strange gas tank problem

[Southin]

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.)

[psfred]

I would guess a plugged line or lines for the charcoal canister.

However, if you fill up from a station with a large underground storage tank (50F) and the air temp is more like 100F, believe me, that gasoline "grows" quite a bit. What you are seeing is typical of the expansion I would expect.

Don't know what kind of filler nozzles you all have, but the ones up here won't go into a Benz tank very far -- the diesel is the worst, it is the old large diameter one -- so you may be overfilling.

Run the fuel in as fast as it will go, not slowly. This will trap some air in the tank and evap system, stopping the pump "early" and leaving you more room in the tank.

I'd also try to fill up in the evening on the way home, so that you drive some out before you park it.

Peter