Been there done that on my 1998 E320. I replaced the "crank position sensor" and problem was solved. You can definitely rule out "fiber optic cables" as they have nothing to do with the engine controls. Get a new mechanic!

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Donwo
1999 SL 500 Sport (30K miles)
1998 E320 (SOLD)
1989 560SL (SOLD)
1987 300E (TOTALED)
1986 300E (SOLD)
1984 300SD (SOLD)