Alfa 145 Boxer 1,4 problem

I ve got a gas instalation put into my car (i didn't do it) and
recently i am experiencing much truble with the car - first of all
lambda probe isn't working (even i exchange it) but still the car uses
much fuel 13-14 l of gas driving inside the city. Second when i open
the fuel valve there is a lot of underpressure so when i open it it
suck into the fuel tank a lot of air is this normal?

Please help what to do what is wrong with the car??
tried going to Alfa romeo servis but they are saying its computers
fault

Mchmiel