The o-ring at the base of the fuel distributer seals vacuum.
Where do you have a fuel leak? Is it at the EHA? I would always advise fixing fuel leaks first. If the leak is at the EHA I don't think it would keep you from doing these tests. If its at the EHA I would worry that the EHA wasn't doing its job, But I wouldn't worry long because fuel pressures tell the whole story.
Mixture correction at the tower is usually done within a few degrees of rotation. basic calibration after a fuel distributor exchange might require a full turn or two. I have turned that screw many, many turns and have never reached an END. How much are you turning the screw to go from poor running rich through good running to poor running lean?? Does this even happen?? Forget about cold till we get warm right!! First warm, then closed loop and then maybe cold.
Bosch Master, ASE Master, L1
33 years MB technician