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
Conventional Fuel Systems
A conventional EFI fuel system uses the following components:
pressure roller cell pump (either internal to the tank or mounted
pressure fuel line from the filter to the injector rail
line to the tank
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
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:
pressure in-tank roller cell pump
pressure in-tank regulator
filter (may be in-tank)
pressure fuel line from the filter to the injector rail
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
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.
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
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
There are two ways of increasing the flow output of the pump in a returnless
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
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
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
Next week: installing a new hi-flow
pump and an external, adjustable regulator on a returnless system.
Rising Rate vs Boost Sensing
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