the distance between the plates does matter right?
No the distance between the plates along with the area of the plates define the size of the capacitor.
What you are doing are trying to (I think ) is to detect the change in capacitance induced by having the fuel as the substance between the plates as opposed to the fuel vapor. This change is due to the change in Relative permittivity ( dielectric constant) between fuel and vapor. This is very small.
Therefore the effect you are trying to measure is small compared to the size of the "standing" capacitance you have. That is why you are not seeing any change in the measurement.
Are you actually putting the plates inside the fuel tank or are the plates on the outside of the tank?
If the latter and the tank is made from metal then you won't see any difference anyway as the tank sort of shorts out any effect.
i was hoping that you would help me out.
Biggest help I can give you is to advise you that this approach is not going to work.