I gassed up my 91 560SEC a week ago, and did not fill beyond the point when the pump stopped on its own (I never do). There was no gas in the upper neck of the filler. I drove less than 2kms and bought some injector cleaner. When I opened the gas cap, a HUGE amount of gasoline squirted back at me (like half a gallon or more) and it continued slowly draining out of the open filler for the next 20 minutes (making quite an embarrassing mess of the Wal-Mart parking lot). I know liquids expand on hot days, but not this much, surely.
Yesterday, I gassed up at the same station, again careful not to over-fill. I put about 8 gallons in (the tank was only half-empty at the beginning). I drove less than a km and suddenly the motor stopped dead without warning. I checked the gas filler and when I removed the cap, again half a gallon gushed out and continued slowly running out for 20 minutes.
The motor died, it was later determined, because of a failed fuel pump relay. So...
Is there a relationship between the excess gas and the relay failure? What took up all that pressure before I removed the cap? Where is all this gas coming from? Is it harmful to any of the fuel system or anti-pollution components? Is this common and I just haven't had an occasion to notice it before? (Does my car manufacture gasoline?--I wish.)