3 years ago my 89 740 Turbo developed a serious knock at start-up, after having been hard to start. The car was flat-bedded home, then looked at by a Volvo tech who said either the rod was bent or a engine bearing destroyed from the cylinder filling with fuel caused by a stuck injector. The injectors were clean and balanced 6 months prior. I ended up replacing the engine with a 94. The Volvo tech had seen this happen twice before, once on an 850 when it was in for service. It is extremely rare, but it does happen. Go on google or yahoo and type in "injector stuck open". My condolences. Been there, done that. Mine happened on Jan. 2. Not a good start for a new year.