co55ie69

help i put a erst s2 mfi engine into a mk5 it was running perfect with the exrtenal erst fuel pump on then i changed it to a intank uprated cosworth fuel pump and now its doing 15mpg it was doing double this before i changed the pump does any one now why this is and how to solve my proplem will a rolling road set up cure my car is it the higher pressure of the new pump i dont have a clue and would love to now before i take it to be set up and may waste £100 cheers tim