My set up consists of a Ford Racing Puma ( 1.7 litre/155PS ) engine, TAPE ECU from donor vehicle, engine compartment wiring harness from compatible MY non FRP vehicle. All sensors are connected except PAS, AC, Clutch pedal depress. Engine starts and runs in all drive modes except the following. When rolling to a stop at a road junction the engine undershoots idle speed to approx. 300 rpm and then stalls ( 4 out of 5 ) or miraculously recovers to idle. Hot or cold idle is normally rock steady and stabilises at 845 +/- 10 rpm. You can see the stabisation process working on the digital read out, so I assume related functions including IAC are working. More recently I have discovered that I can recreate the same stall condition simply by blipping the throttle when stationary in neutral.
The latter rather destroyed my nascent theory that the decal fuel cut off might be responsible ( VSS is connected ). I have replaced the IAC with a new one to no effect. The problem seems to have occurred following initial track test ( pre IVA ). My notes on the latter test do not record the problem. That rather rules out a basic software fault.
If anyone has insights into what might be the cause of the problem I would be exceedingly grateful.

I would be pretty sure it is the lack of a clutch pedal switch, the idle speed should be slightly higher than normal (1000ish) when the clutch is down coasting up to a junction, wire up a manual switch to check this before making up brackets etc (iirc switch closed is pedal down, switch open is pedal released).
I assume that you are using the gearbox speed sensor and the signal is fine (if you have wired the diagnostic plug, you can check the live data for speed signal and clutch position) as this not working correctly can also cause the idle speed to drop while coasting, the sensors on the ib5 boxes do fail and don't like being removed either.

If you bridge the pas switch wires, it will force up the idle speed to about 1100, i think it will throw a fault code if left this way though.

Dave

Yes as above all the sensors you have disconnected have an effect on requested idle speed.
Generally the power steering sensor can be either a switch or a pressure sensor -- if it has two wires (not 3) it is probably a switch and you could try bridging the wires to request idle anti-stall.

However it it could also be as simple as a throttle position sensor that is out of synch or a an ir leak. I don't work on these engines but if it uses a MAF sensor an air leak anywhere on the inlet trunking or manifold can throw things out.

If you have and OBD II socket it would help a lot.

[Edited on 22/3/16 by britishtrident]

On my RIOT I did wire up the Clutch pedal sensor and bridged the Power Steering sensor.

IIRC if you look at the wiring diagram you will find that cars without power steering are bridged.

Nine out of ten of these that I've come across have been air leaks.