Are you running a manual or auto in your car? Cause if the physics of the ECU are the same between VW and BMW a auto setup relays info from the speed sensor to the tcm/ecu so that the ecu can monitor the true speed (from sensors) to the tcm speed and make sure that they match up.
Putting a auto ECU in a manual ECU and the car tends to buck at certain RPM's, not rev out to full RPM or put the car into limp mode. If you clear the code it runs great for one pull and then the ECU goes woah wtf! and you go back into limp mode and those symptoms happen again. All because there's no TCM to talk to. This happened in a swap of my friends GTI, so I know it is a pain. Now what happens if you run a manual in an auto........not sure, but possibly the same thing or it wont move at all. I will also note that with our official VW's scan tool we can't recode the ECU of a auto to match a manual with any car that is '99+
Not sure if that helps you at all but my first big question would be is the ECU from the SI with a matching transmission to yours (manual or Auto).
Also, great work to you for even attempting this!
and even a lil
