I have about 5 IACs which all dont react fast enough when ltting off the gas above 2k rpm and allow the engine to stall. I have one which does work, but keeps the idle around 1100 all the time and reacts fast enough, but doesnt provide a nice high idle when cold. Didnt make much of a difference when I converted a LH2.2 780 to M90, however the 1991 780 LH2.4 seems a little more touchy with the M90...or all 5 of my spare IACs are lazy.
Most automatic transmission ECUs mappings cut the fuel on the overrun to save fuel, relying on the drag of the auto trans to keep the revs from falling too quickly when coasting. With a free revving manual, this can cause it to drop too quickly to recover.
I went thru this when I converted my V12 Jaguar to manual about 15 years ago and solved it by disabling the fuel cut out.
Looking at the pin outs, and wiring diagrams, it apppears the bridge connector adjacent to the ECU (BN and SN-BN) is the auto vs manual signal for the ECU ( pin 26?). I believe pulling the connector is for manual and leaving it in place signifies automatic....though I would like confirmation.
Patrick