my 1998 ,120 excavator , will run fine as can be but when I move my joy sticks to dig or pick up heavy things it overload the engine and smoke black smoke and go dead !!!! I have changed fuel, oil , hyd & air filter, serviced them all up , check all fuel line connections for any leaks or drawing in air, was wondering if it could be in fuel pump, or needs new injectors ?? Need to get up and running bad ????
Probably your hydraulic pump control system. What series machine is it?
if its a weak motor check banjo filter and turbo system if motor recovers quickly after you release the load then its probably not fuel probably pump control issue there is a ton of great info on this site you need to start digging into old threads you could start by searching for "hitachi ex120 problem"
It could be the high speed solunoides on top of the pump if it is a bar 2 this happened to my digger