Help diagnosing an issue

[jam135jam]

Hi there

New member here with a poorly 135i (N55 engine) and hoping for some wisdom to help diagnose the problem.

My car has had a rough cold idle since I got it, if I used low quality fuel and it was really cold it was worse. The car would very occasionally stall at first but generally was fine if left it a little to warm up first or gave a little bit of throttle. Once warm car drove fine. I noticed the revs went to 1.2k ish then very quickly go down to 700 rpm which I thought was odd as usually a car will stay at higher revs when cold.

Recently I decided to unplug the MAF, thinking that it might reset and solve the problem. However now the car is much worse and stalls on cold start and requires manually keeping the revs at 1.5k or so for a little while and/or giving it more throttle until it warms up, then it seems to drive just fine. Driving without the MAF improves the cold start but is still worse than it was before I messed with it.

Some context:
- The car is remapped (340bhp) and has a FMIC and hard pipe.
- The car had an oil leak from the rocker cover which has now been fixed
- The throttle recently gave an error code and had to be replaced.

I removed the MAF after the above, those changes made no difference to the original issue.

I feel that there must be a fairly easy fix to at least get it back to how it was as all I did is remove the MAF temporarily - nothing else has changed on the car since I did that.

I tried unplugging the battery to see if that reset anything but it didn't seem to make any difference.

Thank you in advance for any help.

[jwolf]

You scanned for faults?

[swellengear]

Rough idle at cold start could point to a leaky injector. Fuel leaks on to your sparkplug, causing it to not fire correctly (or at all) = misfire. A simple test is to drive the car for a bit, let it sit overnight, pull your sparkplugs, and look for one (or more) that is wet with fuel. *I am not entirely sure what the injector failure rate is on the N55, but it is quite common on the N54.

And as jwolf mentioned, get the codes read.

[1and1]

You noted your n55 135 is tuned, are you able to get logs of these symptoms?

[jam135jam]

Thanks for the help guys really appreciate it.

I scanned but no faults come up unless I remove the MAF.

I did think it could be the spark plugs, I thought perhaps oil got in to them but a garage did take a look and said they were fine.

Regarding the tune, I don't have any equipment. I only know its tuned as the car had a dyno readout when I got it.

What puzzles me is how could there be anything mechanical as all I did was disconnect and reconnect the map?

[jam135jam]

I thought I'd follow up to say I resolved the issue!

All I had to do is disconnect the battery.

After that the car struggled at first but I left it to idle and after a while it sorted itself put and its better than ever.

Thanks for your help.

[jwolf]

The battery disconnect may have forced a new hunt for corrective fuel trims or other adaptive measures. It could be very very close to the edge of not running well again. Don't assume you fixed it.