(Sorry for a long story) I've had my car for almost 2 years, and I've had the CEL since day 1...
History: Bought the car from a volvo dealer with 55k miles, that had a new clutch put in, it also had like a 3" TBE (catless), CAI, and BOV. Driving home and the CEL comes on. Had it checked to be something like "inadequate back pressure", probably due to the missing cats. THe only other issue was a gear grind btwn neutral and 1st, which is when I learned that a 12 lbs flywheel was put in during the clutch install. I also took it to a Subaru performance place called P &L here in Illinois, where they cited no particular mechanical issues.
At 60k I get the service done at Subaru, and things are still running well. I also put on the stock BPV, air box, and exhaust. The CEL did not go off, but I'm not worried because the car runs great, and I figured I just needed to reset the ECU. I THINK someone mentioned something about a Cyl 1 misfire when I had the stock exhaust put on, but I didn't pay it much attention (denial).
Fast forward about 1 year and now at 90k. The car runs pretty rough, in cold winter start up (Chicago). The mechanic at subaru did the fuel line recall and said the car is running like dog- basically the engine shakes at idle, and during start up it sounds like it is missing and the car shakes. Went to autozone and they said I have a P0301 and P0303 misfire. The plugs looked great, but I noticed cyl 1 coilpack was a little beat. SO I replaced cyl 1 coil pack and I thought I noticed a slight improvement, and so I later replaced the cyl 3 coildpack. I also cleaned the MAF. Still I had the CEL, rough start, and the engine shake.
Now, the car starts a lot better in warmer weather...on cold start up (8+ hours of sitting), the engine will rev smoothly to 2k rpm then quickly fall to a stable 1.5k. As opposed to in winter it would rev very roughly to 1.5k and bounce around a little, and I can hear some exhaust backfire. Today I went back to autozone - All 4 cylinders have the misfire code.
I put in a bottle of 20gallon Chevron fuel injector cleaner today. I am positive the car is running smoother than I have seen in a long time. I looked at the fuel filter, and it looked like it could be the original one (caked in oil residue). I also changed to Royal Purple oil from Mobil 1. In the current state I drove it today (it was cold), the car doesnt seem to be misfiring at start (but not cold start) or during driving.
I am hesitant to take it back to Subaru. The service writer was trying to talk me into replacing the Head gasket at a cost of $2500 because of a coolant smell. THe coolant smell (coolant was smoking) was the fault of the mechanic who forgot to put on a coolant hose, which was why I had to go back there in the first place after I had the fuel line recall fixed. Additionally, they stated there were air bubbles in my coolant, trying to get me to agree to a head gasket replacement... I have had no leaks (until they forgot to put on the hose), no oil burning, no overheating, etc. Also, I just don't have piece of mind after they fix something...and I haven't come across any posts about Subaru fixing someone's misfire code.
So is it Fuel, head gasket, ecu, lightened flywheel? Any thoughts or recommendations would be greatly appreciated. Thanks.