2006 XT rough idle misfire mayhem

Jun 22, 2023
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Howdy! First post here. I bought my 2006 Forester XT this past March. I have been having a cylinder 4 misfire since day one, though it didn't show up when I test drove it the week earlier. The misfire seemed to only come about when the car would sit and idle for a couple minutes. It has now progressed into a noticeable rough idle and many more codes, which reappear after clearing. Here's the info and timeline:

2006 Forester XT
5SP Manual
136k miles
  • some aftermarket clutch, very heavy and short.
  • strange cut wires sticking out from under-behind pedals.
  • places where it looks like gauges were mounted
  • exhaust system probably not fully original, as cat-back is far more rusted than midpipe and headers.

mar 17 130k miles: purchase vehicle. P0304 shows up during idle after ~5 mins. CEL comes back after about 4-5 trips.
mar 24: dealer performs oil change, rear diff fluid change, brake service, fuel sys service, new spark plugs, compression test, passed. Values were all around 120psi if I recall correctly. (I can call dealership and have them pull paperwork if anyone thinks that is necessary). Cylinder 4 has a dissimilar spark plug in comparison to the others. Dealer moves cyl 4 plug to another cylinder and the misfire follows.
since then: progressively shortened time between code coming back and trips. Now the code will come back after a trip.
june 12, 134k miles: take care back to dealer to diagnose issue. Here is word for word what the service notes say from :

Low Compression: spark plugs were done last visit. I had p0304 misfire cylinder 4. I pulled the cylinder 4 plug and coil and found that all was good. Team lead had me check throttle body and clean it. then drove again and found vehicle still running rough at idle. check live data found that mass air data was off. checked MAF found it was caked with dirt. CLeaned MAF and drove again. Found vehicle still runs rough at idle. The checked roughness monitors when reving found very little misfire data maybe 1 would happen on 4. When holding 2k all cylinders would misfire. Team lead then had me look into fuel. I took fuel sample. Visually everything looked good, I then did a fuel test and found ethanol is below 10% good. Called customer to get more info. Ruled out bad gas since he used premium as required. And gets gas from multiple sources and problem continues. I was told to check fuel pump which I didn't fully pull due to state of studs and nuts holding it in. I then consulted shop forman as well as shop mentor. I was asked to check oil level and coolant level, both were good indicating head gaskets are likely fine. I was then asked to repeat a compression test and decided that I would do leak down since I was already there. I tore into it removing all the plugs and performed a compression test and found cylinder 4 has 120psi but cylinders 1, 2, and 3 have 110psi. No greater than 7psi difference between cylinders is recommended. I determine low compression is an issue. I then did a leakdown and found no significant leakdown. The next step would be to tear down the engine. Possible wear in cylinders.

june 21 (today): current logged codes are P0304, P0113, P0102, with another P0304 pending.

I am planning on purchasing another MAF sensor, as these are new codes that appeared ever since June 12. Question: can I use the Denso MAF from the similar year / engined Impreza?

I noticed that my airbox is cracked at the bottom where the little tabs fit in, pre-MAF. Could this be contributing?

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I know that P0304 is a fickle beast. I should mention that the car drives and pulls wonderfully throughout, though I don't get into boost if I can help it due to the issue.

I am working on getting a list of repairs the previous owner did. Until then, I can say that headgaskets were completed around 110k.

I don't drive a lot. I've had many Subarus, and 3 turbo Subarus before this, one being an 03 Bugeye WRX (I still dream of it). I don't drive like an flower pot, and when I do, I know how to properly drive these cars.

Please help!

-CB in Buffalo, NY
UPDATE: I cleared the codes for the first time since getting it back from the dealer. SInce then, I've driven about 250 miles over 10ish trips. No light. I've been checking pending codes meticulously - no pending codes. On my recent long highway venture I was getting 30mpg as well, according to my scanner app on my phone. Rough idle is only really noticeable during open loop. This makes me think perhaps this is a sensor issue between the MAF and o2 sensors.

Please someone help me make sense of this.

1) Use OEM MAF sensor
2) Damaged airbox = un-metered air
3) Test the misfire with oscilloscope
4) Test for internal coolant leak or pressure spikes at the radiator with pressure transducer via oscilloscope.
5) Faulty Intake Air Temperature (IAT) (P0113) back to point 1

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