Oxnardo Ring

Yep, I've used the same guide every time, but the working area is so limited in there for my big-fat hands that it's like threading a needle inside a ketchup bottle with two pairs of vise-grips... in a dark room.

jfrahm

The assembly is not really down in the "lobe" but above the RH one in the main section of the tank. As I have suggested IMO it's better to reprime with a small amount of fuel and get the system running off scavenged fuel right away rather than filling the tank to the point where it floods the fuel cup as you then have a while of driving before you really know it'll scavenge. It's also easier to see the scavenge system moving fuel around with the level lower.

I'm not unhappy with the system, I think the only significant issue is that the long bolt can get stuck and break the blind nut loose. Yes the fuel pump costs $140 but that's not a huge deal once every 120K or so. Having a big fuel tank and a lot of fuel low down in otherwise wasted space is great, I love how big the trunk is and how much gas fits into these cars. With a working and correct fuel pump the scavenge system seems 100% reliable, a twin-pump setup might be more frustrating and have some stranger failure modes. If you want to carry fuel under the rear passenger's seat, you need a way to supply that fuel and this system seems like an OK way to do that (to me).

If the hoses start to degrade down there my opinion might change.

sleepingvr4

I ended up adding almost a gallon of fuel to get the pump to prime and for the car to start. It would of never entered my mind that I needed to add more fuel. I've never worked on a car with a scavenging fuel system before. The car had a quarter tank and I assumed that would of been plenty to start the car, obviously not.

I'm used to working on my Galant VR4 which has the simplest fuel pump/sending using setup ever.

Thanks for everyone's help!

jfrahm

--nevermind, posted that tip in a different thread I guess---

Oxnardo Ring

Hey, great work! Glad you stuck with it and got it going. Now for the fun part-- enjoying the car again.

You'll find as many opinions about the fuel system quirks as people who've worked on them. And, it Is a weird system, between the two lobes, the scavenge-siphon, the "cup" with seals and even the return line routing.

I'll write up my recent S8 experience, but the short version is that I had ruled out EVERY other item-- including replacing the relay, fuse and the filter. I'd done TWO rounds of 4.5 gallon (I knew it was "low" when it stopped and assumed I'd run it completely out, killing the pump) jerry cans. And, yet, it simply would not fire. I don't know why, but as one last "check" before actually pulling the clamps, retaining ring, the banjo bolt and the 7 foot long hold-down bolt ... I did a quick hook-up on the pump's electrical contacts up top. Give it the battery jolt for 5 seconds "Gurgle-gurgle-gurgle". ME: "WTF? What's the gurgle?!?" Did it again "gurgle", "whir". Hooked the harness connector back up, went to the key... turn, fire, Joy to The World. It's running.

So, my new rule is "Before you give up on the pump. 1) Plenty of fuel 2) Prime it, prime it again"

Funny, I was working on my son's older A8 over the weekend-- thought I'd prime it (it had run out of fuel) and bring it back to life to prove the point... And... No Joy.
The fuel pump's toast. Go figure.

Also, it's clearly not good to run them low on fuel. So, I'd basically recommend never letting 'em go below 1/8 of a tank.

Oxnardo Ring

Nomenclature I guess-- I think of the larger "right" section, what you've called the main section, as the right lobe. The pump nestles in the snap-together "cup" in the bottom of the assembly lowered into the right lobe. The left lobe is definitely smaller, but I think the D3 and later kept that layout but used a lift pump out of the well in the left lobe rather than rely on scavenge.

Every time I work on one I think of the complexity Audi put into the two-lobe scheme, probably as you mention to use the space better and lower CG.
I do have a Lancia Stratos with a dual-tank-lobe scheme like that. Rather than the complex scavenge-pump-siphon set-up the Germans put thousands of engineering man hours into... well, the Italians just used an interconnect pipe. which is better?

BrianC72gt

The Fuel tank is really not all that difficult. Odds are it died when low on fuel. Once installed add a pint or so of gas to prime the pump. For testing, the guaranteed method I use is to screw in a brass hose barb on the top of the fuel pump assembly after installing it; 6 feet or so of low pressure fuel safe clear plastic hose... the cheap stuff; pop the plastic cover off the fuel pump relay under the front passenger's feet, push the relay contact points closed; switch ignition to run and pump out the tank.. Now that the tank is bone dry, add one gallon. Pump out as much as you can. you should get about 3/4 of it back out. You're good to go. fill it up and drive it like you stole it.

I need to check the threads on the banjo bolt and post an amazon ling to a threaded barb, but I'm too damn lazy.

Oxnardo Ring

Funny, the first one I had die (and I was lucky, since I could easily have been in rural Nebraska) was about 10 years ago on a '97 PFL. I had just filled at the local supermarket Qwik-Stop, drove exactly 8 miles, turned into my neighborhood and it coughed, then coughed harder as it shuddered to a stop. I pulled over as it died. Dead.

That's was the only one that was easy to diagnosis. The next 5 I've had to pull have always been in some low fuel level, battery dying, CEL ON, mis-fire combination of other "stuff" going on that made it tough to pinpoint. They seem to die more with low fuel level, and since that's what I had going on with the S8 recently, I would have bet any amount of money that fuel pump had gone as it was running out of fuel. Except that as last resort I primed it manually after putting plenty of fuel in it-- and it started right up. That was 1500 miles ago, so I think the pump's fine.

Zumkopf

I did the fuel pump replacement job today (well, finished the installation today anyway). Getting the assembly back in was by far the most frustrating part of the job. You have to do it totally blind, of course. I got the assembly close to where it was supposed to be, then inserted a wooden dowel down the long bolt hole as a guide. I thought that would help, but it didn't seem to make much difference at first. I kept twisting the assembly, bouncing it up and down, pressing, tilting, cursing, pulling it out to check for some unseen obstruction, nothing... until all at once, for no obvious reason, it just dropped into place. After I verified I couldn't twist the assembly in either direction, I installed the main seal. I did a variation on the above seal procedure, getting the assembly aligned first, then worked the big seal into place around the assembly neck, then slowly worked the assembly and the seal together into the hole. It took some patience but that part went easily enough.

I had thought to use the long bolt to pull the assembly down into position, but apparently the nut below isn't fastened to the tank itself, but some assembly in the tank. I fully tightened the long bolt without drawing the pump assembly down into place; I'm guessing it pulled the nut and its assembly up instead. Even with the bolt tightened, the pump housing could be pulled back up. So I removed the long bolt and put the dowel back in, then pushed the assembly down into place. The tip about using a pry bar to pop the assembly down the last half inch is a good one. Then the long bolt went in and tightened as expected. Everything else buttoned up easily. I have the battery recharging while I go for fresh fuel.

That raises my question: I didn't fill the fuel pump housing cup with anything, just put the assembly in dry, figuring that if I added enough fuel through the normal filler, it would overflow into the cup, and I could test it then. How much fuel do I need to add to make it do that, overflow and fill the dry fuel pump cup? Five gallons? Fifteen? The tank is close to dry now, I had siphoned out all I could before attempting the repair.

Tanks for any advice, Z
1998 A8Q (D2) 140k

Oxnardo Ring

Ahem. This is highly controversial.
Some people think just a few cups are enough, via the filler pipe, to get it to prime enough to start.

After that S8 awhile back, I'm on the other end of the spectrum. I think, if it's dry, worst case you need to put in enough to be certain it will overflow the pump basin in the right lobe. I've done 3 gallons and found it wasn't enough, so anymore I do TWO fills from my 4-5 gallon WalMart jerry can... and only then "PRIME" it by giving the pump several 10 second ON cycles sufficient to make the pump housing stop gurgling. If there is still air in the system you will get sort of a muffled "gurgle" from back there.
I think it's harder to pull the passenger side mat, pull the carpet insert, remove those weird wood-into-plastic screws on the foot cover plate and then jumper the fuel relay, so I just jumper it in the trunk via a couple of spade lug contacts to the pump contacts in the fuel pump assembly connector. Once it's primed, if the pump's been replaced, it will generally fire right up.

I think the issue is that the "timeout" the ECU gives the fuel pump to bring pressure up to the mark is insufficient time to clear the air out of the system and scavenge pipes, so the ECU gives up on the pump before it can build pressure. Thus, if coming from a dry tank scenario it's necessary to "override" the timeout by either jumpering the pump relay (which I think is harder) or applying power directly to the pump (preferably via a fused link from a battery)