Here's the situation:
06 WRX wagon - stock. 97k miles. Runs like a dream, always has. It's a smidge less boosty than at 4,00 miles, but I ain't complaining. I almost never drive below 9,000 ft elevation and on recommendation from Silverthorne Motors (Vista) service manager, I start using mid-grade gas instead of premium. Whenever I go to Denver, I throw in the good stuff. I don't have knocking issues. I frequently run at lowest possible RPM to save on gas. I've been doing this for 50k miles with no issues. I live 5 miles up a mountain and probably have ~8k miles on bumpy dirt roads to get to my house. Vibration and dirt are big issues. (once had a plug work out on my pickup)
I do this myself, with a mechanic buddy helping me:
I replaced the spark plugs for the first time. NGK plats, oem gaps. I know, too late, but the old plugs looked fine. White tips when pulled out.
I replace the easy belts and check the timing belt and tensioner. All good. Better than expected. (hopefully not relevant, but I also did pads and rotors)
I drive ~100 miles with no issues whatsoever.
So, I'm driving up Trout Creek Pass in traffic, and RPMs drop, I pull over and it stalls out. I can get it started again, but it's really hard. The first few tries fail, but if I pump the throttle rapidly and I can get it started. On the first rev, I can get it to 6k RPM, but once it settles down, I can't get it past 4k RPM. Unless I flutter the gas pedal, it wants to stall. Eventually there is nothing I can do but either let it stall or turn it off. I get towed back home. This behavior hasn't changed, except maybe it's been getting harder and harder to get it started. It's not drivable. If I put it in gear, I have so little power, that I can't get up my driveway (kinda steep) and it wants to stall. It doesn't sound like a cylinder is missing, and I don't smell gas while someone else is keeping it running.
No check engine light. I talk to the dealer, auto parts guys, mechanic friend, and a general Subaru mechanic that I trust. Respectively they suggest; fuel pump, knock sensor, MAF sensor, and IAC motor.
The two little wires in the MAF look perfect. I can't really warm it up, but the car doesn't act any different with the MAF disconnected. Of course, it throws a code when I disconnect the MAF, but it goes away after I disconnect the battery.
I took apart the IAC. It was a little grimy, I cleaned it, no change after I reinstall it.
My neighbor suggests that I disconnect the fuel line somewhere, start it, and if it pushes gas out several feet, that's probably not it. I haven't tried it yet. I'm not sure where I would disconnect it.
PLEASE HELP!! I can't afford to just buy parts as a simple troubleshooting step. Can I assume that there is a 0% chance of a sensor or IAC failure since there is no code? Does that leave me with only the fuel pump / filter assembly? Would any of you risk the $200 and replace the fuel pump/filter?
Thanks in advance for your help.