driving down motorway and engine is pluming white/grey smoke out of exhausts. only during 2300 - 2900rpm. haven't noticed any loss of power. wondering where to start for troubleshooting? haven't had the car very long, but this is the first trouble it has given me. done 145,000kms. had new piston rings, full custom stainless free-flow exhaust and all engine gaskets replaced, turbos were checked. however this was in 2001, car was used as a sunday drive to the shops by an old man who got bored of it and got into porsches. car is a 1990 vg30dett auto.