BMW 523I N53B25 2nd bank missfire

[zyszek]

Good morning everyone.

This is my first post so i will introduce myself, I'm a 35years old passionate motorhead who like working on cars and is not afraid of getting my hands dirty and have done a lot of car work (including engine rebuilds / replacements).

Hopefully i post this is the correct section of the forum.

Hopefully you guys will be able to help me on my journey with this BMW.

I bought a BMW 523I 2009 with the N53B25 engine and manual transmission. The car had a misfire and it is hard to be driven (also out of MOT so cannot drive it on the road now).

I have found the car history (after the purchase) and it looks like this car had this misfire problem for a while - previous owner replaced all injectors to brand new 2 years ago and tried to sell the car after that failed to resolve it.

I have replaced the plugs and coils but that didn't help at all.

I got to the point where i bout a second engine and replaced it completely (engine come from a running vehicle and it was a complete unit with fuel pump / injectors / coils and the engine electrical harness.

After i replaced the engine everything started up fine but i had a leak on exhaust manifold. Replaced the manifold gaskets and back to square 1 - misfire on 2nd bank of cylinder. same as on the original engine.

After many hours of research and reading various forums, my next step was to check the NOX sensors so I disconnected the NOX sensors (pre-cat and post-cat) to see if the engine will run correctly however engine still run rough (no misfire codes but i can hear it is misfiring and engine is shaking a lot, especially when took it to about 3k rpm).

I tried to look at the injectors signal with oscilloscope (unfortunately I don't' have a current probe) but looking at the voltage signal it is all the same across all 6 cylinders. also the signal to the coil is same across all cylinders.

Ii have coded all injectors with the INPA software, and I did try to move the injectors form bank 1 to bank 2 and also use the injectors form the original engine, but that didn't resolve anything.

I also have MSD81 so there should be no issue with MOSFET's - which is in line with the voltage signal from oscilloscope being correct.

Also what i notice when monitoring the injector and coil signal the coil signal is generated after every 2nd - 3rd injector signal which I though is strange as I presume it should been after every injector signa could it be a corrupted ECU map?

I'm loosing my mind now so please any help will be much appreciated.

If you need any more details please let me know I will test and post it back here.

[MysticRob]

Likely misfire causes:
intake runners gummed up (walnut blast)
spark plugs
coils
injectors (check for wet gassy plugs if leaking) leaking or clogged
fuel pump
O2 sensors
catalytic converters clogged or breaking apart

I find it odd you say it ran fine AFTER engine replacement, but after fixing a leaky exhaust manifold began to run badly again. Are you sure that was causal, and not just correlative? If causal I'd look more closely at the exhaust stream and how it's affecting the engine.

[zyszek]

Thank you for your comments MysticRob.

Spar plugs were replaced with brand new dedicated NGK plugs.

I bout 3 new coils to replace on the old engine - that didn't help. (also put them on to the new engine but no joy).

Injectors - i got 12 injectors in total (6 index 11 that were replaced by previous owner and 6 index 9 that came with the second engine). Currently I've replaced the second bank of injectors with the first bank of injectors from the old engine (3 x index 11 on 2nd bank and 3x index 9 on the first bank) and coded them but that dint' help.

HPFP came with second engine and looking at the fuel pressure in INPA it gives correct (pressure was around 160-180 bars)
O2 sensor - wouldn't that be causing misfire and rough running only when the o2 sensor is connected as form what I read if the sensors are disconnected the ECU is working in different mode and use some sort of a emergency map?

I also find it very strange that it was running fine first time after I replaced the engine but this was only once and the only part I have changed since was the exhaust seals.

It is also puzzling that it give errors on the same bank of cylinders - could it be just one catalytic converter that have failed? but in that case how could a tiny exhaust leak case it to work correctly?

I'm really clueless on this one and don't want to keep replacing parts as I already spend over £1000 on engine and other bits.

[MysticRob]

Hard to tell what the issue is, especially considering the different engine.
I have to assume all sensors and larger bits came with the new engine, at which point I'd start looking at those things that weren't changed.
Hard to tell though, you might've just inherited completely new issues with the replacement engine.

Are you tuned?

How do the plugs actually look? Any gas on them? Oil fouled, gassy, are they real OEM plugs or Chinese crap?

Injector index doesn't really matter, as they can all leak and/or get clogged. For instance, I have one bank of all -12s now, after replacing 1 horribly leaking -11 recently.
My second bank has one each of -5, -6, and -9, though the car runs smooth as silk. Historically -9 has been the most troublesome according to the vast majority of forums I've read.

How do the intake runners look? Direct injection engines can end up running pretty bad with gummed up valves.

Spitballing here, but an exhaust leak might've allowed enough back pressure to escape the affected cylinder(s) so as to make a partially clogged cat less significant. That's reaching though, especially if the leak wasn't that big. I have no way of knowing specifics but figured I'd put it out there.

[zyszek]

Hi,

I have just done a quick check - removed the pre-cat oxygen sensor and engine works much better and smother plus it doesn't misfire when I rev it up.

That pint to a blocked cat from 2nd bank of cylinders.

Question is there any cleaning process that would allow to test this diagnosis / theory, or if it have to be replaced should I replace both of them or just the faulty one bearing in mind it is £300 for 1?

Again many thanks for your thoughts.

[MysticRob]

I'm not sure why when I read your posts I just glossed over the "entire bank" misfires you continuously talked about. Man, I'm a bit thick today.
If you're pretty sure you have at least a partially collapsed substrate you can always run a borescope camera into the exhaust and inspect before spending cat money.
Or just try tapping on the cat (not hard though, as you might actually break it if wasn't prior) and hearing the rattle or broken pieces.
I think you're on the right track though, as one entire bank misfiring is very likely the cat.