1. Fuel injector issues: The fuel injectors on these engines were prone to failure, which could cause a loss of power, rough idling, and increased fuel consumption.
2. Turbocharger problems: The turbochargers on these engines could also fail, which would lead to a loss of power and decreased fuel efficiency.
3. Head gasket failure: Head gasket failure was another common issue with these engines, which could cause the engine to overheat and lose coolant.
4. Oil leaks: Oil leaks were also common on these engines, particularly around the oil pan and valve covers.
As for fuel mileage, the 1991 diesel 8.3L Cummins 300 hp engine typically got between 12-15 miles per gallon (MPG) in highway driving and 8-10 MPG in city driving. However, fuel mileage could vary depending on the specific vehicle and driving conditions.
