The line carrying the boost signal from the intake to the aneroid (top/rear of injection pump)is probably plugged.
It is probably plugged where it goes through the overboost protection switch-over valve. If you have a vacuum/pressure gauge, verify that you get 10psi of boost pressure at the aneroid under full throttle.
If you do not, then back track the line to the hollow bolt where it comes from the manifold, testing each section for boost pressure.
Bosch Master, ASE Master, L1
33 years MB technician