P0304 - Misfire and I'm completely lost...

2012 Gt

 

All I have is the stage 1 OTS from Cobb, with the accessport obviously.

 

I bought the car with a fresh rebuild at 13X,XXX, and after about a month and a half got a CEL for a miss on 4. Thought nothing of it and cleared it. Then it came back in 2 weeks, then a week. Now its every time I let the car idle. Cold start is fine, then it gets to about 1000 RPMs and is really rough. When drops to usual idle speed it's not as bad but still rough and noticeable from exhaust. This started before the tune and I hoped the tune would help (I know the stock subaru tunes are trash). I replaced the plugs and coil on the cylinder and it made no difference. The following weekend I replaced the injector and it ran perfect for an hour and a half. Then when I went to drive it again the miss came back with the code. I've ran a bunch of fuel treatments, and rislone additives through it to try and clean up the valves/head incase it was a dirty head or something. And nothing. No difference. Went to the turning shop I'm going to later for my downpipe, and they pulled the data sheets. I have detonation only on cylinder 4, and it sets the roughness sensor off until it maxes out and resets. The CEL only pops up on warm idle, and occasionally when I'm driving it I get to a light and it will give me flashing CEL until I get on the throttle again. I'm not burning any oil or leaking any. Only ran Costco 92 or Chevron 92 since I've had it. I'm just hoping it's not a pull the head issue because I just bought the freaking car... i was planning a compression test this weekend, but dont know if time or the weather will agree with me. If anyone has any input to try and help it would be much appreciated, I just am completely lost on what it is.

Check the compression. So far most of these issues have been either a dropped valve, incorrect valve lash, or broke valve. Who put the engine in? If it was a dealer then it should carry a 3yr/36k mile warranty.

 

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was the cobb included in the purchased?

The Cobb AP should not have detonation or DAM much less than 1. You need to figure that out before doing anymore mods. You may want to put it back on the stock tune with AP. The 2010-2012 weren't as terrible as some of Subaru's other tunes.

 

Look at the fuel trims, maybe you have intake leak. I bought a 2011 GT with a burnt valve, and it ran bad at idle and it smoothed out at higher RPM's.

Sorry for the late post been busy as hell with work and this virus going on. But an update for the ones who might care, shop who did the work has covered the work and found out it was an exhaust valve that failed (still need to ask what part failed though lol). Car wouldn't hold any compression on cyl 4, but I should have the car back by the end of the week!

When the replaced the short block, someone didn't send check the valve clearance when reassembling. This is pretty common at higher miles. The valve eventually ends up being slightly open all the time which melts the valve. My '11 GT with 127k miles had this issue as well.