We have an 01 c70 convert, with 170000miles. After long trips it goes into a rough idle and large amounts of black smoke come out of the exhaust when accelerating from a stop, with a hint gas? like its running very rich. Its about 90 degrees so thats out of the question. HELP!!

 

Black smoke tends to mean you are running too rich and burning fuel. Have you noticed a drop in MPG? Any check-engine lights? I'd guess you have at least one injector not functioning properly.

 

Acceleration / Black Smoke -> Over Fueling! - should light up lambda light (O2 sensor code) Lamda light (it's the upside down Y, on dash light) Use a OB2 code reader and should tell you the problem(s) you need to fix

Accelleration / Blue Smoke -> Burring oil! wore ring(s) causing oil blow-by. Wore Ring's self explanatory...

Start up / Blue smoke -> wore valve seals, Oil leaks past valve seal pools on valves and smokes when started after sitting. Valve seals can be replaced about $1 each.