Chasing air issues now in low pressure side of fuel delivery system. There is a check valve of sorts in the return line just below the filter housing that was added by MB due to "starting issues"--This $70 gem that might be the culprit here. It holds fuel in the return line and keeps it fuel from falling back to the tank when the vehicle is off. If it has failed, any air leak up top would allow the a huge bubble to form in the filter and would cause massive smoking and missing until it cleared. . . More later.