This trick best done at night.

Get a strong flashlight (preferably one of those LED light bar style with a back shroud) and hold it horizontally just below eye level, bend down at the fender, and start hunting for the smoke with the engine running. This is before any of the pollution controls so you will be able to spot the rising smoke and probably can smell it too.

Found my 90 degree EGR pipe had a loose joint, needed to be properly torqued. Someone was there before I got the car.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech