I spent a bundle of money on my 2011 GL350 Bluetec on this issue and finally went backwards to a 06 CDI and never have looked. Never again anything with DEF for me.

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06 MB E320 CDI 123k miles
14 Sienna (wife's DD) 36k miles
11 GL350 Bluetec 136k (Sold)
96 E300; 292k+ miles (Donated)
11 Sprinter 22K+ miles (Sold)