2010 Impreza 2.5 NA misfire under load

[TheCaddyMan99]

Hey all, i've got this really strange intermittent issue. It started seemingly pretty random, in the morning i had an issue going up the hill that i live on, i would get to the top of the hill and it'd start misfiring. then i'd bring it back down to the driveway and it was fine. it did this while it was cold at first, then a week later, it did the same thing, but this time after driving to my office, car was warm. Both times, it set a code for misfire. I have managed to avoid any misfiring by using higher octane fuel, preferably 93. and that has worked for three weeks, but now it's catching up to me, it was never really a fix, really a bandaid. Today I could feel a slight misfire on the highway, not enough to set a code, but it definitely wasn't firing correctly. Nothing has completely gone yet, but I'm not sure where to start, based on my little premium fuel experiment, I think it's fuel related. Either the filter or pressure regulator, or maybe even the pump. i'm not too keen on throwing parts at the problem to see what fixes it, i'd rather diagnose it and get to the source. i'm just curious if anyone else has had these symptoms, and what the fix was?

 

[Ecmbuster]

Cooling system pressure leak?
Defective ignition coil?
Defective spark plug?
Defective injector?
Vacuum leak?
Defective MAF?
Miss-reporting O2 sensor?
Low fuel pressure?

 

[TheCaddyMan99]

I have been doing backflips fixing this car from prior neglect. I found a cracked ignition coil, that had been left cracked for so long that one of the plug wires was welded to the terminal! I had to replace spark plugs, only to learn that two of them were only finger tight. And I replaced the air filter just for extra measure. And that all corrected my under load misfire.

This stalling issue started recently. I can't take the car onto the highway if there's traffic, especially on a day that's 85 degrees F or above. It will tolerate a little bit of traffic, maybe 5 mins of it, but after a while the engine will stall at idle. It revs and stays running if it's revved, but if it's idling, it will die. If I let it sit with the engine off for 10 mins, will work fine until the same conditions are met. I brought it to my mechanic and he couldn't reproduce the issue.

Recently the car has also been throwing code P0420 for catalytic converter efficiency. I'm not sure if it's related or not, but my original assumption was that the cat material was expanding and creating a blockage in the exhaust.

The last time that it happened was last night, except last night was the first time that I felt the brake pedal get hard to press and the ABS light came on. I found the nearest shoulder, parked the car and in ten minutes was driving fine and ABS light turned off. My new assumption is that the brake booster failed internally, but is only showing signs when the engine bay gets very hot.

Is there any way that I can confirm or deny these theories?
Any advice is appreciated.