I know this might sound crazy but I just cured my 1991 560SEL by doing this: I bought ten gallons of 91 octane ETHYL FREE gasoline. you need to make sure the gasoline is, in fact, ETHYL FREE gasoline.

I put a bottle of Gum Out All In One System Cleaner into the tank followed by 10 gallons of ETHYL FREE 91 octane gasoline.

The difference was ASTOUNDING, the engine purrs like a kitten!

https://gumout.com/fuel-system-cleaner/all-in-one-fuel-system-cleaner-ethanol-treatment/