I was driving my car yesterday, and after getting out, I noticed a smell of raw diesel. My first thought was that one of the return hoses had started leaking, but I took a look and they all appeared to be okay (they should be replaced, but they weren't actually damp). Then, after getting home, I noticed a wet spot on the ground where the car was parked, near the fuel filler. I dipped my finger in the spot and sure enough, it was diesel. On closer inspection, I Found diesel coating the entire side of the car, below and behind the fuel filler

I think that the rubber gasket on the cap's brittle and isn't really sealing, but what else shoulde I be looking at? To my knowledge, this hasn't happened before, and I'm 99% sure that the tank wasn't overfilled.

Thoughts appreciated...thanks in advance!

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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
Spark-free since 1999