I'm not familiar with the newer models but it sounds to me that if it starts fine with no missing that the plugs might be ok. Maybe the relay is messed up, sending another light signal when it shouldn't? Hopefully someone with more experience on that model can help.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo