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

That is a characteristic of these engines. Mine idles as smooth as glass, but more often than not, you will get the hollow thud of injectors or some loud valve noise.

Which is characteristic, the noise or the stop lever?

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