I think trial and error is a little unfair for MFI. It is however misunderstood, and it can be tricky to grasp. Some patience in learning about how it works through the various literature online will get you closer to diagnosing it.

As said, the WUR should be checked . Alot of the issues on MFI'd cars (Ford or otherwise) tend to be down to bodgery. I fixed a crap running Volvo 740 GLE due to poor maintenance ; the flap was caked in rubbish and was transformed upon cleaning it (people here simply leave the plate dirty and adjust the mixture :facepalm:. The other issue is spares availability. That may be worth giving a go in yours.

Admittedly when tuned, there are superior avenues out there ; If or when mine goes I'll probably go for an aftermarket setup that can cater for my car (boost control, fans, chargecooler control in addition to possible boost limiting in lower cars will be considerations for me; with it being a ZVH it should in theory be easier to convert with the trigger setup already being present. Touchwood mine is still working OK at 225BHP.

A 924 I saw that had to be the worst K-Jet car I had seen ran badly due to laughable bodgery ; wrong fuel pump fitted with zip ties, cooker gas hose to the metering unit (no, I am not kidding!), wrong and different plugs per cylinder amongst other bodgery ; to think the guy wanted £600 for it over a year ago despite getting it for free, yet wanted it fixed on a shoestring.