zhandax, thanks for the comments.

...going further, like the Porsche guys, i.e. a complete FD teardown, was precicely what I wanted to avoid by devising a cleaning method.

The fuel deformed FD membrane is only possible on the later, light alloy FDs. The early, cast iron units (what my car has) have a stainless steel diaphragm.

As an update on my 1979 450SLC, with further analysis: considering the type of performance degradation the car experiences now, the new primary suspect is not the FD, but the (likely rusted and degrading) fuel tank.

The earlier work performed, however, was a methodical elimination of confirmed "bad" components, in the order of natural escalation up the food chain, taking into account MB CIS pressure and other tests and that the car sat for 6+ years. First the FFilter, then FP, then WUR, then the FD, etc.

Finally, the original FD was indeed confirmed to be gone, and the used + cleaned (per method described in the earlier posts) FD makes the car run.

For anyone following this thread, the possibility that a "bad" FD can be cleaned is still viable, although not 100% confirmed, at this stage.

bostonmish