E320 CDi W210 sooty exhaust

[Andrew Moorey]

Recently noticed that after a run the bumper adjacent to the exhaust is getting sooty. Fuel economy is still fine averaging 42 over the last 1000 miles. Currently on 80k miles its never run on supermarket fuel and had its oil changed every 3k.
I'm suspecting EGR valve but before I pull it all apart I wondered if anyone else has had this fault with a difinitive result?

 

[BlackC55]

Give it a good red line up the road a few times to see if the smoke clears.

 

[Andrew Moorey]

Gets that regular like!
No fault codes logged and data logging all looks normal. Noticed that if I operate the EGR valve with a Mityvac, the vacuum builds slowly then the valve seems to jump as if the valve was sticking slightly?