Ferrari 348 stalling when returning to idle

Did you try pulling the ecu codes to see what it says, may be a good place to start.

 

Air bypass screw...? I had a stall issue that seems to have disappeared with a simple slight turn of the screw.
https://www.ferrarichat.com/forum/threads/engine-cuts-off-when-the-ac-is-on.678921/#post-149187176
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You can guess and throw parts and efforts at it. Or you can perform systematic troubleshooting.

If it were my car, I would take weld an extra O2 sensor bung in each of the cat bypass pipes. Then, I connect a full-spectrum AFR sensor to that port and read the air fuel ratio when the car is having problem. It will tell me ...

a) what bank is having problem
b) whether it is running lean or rich, either will cause what you are seeing.

Armed with that information, I will look for vacuum leak, fuel leak, sensors, injectors etc.

 

It seemed like the older guys where messing with me about the screw, but a very small amount of adjustment cured the problem for me.
Also the old unplug the batt for a minute and then let idle for 15.

 

The amount of opening of the bypass screw does not directly set the idle speed but it is there to bring the IAC "within range". The bypass and IAC operate in parallel as a fixed plus variable opening. If the bypass opening is too small, the ICV will not be able to control the idle speed when larger amount of bypass air is required. The IAC may open fully (trying to prevent the idle speed drop) but that opening, combined with too small fixed opening of the screw, may be insufficient to prevent engine stalling.

On my 348, I experimented with bypass screw positions between 1 and 3 turns out. Whatever bypass opening I selected did not change the idle speed at all because it is the ECUs that hold the idle speed at 1050 rpm via the IACs (I finally set the screws at 1.5 turns out, but optimal opening will vary from car to car). If the bypass screw opening is too small or too large, the IAC will not be able to controll the idle speed correctly/timely and the engine may stall (too small bypass opening) or the idle speed may get higher and fluctuate up-down (too large bypass opening).

I will list on Ebay today the manual idle speed controller that I developed. It plugs into the IACs and holds both IACs at exactly the same opening as selected by the potentiometer of the controller. In this way, the idle speed can be fixed at 850 (+/- 50) rpm for the purpose of balancing the vacuums of the banks via the bypass screws.

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What you really need to do is hook up a manometer to the plenums and balance the intake from side to side. I do this by disconnecting the idle air control valves and then adjusting the bypass screws. I also made sure that the throttle bodies are opening/closing in sync 100%. Once everything is dead nuts on the money = then I reconnect the IACVs. I will also disconnect the battery for one minute, reconnect the battery, and then perform the 15–20 minute relearn warm up parameters, letting the car sit and idle with everything turned off and touching absolutely nothing, followed by a 30 minute drive around town and on the highway. That gives the Motronic ECUs a chance to learn the initial start up positions of the throttle position sensors, and the running parameters of the engine.

 

I tried to do plenum balance with the IACVs just disconnected (electrically only, unplugged 3-pin connectors) but the idle speed went to some 1,500 rpm and the manual states that the balancing should be done at 850 (+/- 50) rpm which, I believe, is controlled by the Ferrari tool called "Ferrari Tester" which is connected to the left and right IACVs (pic). This is why I developed "My Tester".

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My idle was also high when I initially balanced everything. I like getting the idle set mechanically first. For starters, my bypass screws were turned out way too far and were not even in there adjustments. I had to turn one if them in a few turns to bring the idle down to around 850 RPM. Then the throttle bodies weren’t opening/closing in sync. Those had to be adjusted via the arms attaching to the throttle linkage. The next thing that needed to be adjusted was the throttle stops underneath the throttle bodies. Making sure that both throttle plates were coming to a rest on the stops at the same time, while paying attention to the throttle plates, making sure they close fully, but without binding on the inner diameter of the throttle body. Then, balancing the plenums via tiny adjustments on either the bypass screws or/and the throttle stops. Once I had everything balanced, idling, revving and coming back down to a rest without stalling or over idling then I hooked the idle air control valve back up. That way when the IACVs are hooked back up the Motronics can controlled the idle easier. The car ran great after that, and had no more issues with the idle.