Fuel confusion S8 D2 FL...

[Rightthen]

Looking for things I may have missed concerning fueling issue before i start investigating the rats nest in the tank. I'll try to get as much relevant info as I can....

My S8 D2 FL has been suffering intermittent fuel starvation for a while. Finally it gave out. I figured fuel pump, so I ended up with a running donor car in the form of a FL 4.2 A. I changed fuel pump (the old one was turning when I put 12v direct but thought I'd take no chances), the relevant relays and fuel pressure regulator.

I filled tank to 3/4 and it ran sweet.... for 10 minutes.

Is there a situation where the housing gets primed by fuel coming down the filler pipe then runs out? And if so, what is the fix? Things I'm missing?

Any suggestions appreciated before I delve back into the fumey boot

[Ronin]

Have you replaced the o rings on the fuel pump assembly?
Have you replaced the fuel filter? mine was 20 years old when i changed it
Is the current fuel pump in the car a VDO one?

[Rightthen]

I've not replaced o rings.
Pump that's in there has at some point been replaced (I assume) as the clips from the donor car were replaced badly (bit scary). Whether it's VDO or not I'm not sure as I didn't take it out the housing. The pump that I took out I'm pretty sure was original

[Rightthen]

Where is the fuel filter on the D2?

I had a little look, but I'm used to seeing a little plastic inline thing a la mk 1 and 2 golf, and I know on these it'll be a properly made bolted on situation

[Ronin]

The cheaper non VDO pumps don't have the second inlet to suck fuel from the other half of the tank, so that could be one issue.

Pump fitment itself - it took me 3 attemps to position the pump correctly so that it sucks in from the other half of the tank.

And finally the o-rings, if they are cracked, lost their elasticity or shrunk due to age they will not seal up properly and once again causing the pump not to 'suck' well.

All the info regarding all this is on the forums, I've attached the fuel pump service manual.

[Rightthen]

That's great. I'll pull it out again, check o rings (they looked good on quick inspection, but I'll scrutinise), check which pump is in there and change filter.

I watched the Russian video with the cut away, before replacing and it felt like it sat in right, but if I get no joy, I'll try resitting again.

Thank you for replies, it's much appreciated

[MikkiJayne]

Its very unusual for a failing pump to cause fuel starvation symptoms for anything more than a few minutes before it fails completely. They tend to work, and then don't. Your description implies something other than the spinny bit - plumbing or similar.

Running out at 1/4 tank is very common - usually either incorrectly fitted, or the wrong pump. Duff seals on the pump assembly can cause the inlet to cavitate at low fuel levels, but the pump is completely submerged at 3/4. Quite odd for it to fail at that level, unless its just a crap pump like a Magnetti Marelli which tends to work for ten minutes then burns out.

Normally after changing the pump at 1/2 tank or less it needs a gallon or so throwing in to prime the pump bowl but then it should be fine.

If you're taking it out again, make sure the return line is tucked in to the fuel pump bowl. If thats just discharging off to the side that can cause issues.

[ainarssems]

With 3/4 of tank full it should be running anyway even if it's not seated correctly or if the o-rings are bad, it has to be something else.

Edit: Mikki beat me to it

[Rightthen]

Figured that it should be submerged, regardless. Just baffling that it ran sweet then stopped. Going to change filter, but I'm not sure it's that alone.
I'll double check the return line, but I was super careful putting it all back and looked at many videos, images etc.
I'll check branding on pump too, but both seem to spin well with direct 12v and as they'd be submerged they'd surely go until it stopped getting in the top of the housing?