1. I've recently added fuel injector cleaner (Valvoline) to a very good running car and it caused skipping and a little shuddering. I think I dislodged some crap on the bottom of the tank.
2. My '94 M104 is very fussy about gas brands - it hates Texaco, Exxon and Citgo but loves Sunoco and Mobil. Shell is not its favorite but it works OK.
Maybe some gas line antifreeze or something for water in the lines and tank would help?
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