Cold start up problem

[TheRuss]

Hi folks I have an odd start up problem from cold that seems to come and go. The car doesn't throw up any fault codes that my Cobb AP can find, but the startup seems pretty awful to me.

Firstly the car revs very high, nearly 2000rpm, then it suddenly drops very quickly to around 1000rpm and lower, then finally rectified itself to just over 1000rpm where it usually sits on initial cold start up.

It's doing it often enough at the moment that I managed to film it on my phone whilst leaving work this evening:

The car is a 2009 135i, N54 with around 45k miles on the clock. Mods are Cobb stage 2, AR Design catless DPs and BMWP axle back. I also had some sports cats fitted into some N55 mids to sit in between.

FMIC, air filter, etc is all standard and the car always runs on premium Shell fuel.

Any ideas what the issue could be? With a start like that I'm surprised no codes are being thrown.

Once the car is started it runs fine, except at idle it shudders slightly, and occasionally gives a more violent shudder.

HPFP was changed in 2009 and so were the injectors, which now all have the "good" model number on them. Spark plugs were done approx 10k miles ago or less and oil/filter etc done a couple of weeks ago.

Help!

[TheRuss]

Ah damn - just noticed that I've put this in the 1M forum by mistake - can a mod move to the coupe forum please?

[FactorX81]

Injector

[TheRuss]

Shouldn't that throw some sort've code though? BMW had it in before and declared everything was perfect, even with my suggestion that injectors were a possible area of concern. That said, my dealer is full of dunces and generally screw up the most simple tasks.

[Hobbe]

Looks like a normal cold start up.
Your mods may have added some rpm to it, but what´s the problem?

(...if you can call 9.5 C cold. Still subzero over here )

[TheRuss]

What? It revs far higher than normal, then drops too low, causing the whole car to shake and it to cough and splutter out of the exhaust, then finally flicks back up to the correct cold start rpms. I wouldn't say there was anything normal about it.

Despite my mods, it doesn't usually rev that high from cold (also, I'm not talking cold weather start up, I'm talking starting from a fully cooled down engine, as opposed to a hot start).

Then there's the sudden drop from almost 2000rpm to under 1000rpm - there's nothing normal about that, it should drop more slowly down and sit just above 1000rpms for about 30 secs (which is where the revs end up when it sorts itself out finally). I can't see how you think any of that is normal...?

[M3 Adjuster]

possibly a coil on plug ignition coil in early stages of failing?

check for vaccum leaks from any hoses or intake items that you may have disturbed when changing the oil.

[MDORPHN]

Carbon build-up on intake valves. Walnut blast!

Neil

[TheRuss]

Already done, less than a year ago - didn't improve anything at the time, just gave my BMW dealer an excuse to break all the clips under the bonnet whilst getting to the intake manifold.