I had the exact symptoms on my 87 560SL. It turned out to be the locking unit for the fuel door. $23 and I replaced it myself. The housing on the old one had cracked, probably due to the way they are mounted. I found it by leaving the trunk lid open and then locking the car with the key. Since the pump kept running for a while, I was lucky enough to find the source of the leak before it shut off.

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Kevin Arburn
Overland Park, KS
1987 560SL