It has been a week since my last post. Other than contacting a seller who has a -949 ECU for sale (supposedly from a B200F 940), I have done nothing about the problem. It's actually more of a nuiscance than a problem because all other aspects of driving seem fine.

I have done some more thinking, and my problem seems to point towards the engine load as measured by the AMM. Mainly because all the other signals measure fine.
The AMM signal does measure fine, though, but I'm not sure if the "wrong" ECU for the engine will lead to the desired result. Surely the smaller displacement results in a smaller airflow through it?

The IAC signal clearly rises suddenly when the clutch is disengaged during deceleration.
The ECU must know whether the car is engine braking or coasting, otherwise it would be impossible to regulate intake manifold pressure and with it crankcase ventilation.

At first I thought maybe the fixed ratio between engine rpm and VSS when the clutch is engaged is used to determine that, but quickly discarded that idea.
This is what I think now:
During engine braking the car's momentum has an effect on the engine load. It cannot idle freely. Lift that load by disengaging the clutch and it can.
Since there's no clutch position sensor, that only leaves the AMM and engine rpm signals to derive it from. Depressing the clutch results in a change of both.



As an aside: while searching for LH2.4 Jetronic info I found that it's not a widespread system. Mainly only found on Volvos, Saabs and Porsches.
I also stumbled on this site (https://home.kpn.nl/mirjam_paul/928_lh_repair.html) of someone repairing faulty LH2.4 ECUs from Porsche 928s. Seems that the hybrid module is a common cause of problems.

To be continued.