Datalog BMW 135i Cold Start Rough Idle (fine when JB4/MHD removed)


Dec 6, 2021
Hi Guys,

I've been having a problem with my car for a while and have replaced loads of things to no avail.

On starting only when cold and left for over 24 hours the first start up is rough and stumbles and misfires badly for 10 seconds.

I've replaced Injectors to Index 12
HPFP replaced
Low pressure Sensor replaced
Sparkplugs replaced
Coil replaced
Vanos Solenoids replaced

I don't get any codes, I notice that when starting it's fine until the JB4 wakes up (Needle Sweep) then starts running poorly so I removed the JB4 and it would start fine every time. I then Mapped the car with MHD and the problem is back, flash back to stock starts perfect again.

JB4 Issue? - YouTube - Despite the title the issue isn't specific to JB4

Is there something in the fuel tables on MHD/JB4 that effects start up different to stock?

Thank you in advance for your help!



New Member
Oct 2, 2020
I realize this post is a few months old, but I wanted to see if you had any luck troubleshooting your cold start issue? My 09 135i seems to be suffering from a very similar affliction. I've also replaced all the same parts you have, and then some. I'm currently running MHD v10 Stage 2+ ots.