I have a 1991 350SDL (original owner) with 146K miles. It has a rough idle problem. Local shop wants to replace injector tips and calibrate the injectors, they say they have the correct Bosch test equipment to do this and that the worn injector tips are likely the source of the rough idle. They say that the injector tips should be replaced every 90K miles. Can anyone comment on this based on experience?

Thank you.