I have this same problem .And i have replaced everything in the ht system also fuel injectors seals o rings sensors filter accumulator only thing now is fuel pressure .What points me in this direction is on the day i reoplaced the injectors i forgot to release fuel pressure and removing the hard line to injector i found that no fuel came out .This told me it was low on pressure . The car is in staorage now untill March ,,then soon after i get the car back then it will let me check the fuel pressure,. My M103 has twin pumps i am now thinking that one is not doing its job or fuel is returning via the non return valve .