Why the random codes/misfires–what is the cause?

A6 All/ S6 All
#1

Some days the engine seems to purr at idle like a happy kitten––even on cold starts. And other times it’s just janky. I can’t seem to get a bead on the pattern or the cause. But I’m wondering if the random nature of these faults suggests and electrical problem and not mechanical. (I should add that the engine seems to always run fine at speed.) Any thoughts?
see: Intermittent codes - diagnose electrical or mechanical problem?

A couple of weekends ago, the car was running so smoothly I even commented about it out loud (and neglected to knock on wood…) Then I stopped by the roadside fruit stand to buy some strawberries, and 5m later, on restart, I get a rough idle with these codes:

P0171 (pending) - System Too Lean Bank 1
(confirmed) - System Too Lean Bank 1
P0174 (pending) - System Too Lean Bank 2
P1075 (pending)
P1077 (pending)

see: System Too Lean codes
I’ve checked for vacuum leaks around the oil separator and TBs, but nothing shows up with the carb cleaner test.

Then yesterday I got an EPC fault/limp mode after shopping at the grocery store (on the way out the car was perfect) I couldn’t even clear the fault with the Carista:

P0638 (pending) - Throttle Actuator Control Range/Performance Bank 1
P2101 (pending) - Throttle Actuator “A” Control Motor Circuit Range/Performance
(pending) - Throttle Actuator “A” Control Motor Circuit Range/Performance
P2106 (pending) - Throttle Actuator Control System - Forced Limited Power

I had an earlier battle with these codes that ultimately seemed to be solved by zip ties to the electrical harness:

P0023 - “B” Camshaft Position - Actuator Circuit/Open Bank 2
P1060 - Bank 2, cam shaft adjustment discharge opening, interruption

see: [SOLVED?] P0023 - “B” Camshaft Position - Actuator Circuit/Open Bank 2

With that in mind, I checked all the electrical connections again. I cleaned the electrical connector to the Bank1 TB, and also checked ECU2 because I know it is bouncing around unsecured. I think a tab must have broken off somewhere.

QUESTION:
Can someone tell me what the ECU is sitting on? It feels like some kind of air box and I can tell that the back right corner of the “lid” is loose. It feels like a screw is missing back there and I’m wondering if that is contributing to the “engine too lean” fault.

IMG_6207
IMG_62071574×2100 924 KB

Otherwise, I was able to shove a wine cork on the front left corner of ECU2 to keep the unit pinned under the remaining tabs. We’ll see how that holds up.

But those 2 changes seems to have fixed the TB/limp mode faults and the car seems fine on the first test drive. If anyone has any other ideas, please share. Until then, stay tuned…

#2

Well the ECU moving around isn’t a great idea. You definitely want that to be not moving. I can’t see for sure but that looks like it might be the cover for the blower motor or the cabin filter.

lean codes are generally from vac leaks. it would be worth doing a smoke test just to make sure that’s not the cause.

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#3

(hopefully) solved. Fixed a bad crimp at the wire in position #3 of the 6-pin connector for the Bank 1 throttle body. I suspect that the extreme engine heat was causing intermittent faults with the bad crimp

#4

I would pay special attention to the PCV crossover tube from valve cover to cover, as well as rt side air intake tube at TB, especially the 2 smaller tubes… as for box, it’s actually fuse/relay box in plenum chamber.

#5

Just a follow up: I replaced the oil separator and the system too lean codes seemed to disappear. However, I took the opportunity to open the old oil separator and noticed that the diaphragm was intact.

Now I’m only getting misfires on idle and cold starts.

#6

The warm idle missfires are something that you’re going to want to check the fuel trims on.

The cold start misfires are going to in part be due to if you have the broken intake and these cars even in great condition don’t cold start well. But if you check your LTFT data that should tell you in part why there is cold start missfires.

If you have a +10 on the LTFT then your car is adding that much when the 02s are on line but not giving that on cold start. So this will add to a bad cold start.

#7

So frustrating—still plagued by intermittent/random misfires, mostly at idle, but sometimes under relaxed city driving.

What is most perplexing is the random nature of the misfires accompanied by a flashing CEL. Here is an example from the other day while running errands around town:

  • (2nd trip of the day, engine halfway to warm) the engine starts running smoothly, but after a long-ish idle, begins to misfire with a flashing CEL, and continues to run rough at 40 KPH
  • restart the engine at a stop light––engine is smooth and everything is good again as I reach destination
  • next 2 drives (2h later) the engine is again smooth for the whole time
  • following drive, the engine misfires again, and I can feel it while driving at low speeds

Overall, 2 of 7 trips around town that day suffered from misfires, mostly idle but sometimes also at low speeds. The other trips were just fine, and no changes to the car and no reset of the engine codes.

I have also had the experience where the engine was driving very roughly after a cold start in very cold weather (ski trip). I was only able to pull over after 25m to stop and clear the engine codes (at the time, I thought clearing the codes did something magical, but now I think it’s just the restart…) After that, I was able to complete the next 3 drives spread over 2 days without a CEL––each beginning with a cold engine followed by 1h of aggressive driving up/down twisting mountain roads in S (definitely a fun drive). On our return home, I reset the codes once the engine was warm, and I was also able to drive 9h at highway speeds with only a few pit stops without a CEL.

Can anyone offer some intuition as to why these misfires occur so randomly?

Background

Here’s the data from the latest scan:

Engine:
P2187 - Bank 1; System too Lean at Idle
P1923 - Please Check DTC Memory of ECU Number 2

Engine (2):
P0300 - Random/Multiple Cylinder Misfire Detected
P0308 - Cylinder 8, Misfire Detected
P0307 - Cylinder 7, Misfire Detected
P0301 - Cylinder 1, Misfire Detected
P0306 - Cylinder 6, Misfire Detected
P0309 - Cylinder 9, Misfire Detected
P0303 - Cylinder 3, Misfire Detected

But in general, the misfires most often occur in cyl 1, 6-10. In order to rule out coil packs, I had already put a new pack in cyl 6 and swapped cyl 2 & 7. That didn’t seem to make a difference. Next I’m going to pull a couple of spark plugs and inspect.

  • P2187 - Bank 1; System too Lean at Idle

Also, this system too lean fault finally returned after replacing the oil separator (see above). I think the only difference is that the last scan included about 2h of highway driving, and the other scans were only short city drives. Any connection?

I also recently discovered I can run Windows on Mac using Parallels, so I’m gonna finally get a proper VAG-COM cable and check my LTFT’s.

#8

[UPDATE] I found a vacuum leak(!) In the end, I couldn’t find a garage to do a smoke test, so I made a DIY smoke machine, and voila!

See this post for more details: What are the 2 hoses that connect to the pass air intake hose?

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#9

I also discovered that the real culprit is the warped flanges on the air distribution housing, PN 07L133085A. Top edge of on the driver-side TB and bottom edge on the pass TB.

800×800

Ive got a permatex grey gasket maker in there for now, but I noticed that the bottom back bolts on both TBs are not reaching the threads. Maybe the outer threads in the air distribution housing are stripped?

But I have to say, I just spent 20mins idling and circling in search of parking—and it was a joy because no misfires.

Now I have to either replace the air distribution housing or find a “longer” bolt to fully secure the TBs.

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#10

Wow this is a really great find good work

#11

That’s an interesting find!
Seems odd that I would warp like that. :dizzy_face:‍:dizzy::dizzy_face:

#12

30m of low speed driving and 20m of circling for parking have me encouraged—engine is smooth and no misfires!

But I’m kind of reluctant to put more money into this car until I know it’s solid. And then, the JHM tune before minor body and paint repairs—at the moment the car is a bit of a beater.

The TB bolts are

  • M6 X 55mm P/N N10530304

But they are unthreaded at the tapered tip. This seems to be a common size for motorcycles, see https://www.motorkit.com/en/bolts-and-nuts-for-motorcycle/24326-allen-bolt-m6-x-55mm-1pc.html

Is there any reason not to try a couple of these to see if they can reach the threads in the air distribution housing? Together with the permatex grey gaskets, I can try to go a whole month without a CEL.

#13

Good luck. Progress is progress. Enjoy.

1 Like
#14

I’m not sure if this is a good idea or not, but I did it.

Ordered a couple of M6x55mm and 60mm hex screws from a motorcycle shop. The 55mm screws were long enough to reach past the stripped(?) threads in the air distribution housing for a secure grip.

I used a 60mm screw as shown below because it was long enough to reach past the crack all the way through to the back threads. It seems more secure.

The air distribution housing should probably be replaced, but for now, I’m gonna keep it ghetto and see if anything else pops up. Regardless, with the supra grey permatex gaskets in place, the car has been idling like a dream!

#15

It’s not getto its resourceful. Looks like a good fix to me, did it pass smoke test now?

#16

[UPDATE] I put on about 2000Km since I fixed the leaks at the throttle bodies and all the nasty misfires on idle are gone. The engine idles so smooth now, it’s like a whole new car.

However, I have gotten the occasional CEL with a

P0172 - System Too Rich Bank 1

It seems to happen very occasionally, and somewhere around the time when I smash the accelerator, but I haven’t actually caught it. But the car still drives fine.

Any ideas?

#17

It’s always good to see what cylinders on bank 1. So I would start with getting the LTFT data for those cylinders. Then from there it might not be a bad idea to do another smoke test to see how or if there are any intake leaks.