Does air in gas tank matter for emissions test?

[DUROC]

My 1997 C4S has had "check engine"/emissions test failures for several years. But I have just wire-brushed the corrosion off the screw-threaded brass gas cap and greased the threads with Vaseline to create a seal. The check engine light now seems to be staying off. I ran this experiment thinking -- without knowing anything about it -- that the design of the cap was intended to seal the tank and that if corrosion interfered with the seal, it might affect oxygen sensor readings or something like that. Question: Does this reasoning make any sense? Any advice about this would be very welcome. Thanks.

[hcoles]

I would not be surprised if an air leak at the gas cap caused check eng. light. This is the case on cars other than Porsche and may infact be a mandatory emissions function... e.g. this will happen on some late model BMW's