I drove my 83 380 sel to a nearby town and it ran perfectly. On the way back it drove normally until the second stop light when it started missing slightly. As I continued to drive the missing got worse and worse until by the time I got home, about 30 miles it was missing so bad and losing so much power I was down to less than 40 miles per hour and praying it would make it. There was also a strong smell of fuel.

I finally made it home, and as I pulled to a stop in the driveway I expected the engine to die immediately, but it idled normally. However, when I give it gas, it begins to sputter and miss.

What's most like to be failing and where do I start trying to diagnose it?

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire