As others said, issues are with the PRV6 used in the 80's. I had a 760 with a PRV6 and to change the distributor cap you had to remove the inlet manifold! The engine fit worst in the Volvo, was better in others.

The B20 and B30 use identical parts except for cam and a few other bits, and is a cutdown version of an old Volvo truck engine. The BW35 also comes from a truck. This is why the engine and auto lasts as long as they do.

D-Jet is very simple. The issue is it is an open control loop so there is no monitoring of the mixture just like with a carburettor. There was K-Jet available in the last year(s) which is a mechanical fuel injection, but it is very seldom seen. K-Jet is a great closed loop system and still the most accurate FI around. It disappeared because electronic FI gives much greater control and flexibility (where as K-Jet is accurate for it's one specific tuned curve, not flexible).

I had a 164 project car that was too badly rusted, but the one time I did manage to start it up it had a lot more power and smoother ride than my B20B 144. It is a great car and great engine.