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You read here about the fuel crossover leaking where the two tanks connect. Might be your culprit.
That being said are you getting a leak underneath or just smelling fumes?
I seem to get fumes coming from the passenger rear tire well on really hot days. My cursory searches here indicate there is a "fuel tank vent valve" for pressure above 1psi on that side.
Also, are you getting any codes? These C5's have intricate system of solenoids etc to get rid of gas fumes, a few of which are monitored by the pcm (once again from a somewhat cursory search here).
The two usual causes are the filter as mentioned already and the crossover tube between the tanks. Another place is the fuel return line which can disconnect from the source on the fuel rail. Sorry I don't have a picture of it at the moment but its on the driver's side fuel rail near the back and it looks like a plastic snake like line. It connects to a metal line that runs back to the tank. That connection can come off. The tank pressure valve that was mentioned, is on the passenger side behind the right fender....weird place for it but oh well. You can easily see it by removing either of the right tail lights. Its white and is mounted vertically.
My fuel filter nylon locking tab deteriorated was giving a bath on the left muffler
Took a couple hours to fix this as I didnt want to wreck the fuel line going to the engine. Got a replacement filter and new nylon snap clips and fixed the leak. Started as a smell and turned out to be a guyser.
Luckily was in my driveway when it really failed. Check that as your car is as old an mine and mine failed about 3 years ago.
I would like to thank everyone for their help. I found the passenger side fuel injector closest to the front of the engine is leaking. Do I need to replace all (8) injectors? Can I just replace the (1) injector? Can I get away with replacing the O ring on the (1) injector? Finally, what does it entail to pull that injector? Thanks.
A fuel injector leaking is the first time I've heard this. Leaking internally to the cylinder or leaking at the rail connection? If its just the rail connection, I would imagine a new o-ring will solve that.