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Oscillating fuel pressure

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seeing some huge fuel pressure oscillations e.g 411 kPa to 255 when the expected fuel pressure is 350. (see attached image of one instance ) Generally happens coming off throttle (but not particularly quickly) and still with significant MP (50) and Throttle 20% , but also some other times and as far as i can see NEVER when engine is making a lot of power or during a ramp run.

Engine is a 3L, 4 cyl Porsche 944 turbo making about 440 Hp @ 120 Kpa on E40 and is on an engine dyno. Fuel system is a Bosch 200 LPH 5 Bar pump feeding 15 ft AN-6 lines, to a Haltech flex fuel sensor, then about a 2 foot run to the supply end of a large bore fuel rail with IDC 850 injectors where there is a Marren fuel damper and fuel pressure sensor. At the return end of the fuel rail is an Aeromotive A1000 rising rate EFI FP regulator, set for 3 bar.

image

Attached Files

Replace FPR as a first measure.

Is it only oscillating during sudden drop in MAP and fuel delivery?

As Dom' said, first thing to check/replace is the fuel pressure regulator - the symptoms you report are what I would expect from one that's sticking, and then over-shooting the target pressure when it unsticks.

Second thing would be to go over the hoses, both from the tank to the rails, and from the rails back to the tank, just in case there's a hose that is being restricted as the engine moves around. A pressure side restriction will drop it, a return side restriction will raise it.

so it's a brand new aeromotive FPR....possible it's bad, but most of the time it seems fine. Never at full power , never during an acceleration run. Being a frugal Yankee, at close to $200, I would like to rule out other stuff first.

As far as i can see it occurs fairly consistently when coming gently off the throttle, but still with a fair amount of boost (about 1 second to go from 100 Kpa to 50 Kpa MP).

As i'm conveying this info I'm starting to wonder if the Aeromotive FPR at the return end of the rail and the Marren Damper at the head end are doing "a dance". I think i may try removing the damper and see how that changes things as it's easy to do. Will report back.

You could temporarily fit a known good one secondhand. Since it is not under WOT, this test should tell.

You could send the A1000 to Aeromotive for an overhaul.

Is there a one-way check valve in the fuel system?

Also, try fitting a completely seperate, not T-ed off, vacuum line to the FPR.

FPR and damper going into oscillation with each other ("dance") is unheard of, but you could remove the damper altogether for a test.

It's a possibility there is some positive, and negative, feed-back between the components, but I would expect it to be barely measureable, and a much higher frequency, if present. I assume you're thinking of the 'water-hammer' you can get when rapidly closing a tap?

As an aside, where did you buy your FPR, as there are many counterfeits around - the official site has a guide to identifying genuine and fakes. https://aeromotiveinc.com/buyer-beware-genuine-aeromotive/

Some examples

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