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.
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!
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.
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.
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.
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!
It might be worth while to pull the plugs again. Especially after a recent carbon cleaning.
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.
Will put this on my list, thanks.
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.
Great tip, Pat!
;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!
Awesome man!!!
If you have the part code for the fuel injectors, that would be awesome. As you know, I am doing my best to squash the belief that the S6 is any more unreliable than any other FSI Audi. And I am pretty sure those same injectors are used on other models outside the v10s.
I know you had a long and frutrating ordeal with the car, but I think that with proper troubleshooting, the fuel injectors should have been identified as root cause much earlier in the investigation. Especially before tearing the engine apart and replacing internal seals, at huge labor cost.
I really appreciate you sharing your entire ordeal. It strongly reinforces my advice to people, that a good mechanic is way more important to have, than an ultra clean car. Every car has its issues, and if the guys fixing them dont know what they’re doing, then it could cost you (or in this case your warranty company) thousands and thousands of dollars.
I know what you are going to answer, but are you REALLY sure you still want to sell the car? You could have had this issue on any Audi.
Nevermind on the part number. This is all the other Audis that use the same injector (06E906036C)
Fuel pump fuel rail injector 5.2ltr. ((A6) Audi A6/Avant
(T8E) 5.2L.
(A8Q) Audi A8/S8 quattro
(T8E) 5.2L.); Fuel rail fuel pump high pressure pump injector air distribution housing 3.2ltr. ((A4Q) Audi A4/S4/Avant quattro
06E 998 907.
(T14) 3.2L.
(A6) Audi A6/Avant
06E 998 907.
(T14) 3.2L.
(AA4C) Audi A4/S4 Cabrio./qu.
06E 998 907.
(T14) 3.2L.)
I have been CEL free for 3 days now. There is still a small hesitation but that has become rarer and rarer. Also, it runs rough when cold. I’ve got it going in Wednesday to check fuel pressure, check to see if the IM is the updated version, check the plugs, and inspect the injectors. Hope to have the warranty cover the major stuff.
With all that said, the car is running as smooth as butter. I used another couple of cans of seafom to a full tank of gas, ran a tank with nothing then a tank with the Lucas fuel system cleaner. But I’m done with the additives for a few tanks as I can’t imagine any more improvement.
V8A6, you said it earlier, and I’ve always considered these fuel additives to be more gimmick than anything, but I’m a changed man. It actually made a huge difference to run quality additives through the fuel. I used SeaFoam, Royal Purple and Lucas, and can say that they all delivered some noticeable difference. That feeling of impending doom(aka throwing a code or hard misfires) is gone and I can enjoy driving again.
Geek, that is some good info.
At some point, I plan to put together a guide for the v10 of all the possible things to check when you are experiencing misfires. Both you and Irish have provided valuable info for the guide.
Please keep me updated on your car. Especially if the misfires return.
Also, I dont know if you ever stated it, did you do a carbon clean? How many miles ago?
Yeah, I got the car walnut blasted about 1000mi ago, was hoping for a car that felt like new only for it to be almost the same. After reading about 55hp changes on the dyno after a carbon clean, I was sorely disappointed. But random misfires on multiple cylinders gave way to just two, then one and now none. Like I stated, it still runs rough when cold, takes a few extra revolutions to start, and there is occasional hesitation when at full throttle, my butt dyno is telling me something is still not 100%, but that’s just being picky. As I spent a pretty penny on a warranty, things like injectors and manifolds and starters and O2 sensors are on high alert. I’m just hoping JHM has the headers available by then so I can do everything when the engine is out!
Carbon is at the root of all these issues. Greek Inc very smart plan to get all the little usual suspects checked out. Let us know how the results are
If you aren’t getting any codes and you are seriously down on power, then I would look at the intake manifold first.
PaulF just replaced his. They are known to have internal issues. I know sometimes the issues come with a symptom of ‘rattling’ sound, but Im not sure if that is always the case.
You may have to take it off and pry it open to know for sure.
Justincredible that o2 sensors can go bad, without giving any codes. With all the cleaning fluids, and the resulting gunk, that has been flushed out of the engine, it is possible that the cats and o2s may have an issue now.
Wait for Justin to comment on that, as I am not the expert on that portion. I only know what he has stated before, but he should be the one who says if it is a real concern or not, and what to look for.
Start with listening to the intake. Talk to PaulF, so see what his symptoms were. I think a bad intake can even cause misfires. His new one totally transformed his car.
And heres Justin! Posted while I was typing my response 
Justin, can you give your Expert opinion on the other items I mentioned?