S6 C6 Mysterious Misfiring

A6 All/ S6 All
#1

Hi,
I am a recent and proud owner of a S6 C6, and therefor also new here, so I do apologize beforehand in case I break any rules or regulations. I took over the beast a few months ago and I’ve recently started to experience a blinking CEL. This being scanned and read as P0302, misfire on cylinder 2. After some testing for the occurance of this, it does not happen during (low rpm+low load), not during (high rpm+low load) nor (low rpm+high load), only during both high rpm and high load. I started with the basics to try to figure out the cause of this inconvinience, and tried both swapping coil and plugs to rule out the easy ones. The intakes was carbon cleaned less than 20kkm ago and replaced injectors 7kk ago. Therefor I have made the assumption it is not caused by;
Sparkplug
Coil
Injector
Carbon buildup
Valve spring
Leaving me quite baffled and honestly confused, and thats why I am here seeking assistance and hopefully someone who has experience a similar issue. To be clear, this does only happen on a single cylinder, so it does not appear to be caused by fuel delivery, clogged airfilter, clogged cat.

Any help and suggestions would be greatly appretiated!

#2

Welcome. No rules other than ask if you need help and in general don’t be rude…

If you have a vag com. Get us the data from that. It’s far more helpful than just generic code P code. The Ross tech tool gives WAY more info to help render what is really going on.

The best thing you can do at this point. Is check the LTFT data and STFT data and post that. Chances are if you just have a generic tool and not a ross tech vag com you won’t have access to that.

Phantom misfires can be a number of issues but at the core is some sort of fuel out of range issue. low rpm and load missfires are generally more vac leak oriented.

Mid range mid load lend more to injector plug or coil issue. Don’t rule out the clogged cat as that is an issue on these cars

#3

Thank you, I do have OBDEleven, and if I am not mistaken I should be able to log LTFT/STFT, so I will try to log a trip tomorrow and force the misfire:)

#4

OBD 11 FALLS well short of giving the full info but the codes themselves are shown. The next step in the codes are the capture data that comes with it.

Grab the trim data and then see if it’s specific to that bank. remember there are 4 banks. not just two

#5

Ross Tech has the ability to watch all 10 cylinders and keep a count of which cylinder misfires. Seemed very helpful for me to be able to see exactly which cylinders and at which moment it was happing rather than just seeing a CEL code when it decides to throw a code.

#6

It only misfires on the very same cylinder every time(Cyl2). Which is also why I doubt its the cat, as I would see misfires on Cyl1 aswell then. The freeze frame data that comes with the code is quite usless, just shows date and milages, not sure whether this is because of obdeleven or the ECU.

#7

It’s because its obd11 that freeze data is just about usless. That’s why I was saying vag com shows real data. Load / RPM / Throttle / and other snap shot data of the event that can help track down. In the end. It’s not the end of the world but usually the vag com freeze frame data can give you a window into when the event is really happening other than when it appears its happening.

It still could be the cat but unlikely as mentioned due to not seeing more events on the other cyl still in some cases the misfire buffer on some cylinders is higher than others.

#8

Could it potentially be leaking air through the intake manifold gasket, messing up the intake air volume? And recommended action is a smoke test? (Sorry if its a stupid suggestion, im still quite new at this)

#9

A couple of easy things to do:

  1. Swap Coils
  2. Swap Sparks

If it stays with the cylinder you know neither is the cause. If it moves then you have a possible suspect. Do this one at a time.

Ed

#10

I’ve already done that, no joy, sadly.

#11

OK so you know it’s most likely not the coil or spark plug.

The next swap would be the injector, but that is a PITA.

Ed

#12

Take compressions to see if there is a problem on this cylinder specifically.

#13

At that point an easy next step test before you start taking things apart would be the smoke test. This usually will help give an idea on what your working with. It’s possible to have an entire bank that is getting un metered air and only have one missfire cylinder. This leads back to injectors and the flow rate. It’s not uncommon to have even new non Audi specific replacement injectors under flow by 10 - 20%

#14

have you checked the resistance of the throttle body cables?

#15

I have not, you thinking it does open fully ? Restricting air flow?

PS: Just did compression test on all sylinders, decent results.

#16

just that I had some issues similar to this on my RS5 and it couldn’t even drive round the corner to the shops…was throwing up weird issues too like handbrake warnings etc…turned out to be the throttle body cables.

#17

If the car has a tune, it will need ARP coils and better plugs. You can buy one coil and plug, and test it against the OEM. On tuned cars, the coils and plugs should always be replaced with ARP, as the tune will want more power to light up. Its common to replace them, and misfires do occur with tunes due to this reason.

FYI. My RS7 misfires on 5+6 periodically (I see the error codes), and I intend to replace them with ARP coils and plugs soon.

#18

While it’s always a good idea to replace your plugs and re-gap them down and get a colder plug. The APR coil packs are totally not needed. A good NKG replacement coil pack is more than enough.

The RS7 has a TSB for a misfire on cylinders 5 and 6. They fix it with a ECU update. unless you have the weak spring issue then the ECU update isn’t going to help.

#19

The TSB is on Cyl 5 only … not Cyl 6.

I do not think an APR tune will work with this ECU fix, regardless … I suspect the ECU fix is to turn down the boost at higher RPMs.

Now why would anyone want to put a band-aid on a spring, with a shim … is beyond me.

Back to my issue … its 5+6 … so maybe or maybe not the TSB applies … regardless … I am not putting in a shim … I would rather replace them with new injectors and heavier springs … as the issue noted in the TSB seems to be float issues … by using the spring plate … you shim it … plus the ECU probably tones down the turbo at higher RPMs.

I have not addressed this yet … this summer … but if I am going to dig this deep into the motor … I am going to put back new injectors + APR coils + better NGK plugs + APR or Ferrara Springs

#20

The TSB just desensitizes the misfire trigger for those cylinders in the ECU update for the RS7 and S8. Cylinder 5 and on some cars 6 will be an issue. Regardless of if you see it in the TSB. 6 is less common but still on some cars pre ECU update an issue.

I agree, maybe a shim isn’t a great real fix that I’d want to see.

Lastly I do agree. APR coils are totally not needed. If you like them. Get them but they don’t really make a difference and are totally not needed. Standard NGK replacement coils are more than enough for serious hp. On all cars not just the RS7