35i Misfire Mystery

[Gabz]

Hi all,

I've got a misfire which I can't seem to solve.

The misfire occurs once the car is warm and idles for more than a minute or so. After the odd shakes and vibrations, it starts to shake quite a bit and flashes the CEL (DME fuel mode also switches from 2 to 3). It can also occur while driving slow (low revs, low load) but not when driven enthusiastically.

Codes (always come together):
29CF - Combustion misfires, cylinder 3
29D9 - Misfire at low tank fill level (tank is not actually low)

Plugs and coils were changed around 15,000 miles ago and so was walnut blast to clear carbon deposits.

What I tried:

- New spark plugs (I did notice some oil on cylinder 3 spark plug - possibly valve cover gasket leak)
- Swapped coils 2 and 3
- Upped IDLE rpm to 950
- Changed MHD map back to stock (was stage 2)

Regardless of the changes and brand new plugs, I keep getting cylinder 3 misfire almost straight away.

It's driving me crazy so I thought I make a post here in case anyone can guide me in the right direction. Thanks in advance!

[R.E92]

I think fuel mode 3 is fuel injection cutoff due to misfire. Not certain though.

If you've changed plugs and moved coils around the next possibility would be a bad injector. A bad spray pattern can cause a misfire like that.

Do you have a log of the car idling before the misfire?

[Gabz]

I think fuel mode 3 is fuel injection cutoff due to misfire. Not certain though.

If you've changed plugs and moved coils around the next possibility would be a bad injector. A bad spray pattern can cause a misfire like that.

Do you have a log of the car idling before the misfire?

Thanks R.E92!

Just did a log here on idle: https://datazap.me/u/ctgabz/idle?log=0&data=13-23

It actually switches from fuel map 2 to 3 right at the beginning (and starts vibrating) but doesn't get too bad to produce CEL in this instance. The misfire is always on Cyl 3 consistently.

Not sure if there's anything there to suggest a culprit?

PS: I also get the following codes (I assume these are because of catless downpipes):
2C31 - Oxygen sensor emissions control
2C7E - Oxygen sensor emissions control
2C6A - Oxygen sensors after catalytic converter mixed up

Old 3rd gear pull (pre-issues): https://datazap.me/u/ctgabz/9stg2b?log=0&data=3-23

Once again - thanks for your insights, very helpful.

[Z4M-2006]

Have you tried a compression test ?

I know in some "N" derived engines that the valve guides wear and cause inconsistent seating of the valve.

Would deffo try and injector first though.

[R.E92]

I think fuel mode 3 is fuel injection cutoff due to misfire. Not certain though.

If you've changed plugs and moved coils around the next possibility would be a bad injector. A bad spray pattern can cause a misfire like that.

Do you have a log of the car idling before the misfire?

Thanks R.E92!

Just did a log here on idle: https://datazap.me/u/ctgabz/idle?log=0&data=13-23

It actually switches from fuel map 2 to 3 right at the beginning (and starts vibrating) but doesn't get too bad to produce CEL in this instance. The misfire is always on Cyl 3 consistently.

Not sure if there's anything there to suggest a culprit?

PS: I also get the following codes (I assume these are because of catless downpipes):
2C31 - Oxygen sensor emissions control
2C7E - Oxygen sensor emissions control
2C6A - Oxygen sensors after catalytic converter mixed up

Old 3rd gear pull (pre-issues): https://datazap.me/u/ctgabz/9stg2b?log=0&data=3-23

Once again - thanks for your insights, very helpful.

Those are pretty crucial codes. It's looking like you have a really bad injector in cylinder 3 or that you have the O2 sensors in the incorrect positions. You shouldn't have any codes relating to the downpipes if you're using MHD since the cat efficiency codes are disabled by the tune. Those codes you are seeing are reflective of an issue.

If you look at the lambda for bank 1 and 2 in that log you have a large variance. Bank 1 is running really rich before it switches into fuel mode 3 and cuts injection into cylinder 3 which sends the readings for bank 1 lean.

How long ago were the downpipes fitted and how long have those codes been present?

If I were you I'd be really worried that injector 3 is stuck open. That can cause hydro-lock and engine damage. Is there any soot or fuel leaking out of the exhaust pipes? A stuck open injector can be so bad it lets raw fuel right out the exhaust, I had that on my car not long after buying it and they replaced all the injectors, all O2 sensors and all 4 cats.

[Gabz]

If I were you I'd be really worried that injector 3 is stuck open. That can cause hydro-lock and engine damage. Is there any soot or fuel leaking out of the exhaust pipes? A stuck open injector can be so bad it lets raw fuel right out the exhaust, I had that on my car not long after buying it and they replaced all the injectors, all O2 sensors and all 4 cats.

Interesting - thanks again for the insights. I did another 2 logs - one before and one after resetting adaptations:

Before: https://datazap.me/u/ctgabz/idle2pre?lo ... a=16-17-23
After: https://datazap.me/u/ctgabz/idle2?log=0&data=16-17-23

What I'm going to do is take it back to the garage who installed the downpipes (and may have mixed up o2 sensors) and they'll also have a better idea on injectors / misfire. I feel to much of a noob trying to change injectors (e.g getting the wrong index, coding them incorrectly, etc.) so best if they handle.

PS not sure if this tells anything:


Thanks again for all insights! Hope someone will find this thread useful in the future - will definitely report back in terms of findings.