FPR noise?

[moroza]

I'm chasing an intermittent idle surge issue and rigged up a fuel pressure tester. Jumped the fuelpump and observed the system holding at ~50psi with the pump on, but the fuel pressure regulator was noisy, making something like a fluttering sound. I've tested other (mostly M60) FPR and never heard this before. Any ideas if an M50 FPR is supposed to be silent or if the sound is normal?

[shogun]

Can only tell you from my M70 which runs at 3 bar. Installed last year 2 fuel pumps and tested them w/o engine running, ignition off, and a jumper wire from B+ onto the top of the fuel pump fuse for some seconds. For sure these new pumps have more than 3 bar and good volume and then I also could hear the FPR making a bit of noise as it re-directed the fuel to the fuel tank return line. Would not have heard that 'noise' with engine running.

[Honolulu]

I think it could be *sorta* normal. Think of opening a faucet just slightly - there can be a rushing/hissing noise as water on the high pressure side rushes through a tiny opening. Another noise can occur if the fluid column sets up a hammer condition, a high pressure wave that finds a natural harmonic in the system. It may also occur that high velocity fluid, causing noise, also causes local erosion of the orifice it passes through.

If your supplier will accept, and you've not heard this sound from the FPR before, I'd contact them and request an exchange part.

[moroza]

Well phooey. I can't reproduce the original noise, just a faint whoosh that sounds normal. I just realized the flutter was probably air pockets, formed when I disconnected the supply line to install a gauge, making their way past the regulator.

[shogun]

yes, faint whoosh I also could hear while testing with engine off, and to me it sounds normal, consider the pressure and the volume which goes thru that small FPR, for my 750 it is for example 87-150L/H 780-1650cc3/30s 3.0 bar

[ross1]

I suspect you are hearing the FPR going about it's regular business. Pretty sure it "regulates" by opening and closing rather than restricting. Without any vacuum signal to open it the fuel pressure is acting against the spring holding it closed. Pressure overcomes the spring it opens, pressure then falls, repeat. Like a mechanical fuel injector.

[Honolulu]

Ross1, disagree a bit. If the FPR opens a little bit (and the spring or needle/orifice isn't sticky and the fuel pressure is smooth as it should be from a vane pump) it will do so in direct proportion to the force exceeding the spring force, it will find equilibrium. that equilibrium won't be the FPR needle bouncing in the orifice.

I think Moroza got it right, there may have been air pockets in the system which allowed a rebounding pressure wave to develop. Water hammer is an interesting subject in the fluid mechanics world. The peak pressures can be significant multiples of the steady-state condition, and cause damage. Fortunately said noises appear to have un-manifested themselves.

[moroza]

New problems: with the rail depressurized, I jumped the pump relay and heard fuel flowing, but no pressure built. Bad gauge? Nope; removing the gauge and pressing the fitting's schrader valve got me an unpressurized trickle of gas. Clamped off return line and it quickly built pressure, then bled to zero with clamp removed. But then I pumped again and this time it built and held, descending slowly to ~40 by the time I quit for the day. It's obviously bad, but I wonder if anyone's seen this before?

[ross1]

No, haven't seen this. Perhaps some crap caught in the regulator holding it open.
Interested in the post mortem examination