Along with the vacuum issue, check for moisture in the electricals. Powerwashing if effective in getting out all of the hidden grime, but with that pressure comes the unintended ability to have moisture penetrate components that aren't designed to work under those conditions.

I'm not familiar with diesels per se, but check for cavities at the head where fuel injectors(?) and glow plugs reside to see if water has pooled.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car