Fuel Pump Blues, take TWO.
 

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.

Have you changed fuel filter? toluene is more lubricating then petrol so it should not have negative effect on pump. But it is possible that it lifted gunk from tank an blocked filter.

Yep, sorry I left that part out.

I've changed the fuel filter out, and that had no effect, but there was some "jello" like crap in there, and it was very dark with crud. (I cut it open and inspected the paper media, and very little large sediment was in there, but the paper was dark with fine material that had been trapped. It's either the worn brush material, or metal from the communtator?)

I have no idea what the jello like crud was, it competely evaporated away when I dumped it out onto my concrete driveway.

So, I then pulled the fuel injectors, and had them cleaned and checked. They all passed 100%, no resistance problems, and only one picked up 1cc of flow improvement, so they were not dirty at all. (The shop owner told me there was none of the jello crud there at all, and no debris in the filters at the injectors, so the stock fuel filter had done it's job.)

That just leaves the pump starting to fail, or not producing enough volume to supply the engine under certain loads/RPM from what I can tell.

It does not run out of gas, even when it's down to nearly completely empty. When I tried to pull the pump, and the bolt just turns, I had it down to about 1 gallon. I put 21 gallons back into the tank when I filled up after realizing that this pump would not be coming out without surgery anytime soon.

I drive it nearly every day, around town it's fine, powerful, and runs perfect. But put it on the highway at speed, or race someone at the light, and it will flash the CEL light. (And on the highway, it drops out injectors, and makes the engine run rough until I shut it off, and start it again, then it's smooth as usual.)

Just another thought; is the fuel pressure regulator lifting to early so not enough pressure is being delivered to the injector rail?

Adding a high pressure pump won't help if the regulator is not allowing pressure to build up in the rail.

Good question? I don't have the fitting(s) needed to hook up into the fuel line, and check the pressure at the rail.

If the pressure regulator is not working, that would release too much pressure back to the fuel tank, and create a lean condition under load. (Good call to rule that out before I do anything else.)

Guess It's time I have the pressure checked, or buy the fittings/gauge, and check it myself.

Is it pretty common that the fuel pressure regulator fails on this vehicle? I've never seen one fail on any vehicle, they are pretty simple devices, just a diaphram and spring that regulates the pressure. (I've even seen ways to raise fuel pressure, by crushing the stock Toyota units, on turbo cars. **Using a vice, and socket, you crush the metal housing, compressing the spring more, and raising the base fuel pressure.**)

There are no leaks at the rail, or pressure regulator. And all the vac lines are hooked up at the regulator too, but I'll check the hoses again.

You can get new VDO pump same as original on aftermarket for around £120, no need to buy whole housing.

Have that pump sitting on my desk. :)

Problem is, the long bolt that secures the siphon cup to the pump housing, just spins the captured nut way down in the tank where nobody can get at it.... Without destroying the pump housing in the process...

Thus my problem/question/possible solution thread. :)

And I tried a simple test to see if the pressure regulator was failed, or allowing too much fuel to just flow back to the tank? Easy as can be, two short sections of hose, slipped over the jaws of a needle nose pair of vice grips.. and clamped the return hose down, and after a few tests, completely shut. (So all the fuel, only could go to the fuel rail.)

No dice my friends. The pump is failing to move enough fuel to supply the injectors, even with the return line clamped shut, so it's not the fuel pressure regulator, filter, or injectors.

IT's the pump. (That I can't get to without cutting up the housing...)

What's left of the pump still works pretty good however, I drove about 450 miles yesterday, at 82mph, with the check engine light flashing, and the engine running like crap until I slowed down, shut of the engine and re-started it again, then it runs fine, smooth as can be around town....

I'm sure Graynada replaced his sometime ago, i don't remember if it was a second hand unit from A8 Parts or some other repair. Might be worth asking him

Just a thought. If it's going around you can try turning bolt while pulling delivery unit up, it might pull the nut out of bottom adapter plate if its already loose.

Also here is pdf on testing pump delivery rate if you want to verify that it is definitely pump not delivering, it does require special tools but you can always improvise with whatever is available

Thanks for the test proceedure. That's very interesting that they don't go by pressure, but by volume to an open hose.

Every other one I've ever done was with a pressure meter, and specific pressure range.


So, let's say it puts out the correct amount of fuel? What else could be the problem with my car?

It runs fine around town, but if I try and go faster than 65/70mph for more than about 10 seconds, it flashes the CEL, and then cuts injectors and the engine runs rough.

It also flashes the CEL if I go WOT through the gears for more than a few seconds. And I've noticed that this engine pings, and sounds like it's lean when you get on the throttle, like it needs more fuel. It is however very fast, and smooth around town, even if the CEL flashes on WOT, as soon as I back off to normal cruising speed throttle, it stops flashing, and just remains lit.

The RossTech scan indicates muliple random misfires. I've cleared it, and it comes right back.

And when I bought the car it ran fine up to 120mph, or on the very top end of WOT, it would flash the CEL a very short amount of time, but then went off, and stayed off. (I've seen that happen on my 2004 A8L too, so I figured it was perhaps the MAF? I've seen MAF codes come and go, but did not have the RossTech tool to check this car before I bought it.

If it's not the pump, what could it be? (I've changed out the filter, had the injectors cleaned and tested and I've replaced the bad vac lines etc. There are no vac leaks, and I've gone around the intake with carb cleaner, and there are no gasket/manifold leaks either.)

I'm stumped, but think it's the fuel pump starting to fail.

The amount is measured at the return after pressure regulator so it is flow at full working pressure whatever it is on these engines 3.5 or 4 bar.

I replaced mine with a new VDO pump but I am struggling to think of a way to get the old one out if the captive bolt is no longer captive, beyond what ainarssems has suggested in putting some tension on it. Plan B would be a replacement tank from A8 Parts, salvage whatever you need from the pump housing by brute force from the old one, fit you new pump in there, but given the 'saddle' nature of the tank I can't imagine they are much fun to change :(

I dont know if this is a rubbish suggestion as I dont know how the bolt set up looks, but would it work if you cut the head of the bolt off then somehow push the remaining threaded part through?

Unfortunately no. It's a long bolt that goes through fuel delivery unit and you cannot just pull delivery unit straight up as it's very tight fit and it needs to be tilted and twisted several ways and times as you pull it out.

The long bolt has tapers on it that don't allow it to be removed either.

The solution to removing the bolt, is to make a custom hole saw, that's about 12" long, using a 1.25" hole saw, and arbor, and a 10" steel black pipe. You drill out the hole saw where the arbor normally threads in, to clear the bolt, weld it to the pipe, and then weld the arbor to the pipe on the other end, making a 12" long hole saw.

Then you drill down around the bolt, all the way down to the siphon cup where it's bolted to the fuel pump housing. This of course, destroys the fuel pump housing, and most likely, the sealing surface, and siphon cup unit.

The siphon cup unit is not attached to the tank in any way, but is held in place by the hoses attached to it. (And it can move around, making it difficult to align the two when you put it back together, even if you make marks prior to taking it all apart, assuming your bolt comes out.)

You might have to cut the bolt off anyway, just to get the siphon cup out too.(The guy who came up with this long hole saw idea had to cut the bolt off to just a nub, so it would allow the siphon cup to be removed.)

Good news is the hoses attach with quick disconnects, so they just snap on and off, but bad news is they can just snap off, and if any hose is loose, the siphon pumps and cup will not work correctly, and you will run out of gas before the tank is empty.

The problem I've run into is the cost of all this.

The tank is not cheap, even used.
The fuel pump housing is not cheap either, used or new.
And nobody seems to have the siphon cup used. (Or even know what I'm talking about, but of course the dealer would love to sell me all this stuff new.)

For this sort of money, I can BUILD a custom fuel system using the stock tank, that uses two pumps if need be, one for the fuel siphon system, and the other for the supply to the injector rail, and keep the existing fuel pressure regulator to return fuel to the tank/siphon system/cup. Many years ago, I built a custom system with three pumps, on a road racing car, so it would supply fuel to a central cup, where the main fuel pump was located that supplied the engine. It worked excellent with zero fuel starvation problems common to just a single pump setup. I can see why Audi did this, both for fuel tank design, and it works well if the car is driven hard, as fuel sloshes around, and the siphon system would pick up fuel from any point in the tank at any given moment. (Even better than the electric pumps I used, they ran the risk of being damaged when run "dry" as the fuel moved around the tank.) The tank we used had foam "baffles" in it to try and limit fuel slosh, but still the fuel would move around quite a bit.

So here is my take after all of this:

I'm going to try the cheap and easy route, and put the in-line 255lph Walbro pump just after the stock fuel filter, and have it supply the fuel rail/injectors. What's left of the stock fuel pump seems to be working the siphon system just fine, and it works fine as long as I don't try and go over 70mph. (I just did a nearly 500 mile day at 82mph, and while the engine ran like crap at that speed, just putting the car in neutral, shutting it off, and starting it again as we exited to re-set the ECU and fire all the injectors again, and the engine was smooth as normal around the towns, but would drop injectors again as soon as speeds exceeded 70mph for more than a few seconds.)

If the stock pump FAILS at some point, the fuel tank will run empty as soon as the fuel pump cup falls below the fuel level. But to supply the fuel siphon system, I'm wondering if another smaller in-tank pump can be used, and hoooked up to the siphon supply line, and the pump placed in the left side tank somewhere? It's pretty tight down, there, but I can get a cheap Dodge Airtex pump that's pretty small, has it's own internal FPR, and intake sock, and just pull the fuel level sending unit, and use that access to hook up the hose that normally feeds the siphon system to the cheap small pump, and there you go. Two pumps, and all for about 150.00 USD complete. (And it should allow the system to work like stock again.)

As far as that goes, this idea would work fine if someone wanted to make their own custom two pump replacement setup for the stock pump. Use a Walbro pump with sock, and the cheaper Airtex pump with sock, turn them back to back, and fit them down where the stock pump used to reside. (And of course, you lose the stock fuel filter at the stock pump housing, but you KEEP the fuel cup design that's fed by the siphon cup fixture.) The Walbro feeds the fuel line to the engine, and the Airtex feeds the line that plugs into the siphon cup system.

Should run forever, and I'd think the existing power supply would handle the load of the two pumps just fine too.

For Toyota Supra's, we used to replace the stock pump with twin Walbro's on the over 650rwhp cars, and run them both on stock power supplies just fine. But even hooking up a seperate power supply, and use the stock one to just switch a relay would be no problem, especially since the battery is in the trunk. (Fuse, relay and some wires, and it's done.)

I don't want to spend 1500.00 on fuel pump/tank when I can spend 150.00. :)

I've got the less than 100.00 USD, all metal Walbro external pump on the way, and I'll install it just after the stock fuel filter this next week.

If it solves my random multiple misfire problem, I'll then have a stock VDO/Siemens pump for sale at the same price I paid for it, or I'll try and return it, since It's only been out of the box, and never used.

Greg

This will be an interesting set-up, one never knows when fuel pump failure is around the corner.

Yep, I'm starting to wonder if adding an in-line fuel pump to the fuel system with the change out of a new fuel filter is not great insurance for anyone with a D2.

It should relieve the stock pump of most of it's job, and allow it to live a nice peaceful, and LONG life in the stock housing, just supplying fuel to the siphon system, and to the Walbro pump that's not only cheap, but most likely rated at a higher flow capacity as well. (No problem, as noted, any excess fuel just goes back past the pressure regulator, and into the stock fuel pump cup/assy in the tank.)

I will be thrilled if this adds a few more years of life to my ailing stock pump in it's never to be taken out alive housing... LOL
:ROFL:

Update: Have had the In Line Walbro, 255lph pump on the car for the better part of a week now, and it's running great around town.

1) The low speed engine performance is better. It used to "hesitate" and "ping" when I got on the gas off the line, not it does not do this at all, and the engine sounds, and pulls much stronger than it did before.

2) I can drive at highway speeds without the flashing CEL light, but ONLY if I don't use the cruise control? This has me confused, as clearly the engine is getting enough fuel from this setup, it's running great, but I have not cleared the codes yet, and wonder if that's resulting in the random misfire CEL flashing? (Or if that's what's making it flash, it does still drop injectors, so I'm guessing it is.)

So, crazy thing is I have plenty of fuel at the injectors/rail, but the engine ECU still sees a problem.

Any ideas?

I've been wondering if swapping out to another ECU would be possible. I know that the unit has to be coded to this vehicle, so it's not as easy as just unplugging one, and plugging in another.

But beyond that, I can't figure this out.

Plenty of fuel.
Injectors cleaned.
New fuel filter.

Still get the random misfire when the cruise control is used, and my fuel economy goes from about 20mpg to 16mpg as the engine runs like crap at 83mph.... But if I feather the gas, and slowing speed up, and down every few seconds, the CEL never flashes, and I can go as fast as I want....

ARGH! Love this car, but this fault issue is driving me nuts.
:tuttut:

What are the fault codes?

Something restricting the air flow maybe?

Have you tried running at the same revs as 83 mph in top in a lower gear to see if it revs or speed related. I know the engine will have more load on it at 83 mph but it might show something up.

Could the vehicle speed sensor be on the way out? It might be struggling to give a good signal at higher speeds and this might be confusing the ECU. Also there is a good chance the cruise control will use the same VSS.

I'm with you on the speed sensor, but why would it flash the CEL when I acclerate WOT to redline?

Any time the engine is run hard, it flashes, and any time it's run at a steady speed over about 65, it flashes, and then cuts injectors.

The fault is random misfires. Has a fault for the front parking sensors, but that's not related to this at all.

Cleared the faults, and the random misfire comes right back as soon as it's driven to redline, or at steady speeds on the highway.

Where is the speed sensor located? I'm guessing the transmission?

I doubt it's speed sensor.

Do a misfire log on measuring blocks to see if 1 cylinder or 1 bank give significantly more misfires the the rest and could be culprit.

Could be lots of things starting with MAF, lambda, dodgy coil pack ... Did you actually check fuel pressure and flow? Maybe fuel pressure regulator is leaking and loosing pressure. What are adaption values like in measuring blocks for fuel trim?

I have not run logs, but the fault is multiple random misfires.

I've checked for air leaks, low fuel pressure/flow, and dirty injectors, fuel filter etc.

And even put a secondary fuel pump into the system because I could not get the stock fuel pump out of the tank. (the captured nut in the tank/siphon cup just spins in place, so the fuel pump housing can't be removed without some serious cutting/surgery/cost.)

The in line fuel pump is a Walbro 255lph all metal unit, and is working great. I'm not actually sure the stock pump failed really, since it's not noisy, and clearly, with the Walbro on there, the two of them are supplying PLENTY of fuel.

I've pinched the fuel return line (Using needle nose vice grips with short sections of hose over the jaws, so they would not cut into the fuel return line, I could vary how open, or completely closed it was, while the engine was running, and while driving it) so a failing fuel pressure regulator is not the problem.

It runs totally closed off actually, and while it did not run as well, but ran still, so I'm pretty sure the fuel pressure regulator is working as designed, but I don't have a way to measure pressure, just flow, and I've not done that, as it would require me to remove the fuel line and I also don't have a measured container to use.

It has to be either a failed coil pack, or some sensor that's throwing out a multiple random misfire.

I have swapped the coils around too, just to see if it would show up as a single coil, or even pair of failed coils, and no luck. The CEL still flashes, and pulls the code of "Multiple Random Misfires."

I have not done a log yet, and I'm not sure I know how actually. :) LOL is it a feature of the RossTech program? I've only used it a few times, so I'm not an expert on the system yet, but I've worked on, and built engines for more than 25 years, and this problem has me stumped.

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.