They ARE NOT power house diesel engines, they will not pull like a power stroke, or a Cummins
Ricky is correct on the PMD ( Pump Mounted Driver also known as an FSD or Fuel Solinoid Driver) which is the control module for the injection pump.
The DS4 Injection pump in these engines is fully electronically controlled, and can be problematic. The fuel lift pump ( aka low pressure supply from the tank) can corrode internally, the fragments make it into and thru the fuel filter and cause the injection timing ring to sieze, which will burn out the timing stepper motor, which can fry the PMD
6.5 Engines have also had head gasket issues, head cracking issues, and in 98-99 years casting porosity issues (coolant seepage into the engine oil).
There are oil cooler lines from the block to the radiator, they will develop leaks and can blow out, a definate maintanace check item, if it's seeping, think real hard about replacing them soon ( and it's a REAL fun job, requires lifting engine out of the mounts in some cases )
All that having been said, they are a quiet, good running engine, not all that problematic save for the injection pump and PMD, they make good power, about what a 454 would, only with better mileage.