Possible catalytic converter issue - very bad missfiring

Hello Guys!

Last friday I went on a short trip with my S6 (previously I added a can of Liqui Moly injector cleaner). I was pushing it hard like 220-240km/h, but was running low on petrol (like 30kms remaining).
Filled it up with premium petrol and after about 10 kilometers it started missfiring very badly. At least it was 100meters from home.
I was dumb enough to clear the error codes with a simple OBD reader (without saving the stored ones).
Now the car starts fine, but it has really bad missfires. Once the heating of the cats ends it really struggles to keep the idle rpm. (It happend to stall in reverse.)
I left it run a little bit and checked the error codes with vcds, found the below:
Engine I: nothing (just to check number 2)
Engine 2:

  • 000768 - Random/Multiple Cylinder Misfire Detected - P0300 - 001 -
  • 000784 - Cylinder 10 - P0310 - 001 - Misfire Detected
  • 000774 - Cylinder 6 - P0306 - 001 - Misfire Detected
  • 000773 - Cylinder 5 - P0305 - 001 - Misfire Detected
  • 000777 - Cylinder 9 - P0309 - 001 - Misfire Detected
  • 000772 - Cylinder 4 - P0304 - 001 - Misfire Detected

There were some misfires in the past, but I changed the ignition coils and the spark plugs too (like 2000kms before).

Here are two videos of the issue:
warm startup
smoke on idle (It can be seen that only one exhaust is smoking)

One of my friend said maybe it is a collapsed catalytic converter. Can I check it somehow? He told me to pull out the o2 sensors before the cats to see if it helps. Can I do it without dropping the engine?

Thanks in advance. :slight_smile:

Long time ago, once i put 500ml of Liqui Moly injector cleaner on almost empty tank and did hard style driving, CEL light on, missfire, and end result was multiple faulty O2 sensors. After that im not thinking anymore to use injector cleaner. And once due bad O2 sensor heater it was triggering SAI durring drive and sport cat cells fall in parts, another 1k€ blown out… If O2 sensor have errors, replace them. On my car mechanic relocate connectors so gear box and engine dont need to be dropp down.

The additive was like 300ml and I poured it into a 3/4 tank of petrol. And there is no error codes stored other than which I mentioned. But thanks for the input will try to check the sensors too.

Checking the cats is going to be a bit harder than on most cars when you consider that replacement of several 02s requires you to drop the motor.

The exhaust tone tends to sound different if you have a damaged cat but this too can be misleading as missfires on cold start and shortly after can change the tone of the motor.

The cats on these cars are prone to failure but the error trigger system generally will give you a warning after a while.

While I would monitor things there could be the possible issue that when you filled up with fuel you might have gotten a poor winter blend that is also leading to some of this.

Liqui Moly for gas engines are not designed to be used with small amount of fuels, unlike ones for diesel engines, on which the engine can run entirely on the additive without any fuel. I doubt it has anything to do with the problem. On another note, you have errors on cylinder 4 and 5 which run on one cat, then 9 and 10 which run on another one, and 6 which runs on another one as well, so I highly doubt 3 of 4 cats failed at the same time. I’d start with checking if fuel injectors don’t leak, and if the intake system does not have any leaks as well.

I was thinking about what if I drill a hole before the cat to check if it helps, and once not needed just weld it. I can’t decide if the tone changed because of the misfires it has an unusual sound.
I don’t think the fuel had any issues since (for me) it looks like the right side is working as it should.

I mixed the additive with 3/4 tank of fuel. I think it must be fine. But I heard some stories about these additives killing the cats.
I totally agree with you that 3 of them should not fail. If I see cylinder 4 (for example) as misfiring should I be sure that this cylinder has issues? Can’t they interfere with each other?

Attached vcds scan: s6230210.pdf (161.3 KB)

Just an update if somebody faces the same issue: cylinder 6 - no compression and cylinder 10 - low compression.

Did you post about this on Facebook recently too? I ask because I saw a post looking very similar in one of the V10 groups.

If you have no compression in one cylinder and low compression in another then there is definitely more going on there. I recommend that you do cylinder leakage tests to see if you have a bent valve or damaged piston/rings.

Yeah, was me in the S6 S8 v10 group. Will do compression test on all cylinders but was not able to test all of them properly. Just got a new tool to be able to test them.
Will try to look inside with an endoscope and also check the flaps.
I’m not sure what is going on if it was two cylinders close together then there is no question but these cylinders are far from each other.
Anyway will update the post once I hvae any clue. :slight_smile:

Good luck… it doesn’t smell very good.

It looks like I found the issue.

Check if you have a broken valve spring.

Thanks a lot. Not planed to take off the valve cover, but you were right to check there.

You’re welcome. I actually have been seeing broken valve springs more commonly on the V8s and V10s, especially with higher mileage. Was messaging an APR supercharged V8 S5 coupe owner on Facebook when I saw the notification email for this post. He probably has at least two bad valve springs causing misfires on cylinders 5 and 6.