97 Legacy Outback 2.5 Starts instantly runs several seconds,and dies. Key is still on, turn it starts instantly runs a few seconds and dies..repeat. Already replaced the fuel pump and filter. Ideas ? 

possible IACV hose loose?

1+ disconnected IAC hose, the 1" dia. one going to the air filter box.

Had the air filter cover been removed before this started?

Yes, I believe the air filter had just been replaced .

The IAC hose is not connected to the airbox.  I've had shops to this more than once.

Easy fix, the 1" dia black IAC air hose is thick/stiff but it can be bent enough to get it back on the nipple without removing the upper half of the air filter box.

Well the hose was connected and clamps tight. Also tried cleaning the slightly crappy Idle air unit and checked the gasket which was good.  Still acting the same way.  I think I saw a reference some place about an anti theft system that might act like this ? I don't have an owners manual, so don't know if the car is equipped with one.

Trouble Codes?

 

My first thought is a post-MAF air leak. i.e. hose off, cracked intake tube, etc. But that's a stab in the dark.