Hollow tapping from fuel rail Duff FPR?

[Greybeard]

Tapping follows the revs till they blend into a hum, fpr is not leaking and the note doesn't change when the hose is removed, I have a feeling the fpr issue may be a red herring, it may just be what you suggested earlier, possibly loose (or too tight) fuel pipes making contact with something when things warm up and transmitting the noise through the body.
I'll check this out more thoroughly and get back.
Myron

[Laird Scooby]

Very interesting! The tapping increases with frequency/note of the engine revs increasing?

That almost negates any argument for it being the FPR to be honest as if it's definitely engine speed related then the fuel pressure doesn't vary with engine speed, only engine load, especially if the turbo has kicked in.

I can't visualise the fuel pipe arrangement on the 940, i'm guessing it should be similar to the 740 n/asp but there could be detail differences due to a different injection system (i had K-Jet B230E 740s) but i'm wondering now if the turbo wastegate (or blow-off valve, all turbos i'm aware of have them, just normally they are silenced so you don't hear that "pa-chooosh" sound) is "chattering" slightly causing pressure fluctuations in the inlet manifold and hence the fuel rail.

If you disconnect the air intake trunking between the turbo and the air flow meter/MAF sensor and the tapping disappears it would bear further investigation along these lines. It's a shot in the dark to be honest but it might just be something silly like that.

[Greybeard]

Following Dave's advice I removed the hose and disconnected the maf sensor, the pulsing knocking disappeared! yoohoo!
Poking around the wastegate and I discovered the rod had become disconnected, just flapping in the wind! hadn't heard the "pa-chooosh" sound for a while and just thought it a consequence of the fpr not functioning correctly, wrong was I.
So the wastegate is wasting but may also be chattering as we still have the pulsing knock with the maf connected.
I've redirected the knock away from the cabin by zip tying the fuel return to the dip stick so grounding it through the engine.
MAF connected ... pulsing, MAF disconnected ... no pulsing
Thanks again to Dave for good pointers.
Getting there!
Myron

[Laird Scooby]

As i said Myron it was a shot in the dark - glad it worked out though!

Now maybe one of the "hands-on" turbo owners can explain how to sort that rod out for you, i've only owned three turbo vehicles, an MG Montego Turbo, Rover 218SLD and an Astravan 1.7TD. As such my turbo "hands-on" knowledge/experience is limited but i'm sure someone will explain!

[mocambique-amazone]

Hi Dave, well pointed.
The pressure inside the manifold will raise and fall fast if the waste gate isn't connected and will close-open-close

The Pressure inside the mani regulates the pressure at the fpr. It's atticted to this to raise the fuel pressure at idle.
The MAF will tell the ECU "the engine is at idle" the fuel pressure goes up and down, not the same opinion these parts ;-)

Hopefulle I'm able to write this understandable, not my native language

regards, Kay

[ericbeaumont]

This has me thinking, Myron, because I've had a constant and distant motorboat in the cabin since I bought the car at least five years ago; '97 945 LPT manual. Noise (coming from near the bulkhead) disappears when moving only because (I suppose) other noises drown it, but my guess was that as the car runs well (mild rough idle occasionally) that the noise was something in the aircon (disconnected by a previous owner) or transmission.

New clutch last year and all is in order with no change to the noise.
Turbo was taken apart two years ago during an HG job and that was and is connected properly, so my motorboat is a mystery. Didn't hear it on a previous car, a non-turbo 945 manual.

[Rversteeg]

I also have a kind of faint tapping noise coming from somewhere near the bulkhead. The noise is slightly different when I switch over to LPG, so I guess it must be pressure surges in the fuel supply line, coming from the pressure regulator and injectors. I never gave it much thought, but seem to rememeber having read somewhere about altering the clamps on the fuel line.

[Greybeard]

Motorboat is a new one, someone on a Jag forum suggested helicopter, whatever it's still irritating!
Rversteeg! yes I reckon something going on with the fuel supply/pressure regulator/injectors, although on mine it's more than a faint tapping, tying the return pipe to the dipstick tube channels the noise away from the cabin and left wheel arch (where the fuel lines are) These lines are well clipped! Disconnecting maf and the noise goes on tickover, so I assume the fuel quantities are adjusted to a default value which just so happens to stop the oscillations?? More than I can figure!
Myron

[Greybeard]

Okay, digging further on this site and I came across https://www.volvoforums.org.uk/showthread.php?t=71392 which makes reference to an 'impulse inhibitor' which apparently moderates surge or impulses from the fuel pump. My B230FK has this device! Maybe it's there to prevent impulses from transmitting through the pipes?

[ericbeaumont]

That makes sense, but how to isolate and resolve the problem? Could be a several hours of fiddling around changing parts without much joy, and as the noise hasn't varied at all in five years and makes no apparent difference to driving, is it worth the fiddling?