I called the garage today, seems I went for the wrong end of the fuel rail. It was the FPD (fuel pressure dampener), not the FPR (fuel pressure regulator). It had deteriorated with time and was flooding the engine with pretty much a full on flow of fuel with no pressure dampening. The part comes in tomorrow so should be reassured by then.
Ive shamefully plagerised all this from rennlist, but its good reading and explains it really well so thanks to whoever Greg is.
FPR vs FPD (Edit as Geo mentions below, this is related to the L-jet system, not K-jet)
QUOTE In both systems, the injectors batch fire once per rev, twice for each 4 stroke cycle. In other words, two squirts per putt. Half the fuel is injected each time.
It is a "batch fire" system - that is, ALL the injectors fire at once. "Sequential" (the other system) precisely timed to attempt to fire most of the fuel charge at the open valve *can* get you another 1-2% HP/TQ, an (why manufacturers do it) is because you get slightly better emissions.
On a 928, you'll notice that the "damper" or "dampener" is located at the "IN" end of the fuel rail. This seems odd at first, because the damper is there to absorb pressure waves in the fuel rail from the injectors, therefore it should be at the end of the rail, no? No. Since the 928 does batch fire, the injectors all fire at once , ergo no big waves.
However, the Bosch roller cell style fuel pump does generate some pressure waves - so the damper is really there to absorb those waves, and to act (sort of) like an accumulator. It builds up a "pressure reserve" before all them durn injectors fire again.
The regulator does just what you think it does - it regulates the pressure the injectors 'see' by allowing a certain amount of fuel to be returned to the tank. The vacuum reference allows more fuel to return to the tank during periods of high vacuum (like ~25 kPa absolute) like you see at idle, or during over-run.
How? The damper is just a spring loaded diaphragm and so is the regulator. Though the 928's damper is vac referenced, most aren't. Think of it as a trampoline in a tube. Or (almost exactly) like one of those anti-hammer jugs you put on your hot water lines. Tube contains a flexible membrane on one side, and a spring on the other. Fuel flows under the membrane, membrane flexes at some predetermined rate to absorb the 'jumping' fuel pressure.
Presure regs - though the more pedantic call them 'relief valves', which is technically correct since they are on the "OUT" end, not the "IN" end... I call 'em regulators, because that's what Bosch calls them.
Regs are kinda like dampers. Fuel flows under a diaphragm. This time the spring is a bit more serious in resistance, and the vac ref is really helpful to allow the regulator to keep the fuel at a consistent pressure thoroughout the load range.
Maybe try find a cheapo second hand FPD and see how it runs after..
