I have a 1986 190e 2.3 8v 325,000km (~203,000mi) and want to replace the engine with one from a complete, running 1986 300e (105,000mi) parts car. I have almost no experience with auto mechanics and am doing this project mainly because I want to learn how to do this stuff. The 300e is listed as parts only, so it can't go back on the road. The 190e is currently not being driven because of mechanical issues (hence the engine swap).
I have removed the engine and transmission from the 300e and it is currently on a hoist.
My questions are
(1)What do I need to do mechanically to make it work?
(2)What do I need to do with the wiring/electrical system to make it work?
(3)What problems/possible setbacks/additional costs should I expect?
I know these questions are very broad, but I have to start somewhere!

Dan

you need to make sure that the specs. of the transmission is the same your rear end might not support the torque of the inline 6 and it will just chew it appart. u need the ecu harnes pump everything that can make the engine start. i know you still need more

What transmission specs should I be looking for, and where would I find those?
How will I know if it is compatible?
Also, how do I change the wiring and harness without having to change all of the wiring in the rest of the car? Everything seems connected.

can the engine mount of the 190E support the weight of the inline six. most of the time there is like 20 to 50 lbs. of tourqe that the rear end can take untill it collapes so u can get the rear end of the 300e too i belive its 3.06 or something.
the engine harness on both cars are different the distributor of the 190e only support 4 cylinders. the fuel pump relay is not the same. fuse box might not work. if you can just grab all you see in the bonet of the 300e, grab it just to make sure. just do a lot of research.

im also planning to do the same i got an 84' BMW 318i and the engine kind of gave up but it still starts. and i got a friend who got his rx7 3rd gen. rear ended totaled but the engine is not broken. so this might be the 1st rotary BMW in las vegas

and aslo i dont think the radiator on the 190e wont be able to cool down that 3.0 litter inline six

When MB put the M103 motor into the W201, they modified the radiator support to accommodate the longer engine block. So, you'll need that part and pretty much all the pieces forward of the fan. The A/C system is different, as is the wiring harness and most of the relays for engine management. The front coil springs for the 2.6L accommodate the extra weight.

The auto trans (722.4) is the same, but requires a different torque converter.