B738 Engine QRH
 

Engine failure: leave ISLN valve in auto; turn off effected pack = ISLN valve open and live bleed pressurises both ducts. Effected eng bleed valve closed due lack of puff. Engine fire: Close ISLN valve. Live bleed pressurises only its own duct. Effected engine bleed valve closed; no puff + fire handled pulled. Why the difference with ISLN valve? For consistency why not close it for both cases. In the former, if inflt start required there can be a command in that QRH to reposition ISLN to auto or OPEN. I just have an idea that keeping things the same keeps them simpler, sometimes.

RAT 5,

I have not flown the NG for quite a few years, but perhaps can suggest an answer based on the A320. I think it may be to allow Wing anti icing to be used. The clue for me is the pack being switched off. (precooler performance?) Apologies if i've got the wrong end of the stick.

Possibly keeping it open to attempt a cross bleed start?

There is a note that if WAI is needed ISLN valve to auto: ref Inflt start see in original question.

Maybe smoke, air contamination, :ok:

Doubt it. In both cases the failed engine bleed valve is closed. The bleed and aircond ducts should be isolated from each other.

Don't think so.. the QRH directs you to turn pack switch on affected side off.. this will cause the isolation valve to open..to prevent this the isolation valve is positioned to close, causing the remaining pack to operate in high flow ( in flt with flaps up). Just because the pressure driven bleed may be closed, the systems are not isolated.