Peter, you must really be desperate if you're checking for air leaks. There's a big one called the intake manifold with no throttle

Seriously, when the engine's idling high, unplug the round device at the rear of the IP. This will disable the ECU's control of idle. If idle drops to 500 rpm or so or the engine stalls, something's goofy with the ECU. The idle trimming selector doesn't have the range to support 900 rpm so I expect neither does the ECU. That's why I think it's a mechanical problem. But maybe corrosion in the selector increases resistance. Who knows...

Sixto
87 300D