So i have a 500 EFI that i've been putting together.

I have the mefi 3 harness for it but im missing the ECM and it is very expensive / hard to find

I have a MEFI 1 ECM and harness...

Can i run the motor with a mefi 1 harness and ECM I plan to keep this motor stock and run with a 500 efi bass map?

Will this even work?

you can run it but you will need to tune the ecu.

Perfect thanks

You can do it but it is a HUGE step backwards. I may have a Mefi 3. If so, I can load a 500 efi cal in it.
Eddie

^^^ totally agree and will add MEFI 1 has a totally different mounting pattern and is 6.5 inches long center hole to center hole. Has 4 mounting holes. Also the J1 and J2 connectors mount on the sides of the MEFI 1 ECM.

retail is 2417 bucks for the MEFI conversion from MEFI 3 to MEFI 4 and you got one hell of deal offered to you in your wanted ad and do not even know it. 1600 bucks for the MEFI 4 conversion. FYI MEFI 3 currently NLA unless you buy used.

Again this is even an upgrade doing the MEFI 4 conversion brand new with a warranty for 1600 plug and play.

Buy the time you spend for MEFI 1, then spend someone to dick around with about 4 to 8 recals and who knows if that will be even correct -

then you will need to fab a correct mounting bracket that will accept the side plug ins for the J connectors. By the time it is all said and done plus the months that you and the tuner will take to get it right ??? if even getting it right - plus the devalue of your engines with MEFI 1 - just saying.