Adjuster

Okay, So since I bought my 2001 S8, the CEL would flash on WOT, but then go away, then it flashed at speeds over 120mph, and was fine at "normal" highway speeds.... (Around here, it's common to drive 82/83mph for hours and hours and hours.)

Well, I ran a few gallons of toulene in there to see if that would clean out any gunk, and it sure did. BUT it also took some life out of the pump too it appears, as now it's not happy if I try and go WOT or drive faster than 70mph...

So, I bought the wrong pump, then got the right pump, and when I went to remove the long bolt... POP, it's just turning the captured nut down on the venturi/siphon cup assy....

So, I have a car that runs great around town. And even at 55mph. But want to faster than 70? No dice. Want to acclerate at WOT for the onramp? No dice. (But it's plenty fast the rest of the time, and actually goes just fine, just flashes the CEL lamp.)

Here is my dillema.

The new housing is 800.00 or more.
The used ones are about 350.00 to 375.00.

I can get a in-line 255lph walbro pump for less than half that. (About a third actually.)

The pump in the tank/housing and siphon system all work just fine.

It's just not moving enough volume/pressure to feed the engine's needs at higher RPM, and under sustained load from what I can tell.

The in line Walbro would "suck" on the feed line to the injectors, I figure I'd mount it just after the stock Fuel Filter, in a section of "hose" there where it can be spliced in with minimal fuss.

Hook it up to power with a relay wired up to the stock system so it's only on when the stock pump is on, and it SHOULD be able to keep up with this engine's needs no problem, and I'd hope, the lack of "resistance" now on the old/stock pump in the tank would be able to allow it to feed the new in line Walbro just fine for many years to come.

Any ideas on this?

Has anyone done anything like this, or even considered it?

In my Supra days, we used to put twin in-tank Walbro's, and a few people did external fuel pumps with custom pick-ups etc. (All of them feeding 650rwhp engines needing far more fuel than this 360crank hp rated V8 will ever need.)

I know the in line Walbro's don't like to suck on the tank much. (IE: You want them close to the tank as possible, and have a easy source of fuel, but I've seen them draw through existing fuel pumps/systems with pretty good results, and where the stock pump on this A8/S8 is such a over-engineered mess, I'd rather just use it as the feeder system for a high quality external pump if I have to build my own, or spend 500.00 or more to try and get what's in this car to work correctly.

Thanks in advance for your advice guys.

audinutt

I think you know what the right answer is.. Fix the car correctly, I have no doubt you will have more issues adding the walbro, it's a patch that probably won't give you the results you are looking for. I would start pricing a used fuel tank.

Adjuster

I'm looking around, and I don't need the tank. Just a "Custom" hole saw that's about 12" long, and goes down over the bolt, and cuts not only the top part around the bolt, but down at the siphon cup/nut area too. (I think the trick is to not cut the siphon cup, or you'd have to replace that too, since it seals up there right?)

I've got a call out to a yard who has the pump/housing, but they were not sure if they had the bolt, or the siphon cup fixture that I'd also want to get if I go this used route.

As for replacement of the tank? Seriously, I'll create my own custom fuel pump setup and yes, it would be done right and work fine the way I'd set it up. (I've built a few, even on plastic tanks using a plastic welder in one case, but AN bulkhead fittings and seals the rest of the time.) They don't leak if you do it right, and use the right fittings and seals.

On this tank, with it's funky design, you would need two or perhaps three pick-up points externally at the lowest points, that then feed a good external pump such as the Walbro "in line" pump I've written about here.

These Walbro pumps are rated at 255lph at 60psi. That's way more fuel than this engine will ever need, Seriously.

The other idea is two "internal" pumps, using the "stock" catch can, but two Eclipse pumps with the pickup socks back to back, so they fit into the space available. One pump would be hooked up via hose to the siphon system, and the other would go directly to the fuel rail, and pressure regulator/return line that then dumps excess fuel back into the tank like normal.

This might end up with too much fuel being pumped into the cup, but that's fine, it would just overflow, and dump fuel back into the tank. No big deal, it's what would happen now when the tank is full anyway, or if fuel demand is low, and the siphon system fills up the catch tank the stock fuel pump sits into.

The stock system is nice, but dang, at the cost they want, I can fabricate a much more robust, and less expensive system.

audinutt

Good luck, I wouldn't want to buy a car with a modded fuel system.
Just keep that in mind, the engineers designed it that way for a reason.
If the captive nut it damaged, you might be able to repair it..
It's hard to say.

good luck.

-Richard

Adjuster

I hear you on the mods, and in general, I'd agree, that most mods are often not done well, or even as well as what the stock setup was.

In my research, I've come across a guy who welded up his own "extended hole saw" using a basic hole saw, with a section of 12" long pipe welded into the middle.

The idea/technique is he used that to drill down around the long bolt, through the upper housing, and into the lower housing, where it's bolted into the siphon cup fixture. Looking at the seal/o-ring for this, it would be heck to try and get a good seal if the hole saw cut up the sealing surface of the siphon cup. At a minimum, the entire upper fuel pump housing will have to be replaced, and the guy ended up cutting the bolt off to get the siphon cup fixture out of the tank as well.

He goes into detail on how to remove the hoses, says they are a quick disconnect, you just squeeze them, and they pop off the siphon cup fittings.

From the diagrams, I can see there are three siphon pumps that then dump fuel into the cup, and that in turn feeds the catch can/fuel pump housing.

My real concern is if the OEM pump fails completely, the in-line walbro will not likely move it enough to "push" fuel through the siphon pumps, and the cup will run dry when the tank fuel level is lower than the catch can opening. (As happens when the seal is not good, or the pump is not placed right, and the pressure feed line not inserted into the siphon cup.)

Short of completely going custom, and considering that my fuel pump right now works FINE at less than WOT, or at speeds below 70mph, I think it might have enough life left to keep on supplying the siphon system for a good long while, especially when "helped" along by the external pump that is rated at a much higher flow than the OEM pump from what I can tell. (However, the VDO/Siemens pump does appear to be very close in shape/size/design to the Walbro setup, does anyone have any idea how many LPH it pumps? The Walbro's are rated at 255lph at 60psi.)

Another consideration on this vehicle is it's a prior total loss, so modding the fuel system is the least of it's worries. (It's only worth 1/2 of ACV to start with given the history of the vehicle. It's fully repaired, but some shortcuts were done, and I've found most of them, but it's dang solid mechanically, except for this fuel issue.)

Nobody has tried this before?

I'm not really excited about being the guinea pig, but I'm just as un-excited about spending 500.00 or more on fixing it the "OEM" way, assuming I can even get the siphon cup fixure used, along with the fuel pump housing used. (And put the over 230.00 OEM pump I have sitting here into that.)

Walbro's are 100.00 all day long. Even if I have to custom make my own "siphon" system supply, catch tank and put another walbro into that to supply the fuel injectors, it's going to be robust, and use the existing stock setup to supply fuel to the cup, just like a race car. (I can see why Audi did this, but dang, they made it more complicated than it needed to be. Just put two simple pumps in there folks, one for the fuel rail, and another to supply the siphon pumps to feed the cup. Place them both in the cup, and you are just like stock in that any fuel added to the tank fills the cup first etc.)

And just hook it up with hoses. No idiot o-ring needed. Un hook the siphon pressure hose, and return hose, and the entire fuel pump/catch can and dual pumps comes right out. Easy as pie. Simple as cake.

Totally not German however.. LOL

Adjuster

Well, the results are interesting.

There is ABSOLUTELY MORE FUEL going to the engine now.

I managed to fit the inline Walbro down by the stock Fuel Filter, and just cut the short flexable hose there, and used brass male to male barbs and fuel injection rated hose to put the Walbro into the line so it sucks on the filter and sends the fuel to the injectors.

I wired up the pump to the existing wires, using "Positaps" 8-12ga, so they don't disturb the factory wires, but absolutely are sanitary and a very good connection. (I've used the Positaps and posilocks for years now, and love them.)

So, here is the funny part.

Started right up, checked for leaks, none found.

Took it for a test drive, and WOW! The engine is happy to have the fuel! It's no longer "detonating" under load off the line, and the engine sounds so sweet being fed the right amount of fuel...

But here's the part that confuses me.. The CEL still flashes!

And on the highway? Well I can go 80mph + if I use the throttle.. but engage the Cruise Control, and it starts flashing the CEL, and drops injectors! ARGH!

So, it's not a fuel issue, other than I do think the stock pump was/is failing to get enough fuel there, since it does run so much better now, but could the ECU still keep the codes from the before? (I have not cleared the codes, and have been busy with a trip over to Mountain Home yesterday, laughing about how it I drove without cruise, it works, but on cruise, it flashes, and then runs rough as the injector(s) get dropped out by the ECU.

So, long story short, the Walbro works.

But the CEL is not yet happy with all the fuel it's supplying. (And now I can hear a pump, since my stock one was quiet as can be, and this walbro is noisey as they usually are in my experience.)

My Dad think's I need to find another ECU, and try swapping them out to see if that will cure the problem.

Any other ideas folks.

The fuel filter is new.
The injectors were just cleaned, and were not dirty to start with. (Only one injector picked up 1cc, and all checked out resistance wise.)
The fuel pump can't be changed with the stock pump, since I can't get the pump housing out without cutting it up, and replacing it with another unit due to the captured nut just spinning in the siphon cup.

I hate to take it in to the dealer, but I do trust the local mechanic, Kevin, who's always been very good with my cars over the years, and in Boise, is the ONLY guy I trust to even work on my car. (I've had some bad results with "Audi Experts" and while I did let Aamco work on the transmission, I watched the guy do the work, and he was fine. )

audinutt

If you swap the ECU it will have to go to the dealer unfortunately unless the ecu is sent off to have the immobilizer defeated.

good luck.

-Richard

Adjuster

Update: So, the saga might finally be over.

I had added the in line, external Walbro 255lph metal pump, and it works fine. I have it wired in with the stock pump, seems to work great. It does make noise, while the stock one is totally silent on my 2001 S8, and has been from the time I bought it with 75k.

So, the CEL was still flashing, and the random multiple misfire deal where it drops out a few injetors until you shut off, and restart the engine...

I talked to my local Audi Dealer mechanic, who is the only mechanic I trust locally. (Have worked with a few now, and Kevin is the best one locally, and a really helpful guy as well.)

I went over with him what I've done, and he asked me to pull up measureing blocks 15, 16 and 17 while the engine was running.

He also said the AFM might be bad, and that was block 32 IIRC. (Did not show values that it's failing, so the AFM is fine.)

BACK TO THE STORY So, I pull up the measuring blocks, toggle up to the right numbers, and there you go, all 8 coils in real time, showing any misfires.... And BOOM!

At idle. All zero's.
At 2500RPM, #8 starts to show 50 or more.. At 3k, it's 80, and #1 and #7 have joined in, and are showing misfires as well.... By 5k, they all are over 100, and #8 is 250+...

So, clearly based on this test, I have three bad coils, and the worst of the bunch is #8, followed by #7 and #1 in a tie for second worst...

I have three new coils now on my kitchen counter, from RockAuto.com for just over 15.00 each. They don't have the rubber o-ring, but I can either use the existing ones, or buy new ones for a few bucks tops. The coils are black v/s natural potting material color, so it will be easy to keep tabs on what coils have been replaced.

I re-set the VAG Comm a few times, and cleared all the codes, and guess what folks?

NO MORE CEL FLASHING, and the car runs fine up past 85mph. (Fastest I have pushed it in the past few days around here in the city.) Downshifing into a lower gear, and running it at 6000RPM does not flash the CEL either.

So, I plan on changing out the coils I know are failing just because I know they are failing, and it was less than 55.00 shipped to me from Rockauto.com for new coils. (Standard parts brand.)

Any thoughts? I am VERY happy with the easy external fuel pump fixing the fuel flow/pressure problem, clearly there is no more CEL, but I'll change out the coils that have more misfires just because I think it will smooth out the engine, and perhaps improve fuel economy, and at today's gas prices, the 55.00 will come back pretty quick with a few mpg improvement.

silverd2

No thoughts on the fuel pump situation, but on the coils:

I have never heard of these coils "partially" failing. In my experience, they have been all or nothing...no misfires before failing...just die suddenly.
I'll be curious to see if replacing still working coils makes ANY difference at all...if they do (smoother or better mpg) I'll be shocked and re-educated.

The only change I've ever made on my D2 engine that created new smoothness was new front 02 sensors and going from thick core (OEM) 4 prong Bosch plugs to 4 prong platinum Bosch's.

The only misfires (NEVER intermittent) I've ever had at any rpm, were at the moment of sudden complete failure of a coil...on 2 different occasions...both times "off", never to fire again.

Adjuster

With the codes cleared, the external fuel pump cured the CEL/Misfire issue completely. (I have not been able to get the CEL to flash at all, and I've tried under various loads, but have not gone to top speed either, and that would be the ultimate test of the engine, at high RPM, and wind resistance etc.) Around town, under full throttle, it does not flash at all, just runs/pulls smoothly.

The only way I'd know there was an issue with the coils is by looking at them in real time, all 8 under the 15/16/17 measuring blocks. It shows a misfire count, and they should read all zero's all the time.

Any misfires show the coils are not working as well as they should according to the Audi Mechanic who told me to look at those measuring blocks. He says you can also look at your AFM, and other items, but Audi does not even tell them what each block does, just says look at this block/number, and then gives a range for the results. (Only from looking at the various blocks over time, did he figure out that these three blocks are the coils/misfire counts.)

He says he's seen coils that worked, but failed under load, and created the "multiple" random misfire codes, and also AFM's that are dirty, or failing that will create a code even when they work the rest of the time just fine. (Or within the parameters enough of the time not to trigger the CEL.)

I don't know that I'd ever have swapped out the coils 1, 7 and 8 because there is no more random misfire codes now that the fuel system is working like it should with the external pump supplying fuel. The mechanic says he's seen fuel pumps have the internal check valve fail, and they "bleed" off all the pressure right at the pump, so they work, but don't provide enough pressure/flow for the car to run correctly. He says the only way to find out, is to pull the pump, and pull it apart, and since I can't get the pump out of this S8 without serious surgery/cutting/cost, he's impressed that the external pump works as well as it does. (I showed him the setup, and it's very hidden, under the filter cover/plastic and you can only see the pump peeking out from one corner of the plastic that I had to trim to make it all fit right.)

If that's what happened on this OEM pump in the tank, it would fit the facts that I encountered. It only showed up at WOT and speeds over 120mph at first, but after putting the 5 gallons of toulene in, the car inside of a few miles, was not able to run more than 45mph without serious problems. (Dropped injectors/coils, bucking and poor running.) Shutting it off reset the system, and it would idle fine, and drive fine. After the "crud" cleaned out more, it would run up to 65mph without issues, but still flash the CEL at WOT, and if I used the cruise control, or went faster than 65mph... (But it would go 85mph easy, just running very poorly as the injectors/coils were shut off by the ECU until the system was re-set by turning it off/on again.)

I figure if the check/pressure valve is stuck, dirty or failing, that the pump is bleeding off pressure/volume internally at the pump, so not moving enough fuel to the engine, but not because the brushes are worn out or anything like that. Perhaps even whatever is in the check valve might break down completely and the external pump would not be needed, but I don't have any problem running two pumps like I am now, and I know the 255lph Walbro will supply all the fuel this engine will ever need, and then some. (And it clearly is allowing the siphon system to work fine, as I've run the tank down to nearly empty a few times, with zero issues.)

I'm going to swap out the coils later today, was going to do it yesterday, but wanted to get new o-rings if possible, and the "economy" coils I bought did not come with the o-rings installed. (Other than that, and the potting material color, they are exactly like the OEM coils on the car.)