When it was on MFi it wouldnt of left my workshop running more than 3.5% depending on what mods it had and power it was capable of.

Matt, in general most people who do the EFi conversion on their own ends up with issues similar to what you have with co readings off the scale and the system not working as it should.

This is mainly down to people not using the correct loom and not wiring it in to the car properly, using secondhand parts that dont work and cant be checked easy etc..

I would advise having it checked out before you damage anything.