Bmw e46 rough idle

[Glynr113]

Hi guys, anyone help please. Got e46 2.2 convertible (auto).Starts and runs fine but now and again at a stop it starts running on what I can only describe as 5 cylinders. Turn it off and restart and it clears till the next time (1 mile maybe 10 miles.Took it to an independent bm specialist who diagnosed oxygen sensors,then air leak which also meant replacing inlet manifold, breather and other pipes. Then tried swapping"control unit " ? from another bmw. Now he says could be head gasket.Cost me over £900 so far , now wants another £800 for head gasket but don't know if it will solve problem. Cars now becoming a cash cow. Any ideas anyone,thanks

[SPORTSTER]

Glynr113 said:

Hi guys, anyone help please. Got e46 2.2 convertible (auto).Starts and runs fine but now and again at a stop it starts running on what I can only describe as 5 cylinders. Turn it off and restart and it clears till the next time (1 mile maybe 10 miles.Took it to an independent bm specialist who diagnosed oxygen sensors,then air leak which also meant replacing inlet manifold, breather and other pipes. Then tried swapping"control unit " ? from another bmw. Now he says could be head gasket.Cost me over £900 so far , now wants another £800 for head gasket but don't know if it will solve problem. Cars now becoming a cash cow. Any ideas anyone,thanks

Sounds like a misfire - check the coil packs and spark plugs
Definitely not a HG, very rare on the M54 engine
Check MAF too, vacuum leak
Get the codes read again

Edited by SPORTSTER on Thursday 18th October 11:36

[E-bmw]

SPORTSTER said:

Definitely not a HG, very rare on the M54 engine

Edited by SPORTSTER on Thursday 18th October 11:36

I assume you meant M44 engine as the M54 engine is the 3.0 I6.

My best guess would be concertina hose adjacent to the MAF/throttle body, pull it off to check close enough for splits inside the folds of the concertina.

[SPORTSTER]

E-bmw said:

I assume you meant M44 engine as the M54 engine is the 3.0 I6.

My best guess would be concertina hose adjacent to the MAF/throttle body, pull it off to check close enough for splits inside the folds of the concertina.

No the M54 came in 2.2L, 2.5L and 3.0L, M44 is the E36 engine

[E-bmw]

OK, I stand corrected, the 3.0 is correctly termed the M54B30.

[d_a_n1979]

SPORTSTER said:

Sounds like a misfire - check the coil packs and spark plugs
Definitely not a HG, very rare on the M54 engine
Check MAF too, vacuum leak
Get the codes read again

Edited by SPORTSTER on Thursday 18th October 11:36

This; very much this!

Get the codes read ASAP as they could tell you straight away if a coil is failing and on which cylinder

If not, change the coil packs around and see if the fault follows that coil/plug

Check intake boot from MAF to throttle body, see if there are any cracks/splits. If there are, it needs replacing as it's letting extra air in after the MAF which the ECU won't be happy about etc

[dmarkovina]

Can you read out the short term fuel trim at idle - when it is still running on all 6 cylindes? Any cheap Bluetooth/wireless odb readers would do - free odb apps available for smartphones. If it is consistently well above 0 on both banks 0 say 10+ you probably have a vacuum leak. Oxygen sensors rarely fail (not saying they don't). On an old M54 engine I would also check the valve cover for cracks. The usual culprits are the boot after MAF (both the big one and the small one towards ICV), DISA O ring and of course the infamous oil separator and associated tubing.

[Kettmark]

My 330ci sport sometimes suffers from a similar misfire when damp weather is about. Similar to OP, I turn the engine off and all is well. New plugs at last service hopefully cured any issue.
Have learned to drive round it.