gtreng1

Hey guys,

I really need some help with the folks on the forums. Im pretty handy and usually can figure things out, but in this case Im stumped. I started with sputtering/misfiring on acceleration, lack in power. Replaced the fuel filter, then things got way worse. I barely made it to the dealer (no indy shops in my area that I trust).

The dealer replaced the fuel pump sensors (high and low), did a carbon cleanout service on the intake and valves since the manifold was off, and ended up replacing the high pressure fuel pumps since they were reading at 6psi rather than 25 psi. They also replaced a motor mount - apparently the sensor was bad and it is somehow connected to the fuel pump sensors, and the sensor needed replacement for them to diagnose the system. They comp'd the pumps, and motor mount, and I only had to pay for the sensors and intake service.

Picked up the car this morning, and the car accelerates normal now when lightly accelerating, but I can feel the shutter when I go beyond 50% throttle and the CEL briefly flashed. CEL light is off, so Im thinking its still misfiring.

Plugs are maybe 5k miles old - I did notice oil leaking from an intake man gasket into the plug hole. Not sure if this can cause the issue. Ill plan on replacing gaskets again. In the mean time, it feels like a fuel problem. The only thing I havent replaced is the low pressure fuel pumps.

Is there any way to troubleshoot the fuel pumps more? Dealer couldnt completely solve the problem.I do have Rosstech and will hook up again. Most likely the low fuel pressure code is gone, but the misfire came back.

Thanks in advance for the help!

'07 A8L 4.2q sport (140k miles)
'95.5 urS6 Avant (234k miles)
'99 Audi A8 4.2q (sold at 200k miles)

MP4.2+6.0

Sorry, but honestly can't make enough sense of some of the info. I could believe like 100x of the numbers indicated for HPFP's--hundreds and thousands of PSI, but not 6 to 25. Even if that is in atmospheres/bar, might be low. So, I can't square info.

Start with basic set up. Low pressure pumps in back, whether older port injection or newer FSI. One is a transfer pump (only) over tank saddle IIRC, and one moves fuel forward. Then HPFP's do their thing to raise pressures way up (1000psi plus) for injection into cylinder against the already very high combustion pressures, but only once they get a reliable and steady source of fuel pumped from the tank. Net, a possible guess is the main pump in back needs replacing. If the pressures you mention are feed pressures out of tank pumps and into HPFP's, even those sound low; you could look up to confirm. Historically tank pumps often could not be diagnosed via codes and many failures just seemed to be outright left stranded and no start, but it was also then more obvious without the HPFP's that it just wasn't moving fuel. Like other posts I've been seeing, mechanics on HPFP often seem to just forget about the tank pumps and that fuel still has to be moved forward. Not like they went away on the new ones either; there is still a pump buried in the tank on my D4--separate from the HPFP's right in your face on the cylinder heads when you open hood.

gtreng1

Thanks, just went back to the paperwork. Fuel delivery was checked at 6 bar, within spec. High side was 6 bar (spec is 25 bar at idle). After replacing the fuel pumps and sensors, high pressure side is 25 bar within specification at idle too. Poor signals to the high pressure pumps checked ok at 12v ok. This is an '07 with the fsi 4.2L motor.

MP4.2+6.0

So, I'm staying w/ main fuel tank pump as an operating theory absent any other diagnostic pointers. Yes, data in bars post work sound believable; thanks. HPFP's can't pressurize fuel not being received in suffficient quantity when needed at greater load/revs and they aren't set up to suction fuel AFAIK. Both driving performance and misfire can tie--fuel starvation at higher revs, and then goes lean for misfire.

BTW, tons of these motors/set ups around, but this board is likely the lowest volume for FSI normally aspirated V8 of them all. D4 4.2's (pre-4.0T) also still too new for a lot of these longer term running issues, though likewise I recall one or two with this overlapping HPFP and rear tank pump (on D4's down to 1) issue. For same era of general 4.2 FSI set up, can look at: C6 A6 4.2, (gen 1) Q7 4.2, and S4 and S5 4.2. You might search into those and see if you find other owners with similar issues. Very few 4.2 FSI deep dive posts here over the years, actually for lots of the drivetrain related areas that have differences in the later 4.2 set up.

SS_RTR

Have you diagnosed with VCDS? You can easily see which cylinder(s) are misfiring. I had a similar issue several months ago and it was a bad spark plug. 01-Engine, Measuring Blocks, starting around group 15.

Panelhead

Are you positive this is a fuel problem? Sounds like a coil pack to
me.

gtreng1

Thanks, it was a fuel problem before, but I will check again with VCDS. I replaced all coil packs thinking that was it before, but no dice. I'll check again though.

Shark_V8

> apparently the (motor mount) sensor was bad and it is somehow connected to the fuel pump sensors

What's that about?
CEL flashing there has to be new codes.
Basic compression test?
Fuel filter installed in the right direction?I've never installed one backwards but...

Mohamed Khechen

Hey just wondering if you ever fixed the problem. I have a 2008 Audi A8L and do pretty much all my work myself, but this one has me stumped. Every time I accelerate to 5k rpm I feel like it loses power and CEL starts to flash, but it stores the misfires as intermittent on cylinders 1,3, and sometimes 2 or 4.

amsnooze

check your Intake Manifold LInk Arms, shot in the dark but maybe not enough air is playing a role https://www.audiworld.com/forums/a8-...links-2889638/