newbie to forum hi to all few questions on a4 1.9 tdi

[sgedges]

hi i have a a4 1.9 tdi 96 p reg its high milage but with full history (150000) the car runs realy good except when i excellerate it puffs out grey to black smoke it does not smoke when you start up nor when its idleing nor when you rev it up i have put injector cleaner in today and ill see what that does some times it feels like its holding back and then it clears and is ok it is extremely economical thats why i brought it can anyone help also my heater is not getting hot i have noticed that the aircon belt has been removed will this stop the heater from getting hot and also can anyone tell me where the thermostate is located