I'm still saying that quality of fuel is important on CIS cars. I have a friend with a 420SEL that shook at idle like a dog crappin peach seeds. Turns out his wife is buying her fuel at Costco. After I checked everything according to the MB diagnostic manual, I told him to start adding Techroline treatment to the fuel. He went through about six bottles, and now the car idles quieter than a mouse pissin on cotton. Fuels without the propper additives allow carbon deposits to build on the injectors and the back of the valves. These deposits soak up the injected fuel so the cylinder ends up with a lean misfire. These deposits can cause all kinds of driveability problems. I'm not saying this is the answer, but the possibility has merit.
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