Update,
Decided to start a new thread as the old one is so long now it is time consuming to follow.
Car is stumbling (intermittently) at any rpm below 4000. Will run smooth for approx 1-2 seconds then hit a wall dropping rpm by 200 or so. Occasionally causes stalling when at cruise on highway or at stop lights.
SO, problem seems to only occur near idle circuit. Small throttle or zero throttle opening.
Important note: It is not "surging" up and down like a boat in the ocean. Runs smooth, then hiccups, then smooth again somewhat rhythmically. Condition lets out a deep, airy poof at the muffler when the stumble occurs. Misfire? Backfire? Vacuum leak? Failing sensor? I can't figure this out.
Had car at a local Subaru dealer twice who first replaced intake manifold gaskets first, and then the alternator second, (again.) Last time they suggested that I buy an entire new engine wiring harness based on the fact that the outer coating is cracked on one of the crank sensor wires. Whaaaa???
Decided to take this on myself now:
Have not removed manifold yet. Trying things a little at a time.
-Pulled intake snorkel and inspected. Sealed up any suspect intake leak. = no change
-Pulled and cleaned Throttle body, not very dirty BTW. = no change
-Pulled and cleaned By-Pass air valve, also not very dirty. = no change
-Replaced small diameter vacuum hoses on top of TB to metal tubes near TPS = no change
-Replaced vacuum line between what I believe is the fuel pressure regulator (just above EGR valve) and the intake manifold = no change
-DVOM test on By-pass air = good
-DVOM test on Mass air, both key-on, and engine running conditions, even tapped on it hard and wiggled wires = good.
-DVOM test on knock sensors = good
-DVOM test on injectors = good
-DVOM test on pass side coils = good. Although when I removed the boots, the part attached to the coils, (not the plugs) had little white lightning bolt streaks on the inside. Cleaned off and re-installed. Have not pulled the driver side yet.
Have sprayed starting fluid, propane, MAF cleaner on all suspect vac leak areas with no change in idle. Arrgghhh.
After reassembly, running engine in garage:
Idle would not fall below 900 rpm no matter how hot, no matter how many times the fans cycled, no matter how many times I revved it at the TB. Running Defroster kicked it up to 1100-1200 rpm. Have not road tested yet, I don't like the idea of getting hit if it stalls again.
Still hearing a strange clicking chatter (pretty loud) from under the manifold when throttle is blipped and is returning to idle. Almost like the IAC valve is chattering or a solenoid is freaking out. Only occurs UNTIL car is completely warm.
Hearing a hissing type of noise coming from area around power steering pump/reservoir. The assembly is leaking fluid from somewhere (likely from re-using the o-ring under the reservoir when the timing belt was changed by a tech recently). Could also be the bearing going out on the PS pump shaft. Hard to tell. OR this could be my vacuum leak somewhere around this area.
I still question a burnt catalytic converter or O2 sensor that is not throwing a code. I had a thought to take the car to a muffler shop and by-pass the cats and fab up heat shields, (there are no emissions checks in MI) to help rule them out.
Any help is greatly appreciated.