Misfires on entire cylinder bank in ASN (A6 3.0)

Hello everyone,

I'm constantly getting a message on my A6 4B5 3.0 indicating misfires on cylinders 4, 5, and 6. They're occurring so frequently that the engine should be stalling immediately. When the engine is idling, increasing the RPM to approximately 3500-4000 causes misfires to jump from 0 to greater than 4 within 2 seconds on all three cylinders.
However, the engine runs very smoothly.

The error has been active since our trip to Italy. Since the engine was running smoothly and wasn't displaying any error messages, we even drove it home (including the trailer, so under load).
So, we've been experiencing this issue for about 2000 kilometers.

I assumed the knock sensor on the driver's side was faulty, and I replaced both of them today. Unfortunately, it didn't achieve anything.
Basically, it also has power, possibly a little too little, but I attributed that to a reduction in the ignition timing.


When you search online, you often find references to the catalytic converter, but if there was a problem with it, you would usually notice something while driving.
The engine has a gas system, and the problem occurs both when running on gas and when running on gasoline.
He consumed a relatively high amount of fuel on the trip (15-18 liters), but normally, without a trailer, it averages around 13 liters. Everything is measured in terms of gas.

Since it had been starting poorly for a while, I had checked/replaced a few things to troubleshoot the issue (it also has quite a few miles on it, around 240,000 km).
- Timing belt with water pump.
- Ignition coils
- Spark plugs
Ultimately, the problem was traced back to the evaporator, which had a faulty membrane. Since I replaced these, it starts up really well and does what it's supposed to.


To be honest, I'm a bit lost right now.
I hope someone has another idea about where I can look.

Thank you very much!

I would check the compression and perform a pressure test.

I have already checked the compression. There's nothing unusual about it.

A compression test was not performed, but that should be noticeable as an uneven engine running.

The error occurred suddenly and simultaneously on all 3 cylinders.

By the way, these interruptions don't count as errors when the system is idle. Only when you start driving.
Interestingly, when the vehicle is under a proper load (e.g., pulling a trailer uphill), the blinking of the warning light also stops. As soon as the engine is no longer under full load, it starts blinking again...

It might be noticeable, but it doesn't have to be if the electronics are still managing to contain it... this allows us to rule out certain technical problems.

To the best of my knowledge, in LPG systems, the fuel lines leading to the injectors are routed through the gas control unit (GCU).

I don't know which error code the engine control unit (ECU) sets when there's a minor issue, or how the ECU specifically detects the misfire.

I would temporarily disconnect the injectors of the affected cylinder bank from the LPG control unit, secure them in their original position, and see what happens... of course, while running on gasoline.

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Ultimately, the problem was with the evaporator, which had a defective membrane. Since I replaced these, it starts up really well and does what it's supposed to.

Hm, doesn't it usually start on gasoline, which would explain the defect. Does the type of vaporizer matter when starting out...?

Yes, it usually starts on gasoline, but the vaporizer on the side opposite the gas line has a connection that leads to the intake manifold.
Most likely, this is intended to prevent diffusing gas from being released into the environment in the event of a malfunction.

After being parked, overnight the gas in the evaporator slowly leaked into the intake manifold, which is why it wouldn't start in the morning. After 3 seconds of cranking, the fuel was processed, and the engine could start again.
It took quite a while to find this bug.

Generally, I'm wondering how the engine detects misfires. I had only been considering the knock sensor so far. (which would also make sense in the case of misfires on the entire bank).
It's noteworthy that I can't see any ignition timing retard in VCDS, even though it's registering misfires.


We could try experimenting with those gasoline injectors sometime. However, they will definitely be accessed, and as soon as I disconnect one, error messages for disconnected nozzles will immediately appear.

Hi.

Quote:

Generally, the question that arises for me is how the engine detects misfires?? I had only considered the knock sensor up to this point. (which would also make sense in the case of misfires on the entire bank)

For example, regarding inconsistencies in the rotational angle of the crankshaft per unit of time.
This principle is used, for example, in the idle speed control system for TDI engines.

Best regards, Rainer.

Sorry, I completely forgot to reply.

I've also had instances where the system indicated misfires on the right bank.

However, this cannot be a mechanical problem. As I said, the engine doesn't misfire, it has its power, and the fuel consumption is normal (the increased fuel consumption was actually due to the combination of a trailer and hills).
The error has now been present for approximately 4500 kilometers.

Bypassing the fuel injectors using the gas control unit didn't achieve the desired result.

Of course, it could be the MSTG (Modular System Terminal) or the wiring harness, but neither is particularly easy to check. Since he doesn't detect any misfires at idle, it sounds to me like a sensor issue.

I could replace the crankshaft position sensor (CKP) and the knock sensor (KS) just to be sure, as they aren't particularly expensive. I just expected that I would have to find separate errors in the system to address this.

Does anyone have any other ideas about what could be causing this issue, without it actually being misfires?

Here's a quick update:
After a few weeks, he suddenly lost a significant amount of oil. The culprit was a cap on one of the exhaust manifold water bypass valves. The retaining ring was no longer fully in its position, which meant that the cap could easily slip out of place and sit at an angle in its seat.
As a result, a minimal amount of oil leaked continuously. Due to the rotations, it was naturally well distributed .

I installed a new gasket along with a retaining ring, and I cleaned the whole thing with some brake cleaner. Since then, the error has been gone.
I still don't understand it, but we've definitely driven about 1000 km since then, and the problem hasn't reappeared.

There are some things you just can't understand...

Thank you for the feedback .