Black smoke is normally an indicator of a very rich fuel mixture. The 940 was known to have problems with the fuel pressure regulator. Also while you are looking around under the hood, check for any of the hoses from the turbo charger and the intercooler for holes. Especially the one at the intercooler on the drivers side. They tend to rot out on the bottom causing the boost pressure to be to low to handle the amount of fuel being injected.
If it is a turbo, the turbo needs attention. Bearings? Oil leak? Worse?
It could also be cauesd by the Brain box and air flow meter. So, you also need to make check-ups on those parts.
Turbo chargers that have shaft/bearing problems blow light blue almost white oil smoke, so you can pretty much rule out the turbo itself. The "Brain box" has never been known to cause this on any 940 series Volvo, that pretty much puts it out of the situation for now. Mass air flow sensors/mass meters on turbo 940s usually cause a lean mixture problem not the "black smoke" rich mixture unless the lining of the air filter housing has came apart and the air passage through the mass air flow sensor is blocked. This would give the same problems as the hoses having holes or leaks in them that I spoke of in my first answer about low boost pressure. One very unusual time I experienced a very defective oxygen sensor to cause black smoke. That can be easily checked by connecting a volt meter to the single wire lead of the oxygen sensor and look for a voltage swing from .1 to 1.0 volts. The green wire coming from the computer supplies .5 volts of reference voltage so the sensor moves plus and minus of that voltage as it reads the content of the exhast before it passes through the catalytic convertor. Please don't forget to montior the fuel pressure! In psi it should be around 43. In bars it should be around 2.9. This is of course just my thoughts based on what I have experienced over my past 30 years at a Volvo dealer. Hope this helps.
A customer's auto is producing blue exhaust. What could cause this
blown turbo - worn turbo bearing causing oil to suck through and into exhaust manifold - results in no power and neat oil burning in the exhaust - major black smoke
An exhaust leak could do that.
It could be timing is off. Need more info. Is it Fuel injected or Carburated? Exhaust smoke? Is it black? Black indicates alot of carbon built up. May mean the fuel is burning way to rich.
Could be EGR Valve.
Figure out the cause of the fault, first. It could be physical, or it could be a matter of programming. These things require diagnosis to identify and isolate the exact problem in order to fix them. From my experience with Volvo trucks, they can be an electrical nightmare, and your problem is probably a short in the wiring somewhere.
Exhaust gasses are escaping through a hole or rotted section of exhaust system
It could, if it's leaking.
Could be a diesel? If not then replace your oxygen sensor on your exhaust. A bad oxygen sensor causes the exhaust smoke to be dark in color.
It could.
It could.
Yes, those exhaust gases are extremely hot and can cause the alternator to overheat. Not only that but that carbon monoxide can enter the vehicle and kill you.