[zune_k]

Sounds to me like it might be the MAF sensor

Would that affect only one cylinder though?

 

[zune_k]

That’s the joy of having a Bmw you could be changing something it might not be. Ie an injector which isn’t cheap. What symptoms are you having with the car is it running lumpy or and can you smell a bit of fuel?

Classic BMW.
The car is very rough and "bouncy" on idle, from what I can tell the rev gauge needle is pretty solid and doesn't bounce up and down.
If I pull out the ignition coil on cyl 4 while the car is running nothing happens, if I do it to any other cylinder the car runs much worse.

If I pull out the MAF sensor it's still really rough and bouncy on idle but I atleast get my acceleration back.
Again if I pull out the ignition coil on cyl4 while MAF is also disconnected there's no difference from when the MAF was plugged in.
Weirdly enough I don't get any cyl 4 error codes if the MAF is unplugged, only low voltage on MAF and the ECM RAM Error.

It does smell a bit of fuel in the engine bay and around the vehicle.
No colored exhaust smoke, and doesn't seem to be any coolant mixed with the engine oil.

 

[zune_k]

I would try a new maf there not to expensive and an easy change. The only reason I say about the maf is because it tells the car what air to fuel ratio it needs. Even without the maf plugged in the car should know what it needs but it helps out if the cars needs change and then it will make it right. The my only reason for asking about the maf is the smell of fuel and rough idle. Also just looked up the P0604 error code for the ecm and this is what it says it is:

P0604 is an OBD-II generic code for an internal memory failure between the ECM and the vehicle’s CAN, or controller area network. This is the connection between the ECM and other control modules such as the transmission control module. The ECM constantly monitors its own internal memory as well as circuits and output signals. If the self-check system detects an issue, it will ignite a P0604 code.

I just ordered a new MAF, just in case.
If I recall correctly when the MAF is unplugged the car goes back to default values and run rich, going by that logic cyl 4 shouldn't be misfiring if the MAF is unplugged.
We'll see what happens after MAF replacement, I've also got a fuel pressure gauge coming in the next few days. From my research it could be a soft-failing fuel pump or clogged fuel filter.

I did also order a new valve cover gasket just in case, it's leaking quite a bit of oil on the metal piece just above the exhaust manifold on cylinder bank 2.
I have a hard time to believe it's the gasket that's causing the misfire on cyl 4 as there's more oil in the well on cyl 5 and 6 which running fine - But it's a BMW so you never know :-D

P0604 honestly has me kind of scared, I really don't know where to begin on that one.
Battery is good, connections are good, I've checked the most common spots for a circuit shortage (trunk behind the rubber cable thingie, coil ground wire, cables in the fuse/ecm box and around there) everything looks to be fine and there doesn't seem to be any blown fuses anywhere - Not in the passanger well fuse box nor in the engine bay one.

 

[zune_k]

Can you check if you get a pulse in the plug to cyl-4 coil pack.
Wire could be damaged.

I might've forgotten to post that here.
I did ground cyl 4 spark plug to the chassis of the car with the spark plug connected to the coil pack on cyl 4 and it's sparking.

Actually - Since the valve cover gasket is leaking into the spark plug well, could the oil maybe mess with the ground on cyl 4?

 

[zune_k]

Have you checked valve for cracks when leaking.

Ground should be okay, I think your coil is pencil type?

For rough have you checked thtottle body and ICV for dirt?

1. Isn't that what the compression test is for?
2. Correct, they're pencil type.
3. I have not. I haven't even checked DISA yet.
I've gone off the assumption the rough idle is caused by the misfire therefor all focus has been on the various things directly impacting a specific cylinder i.e coils, plugs and injectors.