Im not sure what the deal is here, about a month to two months ago my car randomly started having this issue:
As soon as i start accelerating onto the road it starts misfiring/backfiring hard under acceleration. idle and slow acceleration are both fine. i replaced the plugs/fuel filter/hpfp and it went away up until a couple days ago. i replaced the valve cover due to a failed pcv and as soon as i started driving again it started missing/backfiring and stumbling under acceleration, anything over around 30% load. i thought maybe i had a bad ground for the coil packs because once i secured that better it completely went away for a couple days, but yesterday it came back like it never left. ive looked/listened for vacuum leaks etc with the new valve cover and see absolutely nothing. it seems to have fixed the issue i was having with cold starts so far as well.
I have two sets of coil packs (oem with probably 100k miles on them, and bosch from amazon with around 15k) and both have the same issue.
I've logged fuel pressure data and i see absolutely nothing wrong with it while it's having this issue so im assuming fuel is fine, hpfp is brand new and oem so if this were the case id suspect maybe the in tank pump not sending enough?
no check engine light, but if i scan all the codes i saw misfire cyl 1 and 2 as well as coil pack faulty cyl 1 and 2. i did not scan codes with both sets of coils independently so im not sure if id get these exact codes with both sets.
I've ordered new delphi coil packs, but does that seem like it will fix the issue? It's hard to imagine two different sets of coils are both bad. is there something im missing like with wiring to the coils? any help would be very appreciated, thanks guys

 

Ouch - sorry to hear:

What kind of Mini do you have, how many miles etc ? Are/were plugs and coils original ?

One thing to try is clear codes, flip plugs/coils from 1 and 2 with 3 and 4 and repeat. See if the problem moves to cylinders 3 and 4. If so - then do plugs alone and coil alone.

Clogged fuel injectors can also cause this ..

Since the problem goes a way and comes back, I think you are close. How do the plugs look after a few weeks ? What kind are you using and gaps ?

 

thanks for the reply, i have a 2010 mini s with 124k miles. its fairly modified so that could obviously have something to do with it, but its odd it just decided to pop up one day. the plugs i replaced were around 12k miles old and looked decent, makes sense since they werent the issue. the coils i was using were bosch from amazon with around 12k on them as well, i tried swapping in the original coils with 100k miles on them and the problem was the same. supposedly my new set of oem coils gets here today so if that doesnt work ill 100% check injectors next. that would make a lot of sense. maybe they get clogged and the clog gets cleared up which is why the problem went away for awhile. im hoping its the coils because i did get "coil faulty cyl 1 and faulty cyl 2 codes," i wish i saved what they were specifically.
im using ngk 1422 plugs with the standard gap that they come with. im wondering if im actually fixing something related to the problem or if it just decides to go away on its own so i think that im fixing it. its really hard to tell. ill see if these coils do anything.

one thing is i never checked if swapping the coils moves the issue, so it could be that both sets of coils have one or two bad ones grouped in. i wouldnt be super surprised. is it possible at all that a wiring issue to the coils would cause them to work sometimes but not others? i figure if its an electrical problem, for example to coil 1 and 2, it would just always be present.

 

well for anyone wondering the new coils seem to have fixed it for the time being. knowing my luck it will likely come right back, but i don't believe its had the issue at all since swapping them in. the quality difference between the delphi coils and the bosch i got off amazon is fairly noticeable.
it seems like the old coils weren't firing under load which caused a misfire and for the fuel to exit those cylinders unburnt, which meant it backfired in the exhaust. hopefully it stays fixed considering its gone away before and came back.