the newer diesel engines are fantastic, they provide mountains of torque and power and have excellent fuel economy. They run quite high fuel pressures and use a diesel control system to run the injectors and other bits. meaning if those cars face ustaad shaagird ustaadee - the owner will have to dump the car and buy a new one. Those cars require good trained techs to repair them.
e.g. the MB Sprinter CDI vans are insanely quick and can haul very heavy load very fast yet give very good fuel economy. There were quite a few ambulances in Karachi too. Most were put to retirement because of bad repair procedures. Usually involving the mechanic guessing the problem.
There is no problem with diesel fuel quality in Pakistan, its the same as everywhere else. Infact look up my pictures of the fuel tank strainer filter of my diesel benz, its run on american diesel fuel - which I can sometimes claim has "potty" mixed in it. Its so bad. I have to fuel at heavy truck fuelling stations outside of the city to get decent fuel.