The freq valve drops the lower chamber pressure to control mixture. usually confirming that it is buzzing is all that is necessary. It does have to flow like an injector though. Try unplugging it. It should run distinctly different on the road without it. It will lack power but otherwise be smooth.
Proper fuel control and proper adjustment is dependant on achieving closed loop control of fuel. It doesn't spound like that is taking place. be sure your fuel pressure is constant and around 70psi.
Bosch Master, ASE Master, L1
33 years MB technician