240 940 turbo engine sputters and dies when you take your lift your foot off the gas

Hi all,
Just finishing swapping 940 turbo engine into a 1993 240. When you lift your foot off the gas at low speeds the engine just sputters and shakes for about 3 seconds and then dies. It starts right back up every time. Once the engine is warmed up it idles pretty good but when its cold it will sputter a bit and almost die while idling. AFR reads around 18 on cold start and then starts to drop and ends up at about 10 when warm. Right now the have a manual boost controller set to 0 boost or at least I think so based on how slow the car is. I don't have a boost gauge so I haven't started tuning that, could running no boost be the cause of this random stalling when I let off the gas?

Low or no boost would not cause it to stall, I would check the wiring to the temperature sending unit. lh2.4 runs rich on start up and slowly leans out during warm up, yours is doing the opposite. Your AFR numbers are out of whack.

air leak

^

First thing I thought of seeing this post was when I did the PCV service on the 854T and the damn thing wouldn't idle. I neglected to hook up the hose to the IAC.

(Pretty sure it was that car. Maybe the S40. Eh, whatever.)

Smoke test

also: check if:
- the idle air control valve is hooked up to the wiring connector
- the mechanical idle level is adjusted OK (with engine at operating temp about 400-500rpm with idle hose clamped shut, once the clamp is taken off the idle air control valve should take care of the extra rpm and rpm should be stable at about 850rpm).
- the throttle position switch is connected, working and mechanically adjusted correctly. With the engine off you should hear a slight clicking sound when you manually turn the throttle valve from the idle position. That clicking sound is the micro switch inside the TPS that switches the idle control on/off.

It would be good to clean out the throttle body+valve and idle air control valve before you make these adjustments.
Also, if there is lots of play on the throttle valve axel it might be the cause of unmeasured air being sucked in by the engine which has a leaning effect which could be influincing the idle quality.