Bit of an odd one, 1999 S3 APY runs beautifully except when under load up hill between 3000 and 3750 rpm, once over 3750, car resumes normal service and goes back to being a pocket rocket. between 3000 and 3750 rpm the car seems to splutter and I get what looks like black smoke appear in rear view mirror, once revs have risen to 3750 everything seems fine again, no smoke no splutter. I think it does it on the flat as well bet less so, on a motorway hill in top its definately not right, below 3000 pulls like a train in all gears. and the same above 3750 rpm. I've not had chance to run VAG-COM as I can't get to my lead at present, I'm thinking the black smoke must be due to massive overfuelling, could coil-packs be the problem? Any thoughts or suggestions most welcome.
Thanks in advance