As TD said, don't be afraid to rev the ZD30. It's not good to lug the motor in low revs. If towing up hills and the motor starts to lose revs, it's much better drop it in 3rd than putting the foot down in 4th.
These engines can over fuel quite easily when the revs are low and the load is high. The ECU just doesn't monitor enough of the engines vital signs to know if something is not as it should be. That's why, even with NADS these motors have been known to melt pistons.