I always appreciate cars with very clean stainless exhaust but why are some full of charcoal-like substance? Does this reflect the state of the engine because funny enough, I see some old or tokunbo cars with clean exhaust but some fairly new cars covered with Soot covered exhaust.

Sooth, charcoal or whatever you want to call it only means that an amount of smoke is coming from the engine and settling in d exhaust line over time. Smoke means incomplete combustion. Incomplete combustion can mean a lot of things, but definitely not good.

OK, tanx but what causes the incomplete combustion?