One thing that seemed curious was the rpms in neutral went to the maximum without any problem but as soon as you put a load on the engine the rpms stayed at around 1200 rpm with no torque at all.
The Cat 3116 Governor controls the fuel rack which in turn decides how much fuel to allow to the injectors according to engine load and speed. The operator controls the throttle and the load is read off of the turbo boost pressure. The boost line goes directly to the governor air fuel ratio control. The more boost, the governor moves the rack for more fuel.
I tested the air/fuel ratio diaphragm with a regulator on the bus and it was leaking into the housing meaning the diaphragm had failed. Once I replaced the diaphragm it was a no-brainer to retest the new assembly and check for leaks. The new diaphragm was holding nicely.
NOTE: Don't test diaphragm over 20 psi