Hi John,
The hissing sound you are hearing might be an opened vacuum line. That might be the cause of your starting problem.

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Peter Chu aka PIKACHU
08 C300 Premium Pkg, MM Pkg, Sport Pkg, Keyless Go
86 300E soon to be E36 with Euro Headlights
400E front & rear brake upgrade
16" CLK wheels with Michelin MXV4 plus Green X