Would check the spark plugs first. Check the air filter isn't clogged from that strange exhaust thermostat under the airbox. Pull codes and if you get 113 and 115, which is what is sounds like, then take the intake manifold off and clean all the negative electrical grounds.

The diagnostic tool now functions and we pulled code 113 and 115. The intake manifold was taken off and all the ground connections were cleaned. Now the black smoke is gone and the ABS light doesn't come on. The car shakes under load, less and less, when it's driven at low RPMs (under 110 km/hour ...

The mint condition 1993 Volvo 960 Executive has a low km engine that ran smoothly for the first 15,000 km after purchase. The OEM1 diagnostic tool doesn't function and now some sensors may need to be cleaned or replaced. They are expensive and I love the car, so the first priority is to have an ECU ...