On the rear bumper after a few miles after being cleaned, these appear.

 

Originally my first thoughts were that it could be oil, but when I wash the bumper with cold water, it doesn't smear like you'd expect from oil, it comes away more as a dust...

Does it sound possible that the engine is just running too rich? When driving the car it does smell of unburnt or rich fuelling.

 

Any thoughts would be appreciated :)

Hi i get this aswell,mine started after my mate thrashed mine up at santa pod one of the turbo pipes came loose and it limped back loads of soot came out.i wondered if it was from the cats or something i really dont know.

ive not washed mine for er a while :/ and there is a definate film across the rear bumper etc, tbh the cars are mapped to run rich on boost in any event, so a little bit of build up unless you never boost is normal. also its a 20 year old engine so some oil etc is gonna seap by now and again, carbination will fall off blah blah blah, unless its actually smoking or the back or the car is black with soot aftera few miles i wouldnt worry too much.

Thanks for the replies. There is no smoke upon start up in the mornings, or under harsh acceleration etc. The only thing I have noticed is that when I did a 200 mile trip, a few times as I slowed to around 5 miles an hour for red lights, there would be a small plume of smoke which would clear after a second.

The engine has done 143k and is as sweet as a nut apart from this...