There is a small S shaped vacuum line on the right side of the throttle body under the white connector if I remember correctly. Make sure that vacuum line isnt disconnected. Are there any codes ?
I also have exact same problem. No acceleration, tilt, or lower. No codes, does try to travel
Check the wiring harness.
Has the unit been outside in a lot of water? Debris in the steps, gravel,mud.... Water can build up in the left step causing it to leak into main cpu, ecm box. Below left fender/step area. This causes the distribution board to corrode several of the plugs and the board itself all interacts with the ecm and without good connection, cause this. Have had several build up with snow and ice. Inside melts outside ices up. Then no hyd or RPM.
A few years ago, I diagnosed a komatsu LPG with the same problem.
Changed the fuel injector and it solved the problem.