I decided to do a couple of the same mods to the '95 EJ22 in our most recently acquired 1997 Legacy GT. Similar to what I did with our '98 Legacy EJ22 wagon, I swapped the air tube resonator for the earlier style '95 air tube (less air restriction). The vacuum lines all install pretty much the same way and it ran fine afterwards, although the idle might have been a light rougher (attributed it to the ECU needing the relearn the difference in air flow).
Drove fine a couple of weeks then last weekend we were going to have my son drive the car on a longish trip and the car idled rough and got a CEL. I checked the connections (especially the large hose to the IACV) and all looked good, but I swapped the originally air tube back on and cleared the code.
The car has a bit of miss at idle (almost has a rumbling sound coming from below the engine when it misses). Also idle is a little erratic.
I went ahead reinstalled the '95 air tube and installed the MSD coil- no real differences or improvements in the way it runs. My son drove it about 40+miles for a driving lesson yesterday and it drove fine
I also played around a little with the TPS adjustment to try to up the idle speed a bit, which may have improved the idle speed a little.
Then this morning my wife went to take it to work and no start at all. The engine cranks and when it ties to catch the engine shakes quite a bit. It feels like some sort of misfire. I tried to read any codes from it and did come up with the P0505 IACV code again. I cleared it, but it does the same thing when you try to start it. (I wouldn't think IACV issues would cause a no start situation anyway).
So I'm thinking some of the recent work done on it is the cause- is the the MSD coil defective possibly- seemed to work fine at first. Totally unrelated but could it have jumped timing? (timing belt and idlers replaced 30K something miles ago when P.O. swapped the EJ22 in). Should whatever happened be throwing a code? (I'm not reading any new ones).