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Ok, so I have a friend that seems to have blown his engine. (Overheated, lent to a friend).

So we are trying to find a 2.2 engine but it is a bit of a problem to get. I can however get a 2.5 fairly cheap. Is it as simple as swap engine and DME, or is there a ton more work to do?

It is an auto BTW.
 

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No, It's even simpler than that.

LEAVE the B22 DME in the car. Install the B25....Get it running.. It will li likely run fine on the B22 tune.

Then, and only then, consider re-flashing the B22 DME with the B25 tune...

Spend some time searching this swap. It's been done before and its nearly identical to the B25 to B30 swap...
 

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I've done just that this week, including auto to manual swap.
Yes, you can swap 2.2 to 2.5 or 3.0 engine. 2.2 to 2.5 is easier because all but the engine block and head is the same. However, there may be some differences based on build date. Early M54 engines have different head cover with different spark coils, I believe that the spark harness is also different. Early M54 doesn't have secondary air pump, valve and solenoid. For that the main harness differs too. All 3 engine harnesses (main, spark, o2sensors&transmission) differs by length depending if the engine is from E46 or E39 because for unknown reason BMW just didn't put ECU box on the same side of engine compartment (facepalm).
If donor engine differs in anything mentioned, just take that part off your current engine.

You can't just take ECU (DME) from donor car, you would need to change VIN & ISN (EWS ID) in donor DME, which is not easy. I've done that because I want to keep my original DME as a golden spare.
You have to flash your current DME with 2.5L (M54B25) software of any version. It's much easier than to virginize or clone DME for VIN&ISN change. You can do that in car through OBD plug and INPA cable using MS4x flasher software from ms4x.net web. Then reset/synchronize DME with EWS using INPA and that's all.
Yes, 2.5L engine will run with 2.2L software. It will even run all right at idle and low load thanks to o2 sensor regulation. But maps are slightly incorrect for different displacement and you risk damaging your cats in the long run. The DME then learns the car configuration and adjusts engine adaptations in first 10 km run.
 

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By the way, ZF 5HP19 auto transmission differs too. Torque converter differs slightly and most clutch packs have one friction plate less. It can slip with slightly higher torque of 2.5L.
I've dumped my auto trans, I've never liked that.
 
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