Fuel Pump Blues, take TWO.

[graynada]

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.

[adjuster]

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?

[ainarssems]

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?

[adjuster]

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.

[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.