Unrelenting misfires S6 V10. New member

A6 All/ S6 All
#13

So I had unrelenting misfires with my B7 A4 only to have the same exact issue with my C6 S6! So many similarities it’s uncanny.

Always happens on mid throttle acceleration, kind of a stumble when trying to pass. Doesn’t seem to happen at low throttle or at WOT, but definitely feel loss of power. Just bought the car, knowing it had misfires but the price I paid was reflective of that knowledge.

I had the valve cover gaskets replaced, new plugs, a thorough decarb, and a new oil pan gasket. I hoped the decarb would take care of the misfires but I am still throwing codes 0300, 0304, 0309. This is good because prior to decarb, it would throw codes from more cylinders, but now it is confined to 4 and 9.

I’ve been running different kinds of fuel additives from day 1 and it hasn’t made a noticeable difference. I’ve used Marvel’s Mystery Oil for 3 tankfulls, and Audi G17 for one. I also swapped out the coil pack from 4 to 1 and still getting misfires on 4.

At this point I’m pretty sure I need a fuel injector or 2, but hoping something easier pops up. I should note that the previous owner took it to an Audi dealer which recommended a decarb and 10 injectors but he wasn’t parting with $3000.

#14

So it seems like a can of SeaFoam on a half a tank of gas has fixed my misfires!

No CELs all afternoon. It’s still not 100% but we are getting there. I just filled up the tank and added Royal Purple Max Fuel System cleaner as I’ve read great things about it. Hoping it gets smoother and stronger over the next few tanks of gas. I’ll update as necessary.

#15

Interesting. I don’t think I have ever heard of Seafoam curing a misfire problem, so I’m interested to see how long the situation lasts for.

Let us know if the problem returns, or if things are still looking good a few months down the road.

#16

Just to clarify, car was just carbon cleaned. The sea foam probably only cleaned the injectors. Since yesterday only have seen pending codes, and still no cel.

I will definitely keep this updated.

#17

Ah, cool. That makes more sense.

Let us know how it goes.

#18

Yeah, my car is still misfiring. My guess is it’s the fuel injectors as well. I think the rest of mine are being replaced next week, that’s literally the only thing left that would cause a misfire.

#19

Please keep us posted Irish. Since you have provided so much info on your case, it would be great to be able to identify root cause(s) of all your issues so that they can be included in the Info Dump.

#20

Well, I may have spoken too soon as they’re not completely gone, but much, much improved. Gonna keep cleaning the injectors via the fuel for now, but I expect that I need new injectors for cylinders 4 and 9 as they keep throwing pending codes. However I’ve gone from throwing codes on every pull from multiple cylinders to a consistent code only from 4 with 9 only throwing pending codes. And there are zero or less than 10 misfires from the other cylinders when I’ve checked.

Carbon clean, new plugs and valve cover gaskets get the majority of the thanks with Seafoam the rest. I still haven’t changed coil packs, but swapping 4 with 1 didn’t yield any changes.

#21

Guys on the rs4 and A4 FSI they have found that aged 02s can cause miss fires. Lot of the time the misfires pop up after a carbon clean or some kind of maintenence of that kind. From what I can tell the debris that gets into the cylinder during the cleaning can contaminate the 02s. They won’t throw any codes but that’s been known for causing hard to find misfires specially if it’s on one bank. It might be worth a shot.

#22

Great info, Justin! With some of the o2s on the S6 requiring a motor drop to access, it might make sense that we see so many of these cars with undiagnosed misfires issues, after all the ‘prime suspects’ have been addressed. Especially when you mention that the o2s might not throw a code. No one is telling their shop, “hey, can you drop the engine and change the perfectly working o2 sensors for me, on the chance that it will resolve my misfire issue?”

The cats rarely go bad, so there are very few reasons to drop the engine other than to replace the engine entirely or the o2s. And even then, the o2s are expensive, so if they arent throwing a code, people are probably just leaving them AS IS.

It would be VERY interesting if we could track some of the guys that had an engine replacement but didnt replace their o2s at the same time. I’d like to see if they still get some random misfires in the logs (maybe not enough to throw a CEL).

On the project S6 with the custom headers we are relocating the o2s to a more accessible location. Why oh why did Audi bury them up there so deep in the first place? Those cats shoved right behind the engine, and the o2 sensors with them, have caused so many issues with this car. All so they could heat up faster for emissions!

#23

both of my O2 sensors were replaced before the misfires, and with my engine as they replaced fuel injectors, the misfires went away from those cylinders, however they popped up in new cylinders. That’s why we think it’s the fuel injectors right now, especially since all of the coils/plugs have been changed as well as new valve cover gaskets. fingers crossed after they are replaced, it’ll be gone.

#24

Irish,

Have you tried Seafoam in the gas? I put a can in 1/2 tank and saw major improvement, then I tried Royal Purple Fuel System Cleaner and it helped some more. I’m almost CEL free, I get occasional flashing CEL but it doesn’t stick. Hoping a few more tanks of cleaner will get me to perfect, unless there is a physical defect in one or more injector.

#25

Fuel injection cleaner is always a good thing to add to the FSI cars.

Keep in mind a few things. If you have cats and are cleaning gunk out of your injectors it cam clog the cats.

Also there are 6 02 sensors on the v10. So if you changed 2 that might have just been on or for one side. If you changed the two on the bank one side and then the misfires popped up on the second bank it could be those 02s need replacement as well.

#26

Great point! I just had a full, walnut blasted carbon clean which is why I am being aggressive with the Seafoam. I’ve eliminated the always on CEL, reduced it to throwing a code every few days. Before carbon cleaning, it was consistently throwing codes at every acceleration. I thought the carbon cleaning would magically fix everything but I was wrong. I was still misfiring, although it reduced it from throwing codes on multiple random cylinders, to just 4 and 9. And now after 1 1/4 tanks of gas with Seafoam with a tank of Royal Purple in the middle(trying to compare the two) I’m down to a code every 2-3 days and so far it’s been limited to just cylinder 9. I’ll keep updating, but when considering that it takes a few extra seconds to start when it’s been sitting for a couple of minutes, I might have a leaky injector.

But there has been a huge difference in power output and smoothness after the carbon clean and injector cleaning. It still has trouble in 6th, mid throttle acceleration between about 75mph and 90mph. There is a slight hesitation and it has thrown codes in that band.

She’s high maintenance but I love her!

#27

It might be worth while to pull the plugs again. Especially after a recent carbon cleaning.

#28

Check your fuel pressure at start up

http://audirevolution.net/category/zip-technical-or-diy

I know the video is for a RS4 but it will be helpful to you. There should be a what the good pressure is and what bad pressure is.

#29

Will put this on my list, thanks.

#30

Guys i have an update in regards to my misfire/CEL, Both my throttle body gaskets did not have a perfect seal as diagnosed by my audi tech, he replaced the gaskets, adapted everything and cleared the codes, so far the last 3 days driving the CEL is off and the car idles a bit more stable. To check to see if your gaskets are leaking, start the vehicle and spray some soapy water around the throttle body gasket, mine was pretty obvious. hope this helps anyone. the parts cost me about $30.

#31

Great tip, Pat!

#32

;D
In the end, it was the fuel injectors. 6/10 replaced from fidelity, I just sucked it up and paid for the other 4 because the warranty covered all new seals. Heading to pick her up in about an hour. So relieved.
Time to get her up for sale!