It all started with a single misfire...

[MaciejMSalach]

Hello SubiFans, I have a riddle that i hope someone here can help me solve. 2014 Impreza 2.0i Sp Prem with now 189K miles. A few months ago, well more like over 4 months now, i had a single misfire reported on my OBD. I cleared it, babied the car for a week or two, then went back to what i would consider 'normal' driving for me. Issue never reappeared, but i never forgot about it, so when i had the chance i changed the plugs, ignition coils - this would have been just over 3 weeks ago by now - and about 3 months after the misfire was reported. Suffered thru the relearning process for a couple of days (only got about 75 miles on it) then I finally changed the fuel pump last wkd. So now we being the relearing once again - each time i did unplug the negative for an extended periods of time which reset the specs on the OBCm - keep in mind that all these parts mentioned above have been in my car since day one, so each part has over 11 yrs and 189K miles on it. Now, THAT nothing out of the ordinary happened until about 2 days ago - after sitting in parking lot with the car running for a bit, i reversed out, put it in drive (CVT tran) and OMG it sounded like a piston was about to blast thru the block. The car was 'chocking-out' so when moving it was stuttering to a point where it almost felt like all the wheel-bearing busted on all 4 tries - and there also was a rather metallic sounding knocking coming from the front, this lasted about 45 seconds. The car never died and it kept moving forward, slightly jerking, it made it up a hill towards the red-light, and came to a stop and didn't die and eventually the engine smoothed out, all the noise went away and it was like nothing ever happened the rest of the driver home - well except for me having lost about 30 LBS from freaking out seeing how i was 30 miles away from home with no other way of getting back home. Got her back in the garage let her idle for about 5 minutes to see if there is something weird going on, listened to the engine.... nothing. So next time to plug in the OBD and see what the heck just happened (during the time stuttering/chocking the CHECK ENGINE light was flashing the entire time, but did not stay on after the car started to run 'normal') - the OBD reported a big fat 0, no codes no anything. Unplugged the coder, plugged in the reader - trim, pressure, injectors, sensors - everything w/i spec! Cool huh? So then since i had to driver the car again this morning, the only thing i could think of was the MAF sensor - this is an aftermarket Denso that comes with K&L long-arm cold intake that was made specifically for these cars - this was installed w/i the first year of owning the car w/ less than 30K on OD - cleaned it plugged everything back in, car start NP, the read-outs did improve just a bit, but this was only in idle when i wanted to check if i did this right. This AM car turn on i got on my 30 mile ride to work (i am being a bit more gentle with it), and no problems. The only other thing i can think of doing is cleaning the throttle-body which i will do this wkd, but I'm at a loss. Subies are notorious for silly-dummy-lights, and yet this ordeal that was the worst thing that i heard and felt from this car since i bought it over a decade ago, reported absolutely nothing - it came out of nowhere and then just dipped out after nearly giving me a heart attack, as of yet to be seen again. So as of right now, we are about 60 miles AFTER the stutter, and 30 miles after the MAF was cleaned. any idea what i might be missing? we have a break in the weather now so its not too bad outside, but this is not something i want to deal with when winter returns, having this happen yesterday was probably the best its gonna get till spring, so i wanted to know if ANYONE here can point me towards what the issue might be. Don't get me wrong, i could so go for a new subi, but i've had this one for 11 yrs, and it never let me down or freaked me out as much as it did yesterday, so to be brutally honest, i don't want to have to get rid of this one, cuz.. you know its a subi, and you know how those owners can get. Help plz?

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[Charlie-III]

When did you last get gas?
Was it from a normal station, or somewhere different?

Any recent work done on it, if so, what?

[MaciejMSalach]

Since I'm still not a 100% sure what happened (thinking it was/is a possible fuel-pump issue) i filled up 50% tank on MON AM before work - same ol gas station as always, like almost to a creepy level of me using the same gas station since i moved up there over 20yrs ago, never had an issue with engine-juice from that station. The work performed on the car was probably the most the car had done to it in over 9-10YRS, and it all fell w/i 3 weeks - this included plug swap, ignition coil swap, an the fuel pump was the last one to get plugged into the car.
So, despite me wanting to clean out the throttle-body over the wkd, i decided to wait and see what happens (or how the car performs) after cleaning the MAF. It's only been a few days of driving after the clean, and it did seemed to level off the N/P idle RPMs to around 700 when warmed up, smooth reduction when slowing down, and a very smooth RPM drop when coming to a stop. I'll go a little bit longer before getting the TB off to clean it, my concern was that unless i can pin point something light RIGHT NOW, I'll give the ECM another week or two to start remapping everything properly - if there is any more issues that come up in these 2 weeks at least i know what was already done and what is left for me to do before crying uncle and taking that car to the dealer. If nothing happens, i will go and clean out the throttle after a week or two just as an additional PM. Keeping my fingers crossed here, given the year and # of miles on it.

[Charlie-III]

Thanks for update.
Hopefully, it was just a glitch.

[MaciejMSalach]

final say: a few weeks have passed since this was posted so I just wanted to give a final follow up, just in case someone else somewhere down the line runs across the same issue - even though please keep in mind that similar symptoms will not necessary be corrected similar actions; after MAF sensor clean, and after the weird studded from the car, everything seemed OK, i got the infamous P2119 code (i hope that's the correct code - throttle body position sensor - these are great as they throw your car into limp home mode right away. Cleared the code and drove back home, drove the next day, and for about a week w/o it going off again - however a brand new symptom showed, and that was the occasional surging while in high gear and higher RPMs. Replaced the throttle body with OEM. Engine runs great, beautiful, smooth idle, after 2 days of driving surging comes back and continues. At this point, since cleaning the MAF sensor did seem to have an effect the first time - i figured why not replace it - so i did with OEM DENSO. again first 2 days, nothing but bliss - then 4th gear about 2800RPMs a lil surge again. keep in mind these are not violent shakes, other people in the car didn't even fill it, but i did and idk if this is because i was driving, but they don't even seem to produce an odd rav-up sound, its more like a little burb. Finally decided to log ODB while driving again which was now getting rather old, but for the first time i was on the 4/5th page of read-out before i noticed anything weird going on - no check engine lights during surges - my o2 sensors, both up and downstream, where at times hitting 0.000V - no i know this can happen with the downstream under 'somewhat normal' conditions, but the upstream was acting bizzaro with it logging anywhere from 1.13V to 0.000V while on the gas-pedal, even though it didn't cause a 'surge' everytime it did this once or twice it was dead on to when i 'felt' the surge in the car. So - the original issue - failing throttle body/dirty (or going bad) MAF sensor - both replaced after initial problem. Surging after replacement - dirty or failing O2 sensors - i don't know why it took me this long to tie it back to that, but misfires, especially the more nasty ones, tend to damage or throw off the sensors on the O2 sensors by covering them in extra soot or whatever else blasts out of our engine and into the exhaust manifold. All in all - for 189K Subaru that is still running all original parts this is something that i can live with. My lil subi is still a mechanic virgin, and even though i never want to do this again, it sure was one heck of a learning experience. if any of this sounds familiar to what you are experiencing, sorry but you'll get it... just need to get all the way there first.