Modifying Returnless Fuel Systems, Part Two

This article was first published in 2005.

Last week in Modifying Returnless Fuel Systems - Part One we looked at how returnless (ie single line) fuel systems differ from conventional fuel systems that use a return line to the tank and have the fuel pressure regulator positioned in the engine bay. As we saw, modifying returnless fuel systems is a completely different ballgame to conventional fuel systems, as the physical packaging of the pump and regulator prevents the easy upgrading of the regulator and the changing of the pump.

But in this story we’ll do just that - upgrade the pump and fit an external, adjustable fuel pressure regulator.

Accessing the Bits

In this Toyota, the fuel module is accessed by lifting the rear seat squab which reveals an inspection plate. The plate is held in place with black mastic and can be prised up by the judicious use of a screwdriver.

Under the inspection plate is access to the top of the fuel tank. The assembly beneath is held in position with 8 bolts that hold down a circular flange which seals against the rim with an O-ring.

The red arrow points to the socket for the fuel pump power supply and tank level sensor; the green arrow points to the fuel line (note the absence of a return line). With all the flange bolts undone, the assembly pulls straight out of the tank.

The Assembly

Returnless fuel systems have the pump, pressure regulator, pressure damper, fuel tank gauge sensor (and sometimes the filter) integrated into the one assembly. The assembly clicks together and so can be pulled apart – here the red arrow shows the plastic cover over the fuel pressure regulator and the green arrow shows where the upper and lower halves separate.

With the lower cover removed, the fuel pressure reg (left) and base of the fuel pump can be seen. Note the very small size of the fuel pressure regulator and how it has only one plumbing connection – it uses an O-ring seal into the passage above it. There’s no manifold pressure connection and the fuel that the pressure reg bypasses feeds directly out into the tank.

The fuel pressure regulator pulls straight out of its O-ringed fitting.

The fuel pump pulls out the top of the lower half of the assembly but before this can be done, this clip needs to be removed so the strainer can be taken off. These clips are generally single-use only, so if you intend re-using it, remove it very carefully.

The New Pump

Usually the in-tank pumps need to be upgraded without a change in dimensions. (Though sometime it’s possible to go a little longer.) The best way of finding a suitably sized upgrade pump is to go to a fuel injection specialist and consult regarding the availability of higher flow pumps that physically appear the same. Here an upgrade pump was easily sourced....

...although it had a slightly different electrical plug.

However, the fuel injection specialist had available the correct plug complete with flying leads, and these leads were crimped to the original power supply wires, making sure that correct polarity was maintained.

The New Fuel Pressure Regulator

When replacing the internal fuel pressure reg with an external one, it’s tempting to go straight to an aftermarket fuel pressure regulator. However, it can be much cheaper to use off-the-shelf factory regulators, either from a fixed pressure system or from one that varies with manifold pressure. (If the manifold pressure connection is not made, these latter factory regs will provide a fixed output pressure suitable for returnless systems.)

This Bosch regulator costs about AUD$80 and was fitted as standard to the Holden JE Camira and the N13 Pulsar (and, we’re sure, to lots of other cars around the world!). It is Bosch part number 0 280 160 221 and provides a pressure of 2.5 Bar (36 psi). As it uses hose fittings for all its connections, it’s easy to plumb into place.

A higher rated pressure regulator is the Bosch 0 280 160 249 unit, which has an output pressure of 3 Bar (44 psi). It is of the type that is designed to fit on the fuel rail and so uses an O-ring connection for its pressure feed – external mounting will require a mounting block with an opening sized for the O-ring and another hole tapped to take a hose barb.

A 3.5 Bar (51 psi) Bosch regulator is also available, in the form of the 0 280 160 263 part. Again, it needs a block for external mounting. It appears that both these regs are fitted as standard to BMWs, however the prices are quite reasonable – about half the price of a brand name aftermarket reg.

If the fuel system pressure needs to be adjustable, better is this Bosch 0 280 160 001. It’s used in the very old Bosch D-Jetronic system. (You’ll find it one the VW Type III 1968-1973, Porsche 914 1970-1976 1.7L or 2 litre, Saab 1969-1974, Volvo, and Renault 1972-1974.) Nearly 40 years after it was first produced, this regulator is an ideal upgrade for returnless fuel systems. It’s still widely available from fuel injection specialists and costs just AUD$77.

The adjustment screw of the D-Jetronic reg allows variation in the fuel pressure from 2.5 – 3.1 Bar (36 – 45 psi), which is enough range for most returnless fuel systems. However, if you remove the jam nut and so allow the screw to be turned further inwards, fuel pressure can be j-u-s-t lifted to 50 psi – any higher and the pressure starts to fluctuate.

Finally, there’s the aftermarket Malpassi regulator. This regulator allows easy adjustment of fuel pressure over a wide range and if required, can be used as a rising rate fuel pressure reg on returnless fuel system turbo cars. If the manifold pressure connection is left open, fuel pressure is fixed – that is, you just adjust it to the desired level and then leave it. But if the manifold pressure connection is made to the intake system before the throttle body, fuel pressure will rise with boost pressure but will remain at the prefixed level when off-boost.

In this Toyota, where a turbo was being fitted to the previously naturally aspirated engine, the Malpassi reg was used to increase fuel pressure from the stock 44-50 psi to 60 psi off-boost. The manifold pressure connection to the reg was taken from before the throttle, so causing fuel pressure to then rise further with boost. By switching out of closed loop as boost rose, this gave the correct mixtures for the application.

Modifying the Standard Reg

If the external fuel pressure reg is to function correctly, the in-tank reg needs to be either removed and the opening blocked by a specific-shaped plug (eg one turned-up from brass), or the internal reg needs to be modified so that its output pressure is much higher than the level maintained by the new external pressure reg. The second approach is easier, and is what was done in this system.

To increase the fuel pressure setting of the internal reg, it needs to be removed and a suitably sized socket used to support the end opposite that containing the internal spring.

A press or a vice can then be used to slightly compress the end of the reg that has the spring inside, so increasing the spring preload.

Here this part of the reg has been shortened by about 2mm. As a result, fuel pressure increased to 90 psi (probably the relief valve setting of the pump) which allows the external reg to perform its function correctly.

Fuel Filter

Many returnless fuel systems either don’t have a filter (they rely on the fuel pump strainer alone) or have the filter buried inside the tank. However, if fitting an external regulator, it’s easy to also install an external filter.

The fuel filter bracket from a Mazda 121 (bubble shape) was sourced from a wrecker. This fitting comes with rubber mounts and suits a variety of EFI fuel filters, including the Z308 and the pictured Z306 designs. The filter is mounted ahead of the regulator and T-piece that sends fuel to the injectors.

Return Line

The installation of the external reg and filter requires a new return line to the tank be installed. The easiest way of doing this is to drill a hole through the tank pump/fuel level sender module’s plastic flange and install a brass bulkhead fitting. That’s what has been done here.

Complete System

The layout of the new fuel system looks like this. (However, this diagram doesn’t show the modified in-tank fuel pressure reg, which is incorporated into the module.) Note that as described above, we chose to boost reference the fuel pressure (though not vacuum reference it) but in most modified returnless systems, no manifold pressure sensing at all would be used.

Finding space for the external reg and fuel filter can be difficult, however here they were able to be installed towards the rear of the fuel tank, positioned above and forward of the beam rear axle. The green arrow shows the Malpassi rising rate fuel pressure regulator and the red arrow the new external fuel filter. Both the pressure reg and the fuel filter were mounted on rubber stand-offs, the filter on the standard ones that came with the Mazda 121 mount and the regulator on mounts salvaged from another car. Rubber mounting these components reduces the likelihood of fuel pressure pulsing noise entering the cabin.

Conclusion

While at first glance it looks near impossible to modify a returnless fuel system to provide more fuel at a user-adjustable pressure, that isn’t the case. By fitting a new in-tank pump, modifying the in-tank regulator and fitting an external reg complete with a new return line, it’s straightforward to modify one of these fuel systems to allow a lot more power capability than standard.