Can someone help please.Peugeot 205 gti fuel pump has been tested.works but doesn't work when starting the car.fuses and tachymetric relay have been checked also.any ideas?

it could be a lot things the first being the e.c.u electronic control unit get it tested the next thing to cheak is weather you are getting a vaccuum in the fuel tank if so a beather tube/pipe is blocked then there are a number of senors to look at and there related wiring and the fuel pressure regulater