I recently got a 2007 S8 and I love it, but I'm having a couple small issues.

The previous owner blew the motor and spent about $25k rebuilding and installing a donor motor with all the available improvements to make it bulletproof. Initially when I got the car from him, I had an issue with a "lean at idle" CEL but I seem to have figured that out (improperly installed catch can allowing unmetered air into the system). Everything was running fine until the last time I filled the gas. I also put in a bottle of injector cleaner to get everything flushed out. But immediately after pulling away from the gas station, I've been having issues with knocking and misfires including CELs for those conditions.

Since everything was working fine until I filled with gas, am I crazy in thinking that I got a bad batch of gas or that the injector cleaner I got doesn't play well with direct injection or was bad itself? Just want to make sure I'm not crazy in thinking that's the most likely scenario before I start dumping money in to coil packs and spark plugs (which honestly, I'm probably going to do anyway since I don't know when they were last done) or HPFPs.

 

Probably water in the gas. Throw in some HEET, see if that sorts it out.

 

Thanks!! I tried HEET and it seemed to do the trick. Still a little bit lopey cruising at certain RPMs but MUCH better and I just did a 50mi drive with no CELs or even DTCs.

Now I just need to figure out why I get random TPS value improbable errors every now and then.

 

Maybe a bad throttle position sensor on the pedal, or the throttle bodies need to be recalibrated.

 

So I think I have a bad TB, did an adaptation and it worked but this morning it went into limp mode with all kinds of TB related errors again, I believe it said it was #2 which is passenger side I think. Tried doing adaptation again after clearing DTCs and it just says "ERROR". So I'm going to grab a used TB and toss it in and see what happens.

Also going to put in a new fuel filter since it's still not running as smooth as I think it should be (ignoring the TB freaking out), and fuel filter makes a lot of sense.

 

OK so if anyone is still here, indulge me in my thought processes.

I'm still having what seems to be throttle position issues, getting the "EPC" limp mode and throwing a bunch of codes. I've finally isolated what I think seems to be happening and see if anyone has anything else to add.

Starting from disconnecting the battery and doing a full reset, so basically from scratch, here's what has been happening.

1. Reset battery
2. Go into accessory mode twice (after a battery disconnect the first time in accessory mode doesn't actually do anything, so I turn it off and then back on again).
3. Perform a TBA on both the main ECU (passenger TB) and ECU 2 (driver TB). This means run TBA until it says "ADP OK" and then wait 30 seconds on both sides.
4. Start car, initially idle is rough for a minute or two but it smooths out and seems to be doing fine.
5. Car idles fine, go for a drive and everything seems to be working fine.
6. Once car is warmed up, go to sport mode, car has some harsh shifts but nothing unusual and seems to be running good.
7. Try to do some spirited acceleration (more than half throttle very quickly) from a stoplight, almost immediately get the EPC error and ESP light with limp mode.
8. Checking the codes immediately gets me the following:
ECU 1:
- P012100 - Throttle/Pedal Pos. Sensor A Circ. Range/Performance
- P022100 - Throttle/Pedal Position Sensor/Switch B Range/Performance
9. Clear codes, try to adapt TBs and only get "Error"
10. "Throttle/Pedal" codes disappear but I get persistent "throttle valve control" errors and doing TBAs again just error out.
11. If I manage to get the TBA to take, the car will start but idle is all over the place, up and down, up and then hold, weird idle sounds, random revving like if I was testing something even though I'm not touching the pedal. At one point it was like a ghost was testing the pedal function because it was doing the exact same actions I would do while testing, but I wasn't touching anything.
Addendum: I swapped the TB sides because it SEEMED on the surface like the TB controlled by ECU 1 (passenger) was having issues. Nothing changed (I was able to run a TBA on both sides successfully even though I wasn't able to originally, I'm guessing because removing them discharged them like if I had reset the battery).

Potential diagnosis: Something about the accelerator pedal is faulty and when used under certain circumstances, it sends faulty voltage ("implausible") ranges to the TBs which cause the TBs to freak out and requires the whole system to be reset.

I have a replacement pedal on the way, I plan on replacing that first before diving into any potential TB issues. I might also do an electrical contact cleaning first just in case its some weird corrosion issue with the pedal in place.

 

Brake light switch... Might not be THE answer, but for $20, it will be a problem soon...

 

Can you post the full VCDS scan?

 

Most likely an ECU issue, or wiring for the throttle body. Common at this point. Check the wiring and for any water damage to the ECU's.

 

I’ll post up the VCDS shortly but I think I solved at least a big part of it. The intake plenum on top of the manifold had a hairline crack on one of the throttle body mounting tabs and I didn’t think much of it but I tried to get the engine running today and it was sucking all kinds of air all around the driver side throttle body. Looks like the crack weakened the area and it’s actually bent meaning the throttle body can’t even seat properly. I’m guessing it wasn’t originally as much of a problem but just got progressively worse over time and then when I swapped the throttle bodies it just did it in completely. The cracked hole is also stripped, all four bolts and all four threaded holes on the driver side look terrible as well. So looks like it’s a new upper plenum, gaskets and bolts for me!

Can anyone confirm if these are the right bolts? The bolts I have are triple-square and have bullet noses on the end, but I’m thinking they might have been changed since I’m seeing an “after xxxxx vin” on these ones.

https://parts.audiusa.com/p/51514933/N10562101.html

Edit: These are the bolts.

N10530303 - Screw - Genuine Audi Part (audiusa.com)