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[mark.adams]

How does this thing actually work? I noticed the small vacuum pipe that connects to the throttle body, if i disconnect that, there is a vacuum, when disconnected the car runs sweeter, however if i close the end of the vacuum pipe with my finger, running gets unstable again... Red Herring??

[Abiton]

As far as I can remember - from reading a book - the idea is that the amount of fuel dispensed by the injectors is to some extent influenced by the pressure on the outlet side of them, as well as the 'pumped' side.

At idle there's a very low pressure in the inlet mani, and so on the outlet side of the injectors. So whenever the injector solenoids open, there's the 'push' from the fuel pump, and the 'pull' from the intake vacuum affecting the flow of fuel through them.

The regulator reduces the pressure in the rail such that there is a constant pressure difference across the injector, and so a predictable rate of fuel flow per unit of time of injector opening.


Mainly affects idle/light throttle running as that's when the inlet is at lowest pressure.


Clear as mud? 

[Kevin Wood]

Yep, flow through the fuel injectors is very dependant on fuel pressure supplied to them, so the fuel pressure must be precisely controlled.

The FPR maintains a constant fuel pressure relative to the pressure at its reference port which is the smaller of the 3 pipes connecting to the FPR.

Inside is simply a diaphragm with the reference port connected one side and the fuel rail connected at the other.

When the fuel pressure reaches the correct value, the diaphragm moves and opens a valve which starts to bleed fuel back to the tank via the return fuel line. This prevents the pressure increasing any further.

On some injection systems (and particularly those fitted to turbocharged cars) the reference port is connected to the intake manifold so the pressure across the injectors is constant regardless of the pressure in the intake manifold (which can vary from a strong vacuum when on the overrun, to several BAR of pressure when a turbocharged engine is on full boost). Effectively the fuel pressure follows the intake manifold pressure.

On other systems, the Omega V6 in particular, the reference port is is exposed to atmospheric pressure (it connects just upstream of the throttle on a V6) so the fuel pressure is constant and the injectors see a variable pressure.

Either way, the way the injection system is mapped takes account of the fuel pressure reference setup.

If your car is running sweeter with this removed, it suggests the fuelling is not quite right, as either changing the fuel pressure, or introducing an air leak, is affecting the running.

It could just be that the air leak is taking the engine out of closed loop lambda control so you are no longer seeing the slight fluctuations in idle that result as the fuelling is corrected based on the lambda sensor outputs.

[mark.adams]

Thanks guys, that has given me food for thought