So just to make sure we are on the same page. With your ECM in the vehicle it does not crank. With your friends ECM in the vehicle, it cranks but does not start and you have the U0100?
If this is the case then yes, your ECM has a short on the circuit board. More then likely it is a coincidence but could be related to you washing the engine. Clearly the connectors for the ECM are weather resistant but if sprayed directly with water, the water can wick inside the connectors.
I would look for water marks on the connectors and ECM where they mate and if you find any then that was the issue.
However, you are correct that it would not start with the other ECM in it because it must be programmed to the vehicle. I have attached that process. Let me know if you have questions. Thanks
Images (Click to enlarge)
Wednesday, May 6th, 2020 AT 3:07 PM