Ok I have searched these forums for months along with NASIOC and others as well, Tested a few theories and had no results so I am sorry if this answer is somewhere else but I have taken a good hard look.
2002 wrx sedan
currently 102k miles
SPT intake w/ heatshield
TGV's deleted (rods left in place sensors functioning)
Up pipe cat deleted (CEL fix in place)
Paralleled fuel rails
Lightly ported throttle body and intercooler inlets
cheap ebay CBE (hey it came with the car)
Oil changes - royal purple 5w30 and Purolator Pure One filter changed religiously at 3000 miles (have gone to 3200 but never further than that)
Tranny fluid - red line super light shockproof changed at factory specified interval
Rear diff fluid - red line 75w90 gear fluid changed at factory specified interval
Spark plugs - NGK iridium changed at factory specified interval
Coolant – subaru green only changed at factory interval
MAF cleaned regularly (every month or two)
91 octane always
Driving habits – daily driven, no redlining at stop lights
Let me start by saying in no way has this car been a problem, I have thankfully (knock on wood) had no major issues with it whatsoever since I purchased it with 77k miles on it.
The problem I have did not start after a major mod. When I bought the car it had a cheap-o ebay intake, I changed it out within a week to the SPT with heatshield – I also got a factory intake scoop and chopped it short to bring cooler air in when moving. It was about 5k miles until the first misfire codes.
1st occurrence – cyl 1 and cyl 3 while cruising on the freeway: no engine stutter, hesitation or difference in feel/noise
Changed plugs, Reset the ECU – car ran the same; perfectly fine.
Approximately 700-800 miles later another misfire this time on the other 2 cylinders.
2nd occurrence – cyl 2 and cyl 4 while cruising at about 55 on a small downhill: no engine stutter, hesitation or difference in feel/noise
Swapped coil packs #1 and #2 with #3 and #4 – reset ecu
Went away for about 4000-5000 miles later another solid CEL.
3rd occurrence – cyl 1 and cyl 4 after accelerating away from a stop sign: no change in engine behavior, no stutter/hesitation.
I didn’t clear codes at this point – don’t really have a reason was just thinking more about the problem then what I should have been doing but that ended up playing a role in the next odd occurrence.
4th occurrence – Solid CEL is lit while cruising on the freeway at night and suddenly I notice – its gone.
Checked with a scanner misfires were still there (presumably a corrected problem since the light had turned off) did not clear codes.
Another 1000 miles later it came back.
5th occurrence – cyl 2 and cyl 3 misfires (1&4 still remain but are from previous occurrence)
Cleared the codes 2 more times in the next few months – car gets in a wreck spends 6 weeks in the shop. After approximately 3000 miles after I get it back (yesterday) another CEL.
8th occurrence – cyl 1,2,3 & 4 misfires while cruising about 40 on a flat road: again no change in engine behavior, no stutter/hesitation.
I have talked to the shop I use for all of the installs I didn’t feel like doing and a couple things I just couldn’t (alignment, suspension components, rear diff plug stuck) which is run by a reputable tuner another guy who owns a rally shop (and a nice subie rally car himself) and they were pretty stumped by it.
At this point I am relatively sure no major damage is being done so It’s not going to stop me from driving my car (for now) but it does raise concerns. Anyone have any ideas? Anything you can throw out there I would appreciate. Again I am sorry this is lengthy but I wanted all the facts to be there.