system uses the following components:

• High pressure in-tank roller cell pump
• Fuel pressure in-tank regulator
• Fuel filter (may be in-tank)
• High pressure fuel line from the filter to the injector rail
• Additionally, there may be one or more pressure dampers

In this system, fuel that is pressurised by the pump flows in two directions – to the injector rail and to the fuel pressure regulator. The greater the fuel pressure regulator opening, the lower will be the fuel pressure as it bleeds fuel from the injector line. In many cases, the fuel pump, local fuel reservoir, pressure damper, pressure regulator and fuel level sender are integrated into the one assembly inside the tank.

In these systems, fuel pressure is not referenced against manifold pressure. In other words, the fuel pressure stays at a fixed value (eg 45 psi) above atmospheric pressure and so the pressure difference across the injectors varies with manifold pressure. These fuel pressure regulators lack the manifold pressure sensing diaphragm and are usually smaller than conventional regulators. Pressure in these regulator designs is therefore regulated solely by the spring and diaphragm. (If the pump is a variable speed design, pressure may also be regulated a little by the pump flow.)

So that fuel continues to pass multiple times through the filter, the filter is often placed between the pump and the regulator, ie in the tank. This prevents (say) rusty particles being sent time and time again through the pump.

Again, as with conventional return-line fuel systems, a one-way valve in the fuel pump maintains pressure in the rail when the engine is off.

Modification

Fuel system modification is carried out because the car is running lean at high loads – it needs more fuel than the standard fuel system can provide. Two modifications can be made: to the fuel pump and to the fuel pressure regulator.

With conventional systems, the fuel pump is easily changed (especially if it’s an external one) or another pump can be added in-line. In the latter case, the second pump is used as a booster, coming into action when high power is being demanded. Because it’s located in front of the pressure reg, the pressure is still maintained at the correct level relative to manifold pressure (that’s if the fuel pressure reg is large enough to cope with the extra flow, anyway).

When fuel pressure needs to be changed in these conventional systems, the pressure regulator can either be swapped for another, or an aftermarket fuel pressure regulator installed. In the latter case, plumbing it after the factory reg is easy and effective – its action in restricting fuel flow back down the return line increases fuel pressure when it’s needed. This approach is used when for example a rising rate regulator is being fitted – this type of reg increases fuel pressure disproportionately with manifold pressure.

But returnless fuel systems are very different – primarily because of the way they’re packaged. Changing the pump to a physically much larger one is nearly impossible (it simply won’t fit into the plastic assembly) and fitting another pump between the factory unit and the pressure reg can’t be done because both devices fit into the plastic housing, using internal passages to make the connections. Because the fuel pressure regulator is built into place, unless the new one is physically identical in shape, it ain’t going to fit – and there’s no easy way of adding a second pressure reg immediately after the first.

Hmm, so can’t fit a bigger pump and can’t add another pressure reg.... that makes things hard!

Returnless Fuel System Pump Upgrades

There are two ways of increasing the flow output of the pump in a returnless fuel system.

The first is to fit a higher capacity pump that is the same physical size as the original. Many in-tank pumps are standardised in size and even if an upgrade of identical size isn’t available, much higher capacity pumps are available in packages the same diameter but just a little longer.

The second way of increasing pump output is to increase its supply voltage. This normally calls for a dedicated electronic module that increases the voltage to as high as 22 volts. However, even a more modest increase of 3-4 volts can make an appreciable difference to fuel flow. The pump isn’t run on the higher voltage all the time; instead a load switch of some kind (eg a voltage switch on the airflow meter output or even a simple boost pressure switch) is used to switch-in the higher pump voltage as required.

Returnless Fuel System Pressure Regulators

If there is a significant increase in fuel pump flow, the small in-tank regulator may no longer be capable of regulating the pressure. In general, the physical packaging of the regulator in the assembly means that a larger in-tank reg cannot be fitted and so if this is the case, plumbing changes need to be made to allow the installation of an external pressure regulator.

The major modifications involved in fitting an external reg are the installation of a return line to the tank (normally a fitting can be placed on the upper plastic flange of the removable pump/reg/fuel gauge sender assembly) and either the modification or removal of the in-tank pressure regulator. If the in-tank reg is removed, the opening needs to be blocked with a plug that matches the shape of original regulator. Alternatively, modification to the in-tank reg to allow the running of an external reg involves increasing its output pressure so that it no longer plays any role in regulating pressure – the external reg takes over this function.

Conclusion

Previously, upgrading the fuel system was a relatively straightforward procedure of installing a new external pump and placing an aftermarket pressure regulator under the bonnet. However, returnless fuel systems require some new approaches.

Next week: installing a new hi-flow pump and an external, adjustable regulator on a returnless system.

Rising Rate vs Boost Sensing Regulators There is some confusion around on the difference between rising rate and boost sensing fuel pressure regulators. As indicated in the main text, a manifold pressure sensing fuel pressure regulator always holds a constant pressure headroom above manifold pressure. Fuel pressure regs fitted to naturally aspirated cars aren’t normally designed to see pressure (ie boost) against the diaphragm in the regulator, and so when a car is being fitted with a turbo or blower, a boost-sensing fuel pressure regulator needs to be fitted. The simplest way of doing this is to use one from a factory forced aspirated car. Rising rate fuel pressure regulators are different beasts. Again, they are primarily used on forced aspirated cars but instead of holding a constant pressure headroom above manifold pressure, the fuel pressure rises more rapidly than manifold pressure. For example, the fuel pressure may rise 4 psi for every 1 psi of boost (a 4:1 rising rate) or 8 psi for every 1 psi (an 8:1 rising rate). As far as we know, rising rate fuel pressure regs are available only in aftermarket designs.

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