Removed the pre-cats and cat(but not the O2 sensor) from a euro 2.5 16v. There are two trimming resistors in euro models, one for the injection and one for the ezl ignition. With the original trimmers, and without the cat, closed-loop operation is achieved (as before the cat was removed). Replacing the trimmers with the ones specified for non-cat cars, duty-cycle remains constant at 45%, and does not sway as it should if it was running closed-loop. These trimmers alter the timing. I assume the mixture is to blame.

Hence my question arises: would the mixture have to be enriched or leaned in order to be in-range for closed-loop operation to occur after the timing has been advanced. I assume it would need to be made leaner (by altering the plunger position in the fuel distributor, because turning only the mixture adjustment screw did not help ), but I am not sure.

Thanks