Thus far I've replaced the intake manifold gasket, injector heat shields and delivery valve seals and o-rings. Idle is a little better but still not close to the SD. The sound of driving a nail into a metal slab is gone but a periodic 'clack' continues. Balance test suggests that #5 is a little weaker than the others but I had to do it a dozen times to convince myself that there was a difference. Will move injectors around next.

The most puzzling thing is that the stop lever rests at the middle of its throw. I haven't checked if it shuts the engine. The engine shuts off as it should when I turn the key to off. Idle doesn't improve when I swing the lever to the rest position. Any ideas?

FWIW, it's smooth at speed and accelerates nicely.

Thanks,
Sixto
91 300SE
87 300SDL
81 300SD