2004 545i engine shaking on cold start

[fanling]

Hi all,

bought this car 30 months ago when it had done 36k km.
since then, i logged up 20k. about 3 weeks ago, weather turned cold to 45-50F.
car has no problem starting in the morning, however, the engine starts to shake as if it is on 5-6 cylinders only. at the same time, engine fault icon comes on plus message warning to drive moderately or else risking overloading catalytic converter. it shook so much i had to turn off engine. strangely, engine shook far less on restart, as a matter of fact, engine was normal and responsive on the third start, there were no more warning icon and message. i could drive the car and start it several times in the same day without further trouble. diagnostic shows #2 and 3 cylinders were at fault. so i changed all ignition coils
and fuel injectors but the same problem still persists.
can anyone advice me what the problem is?

many thanks.

fanling.

[brettski]

Welcome to the Werkz. Check your MAF, sometimes they cause strange issues (particularly when cold). The simplest test is to electically disconnect it and then start the car. The ECU should go into a default mode when it ignores the null response from the MAF and uses the other engine feedback about what the engine is doing. If the engine performs smoothly (albeit without as much power as normal) you have found your likely suspect, sometimes you can simply clean the MAF, otherwise you'll need to replace it. Good luck

[fanling]

thanks Brettski.
i forgot to mention i have a brand new air flow sensor put in already.
also the mechanic replaced all the air tubes.