Mystery CEL indicating cylinder 3 & 4 misfires

Hey all, I have a well maintained 2008 Legacy Limited 2.5i with nearly 200k miles and a very hard to diagnose Check Engine Light that indicates cylinders 3 & 4 misfiring. I've replace both catalytic converters and the ignition coil as recommended by my mechanic, and 2 oxygen sensors (upstream and downstream) at a Suburu dealership to address it to no avail. It only occurs at very high RPMs approaching redline, near or over 6k (e.g. when passing or driving in sport mode), but can be reproduced on demand. The third time it happens it goes into flashing mode and has to be reset. Compression is good and hardly notice when it happens. Engine runs fine otherwise.

 

My mechanic thought it might be a fuel filter/pump issue then recommended going after the air fuel mass sensor; fwiw, Midas thought it might be a crank sensor or timing tensioner? issue without testing it themselves. An online tech recommended I do a fuel pressure test and replace the plugs. I haven't done any of these things yet because I'm way over budget and can't afford to keep throwing money at it blindly. I'm hoping for some better indication before proceeding. I've also read on the forums somewhere that a bad alternator could cause these issues.

 

To make it even more interesting, I connected with a Subaru enthusiast/employee/tech online and we've been texting back and forth because he has the exact same issue (cylinders 3 & 4, CEL at very high rpms). He has spent a lot more time and money chasing it and said he's ruled out the entire fuel system and spark. He's also done everything that's been recommended to me above and a lot more including checking timing, injectors, valves, replacing knock sensor, battery, etc., etc. He's convinced it's related to the longblock or a sensor attached to it. I know every car and issue is unique, but the motor (2008-2009 2.5i) and symptoms are precisely the same, and he knows of at least one other case of the exact same issue happening on another 2.5i. This suggests the problem is not entirely unique.

 

I'd be very interested to know if anyone else has had this same issue (CEL + cylinders 3 & 4 misfire only at very high rpms), what they've ruled out, how they fixed it, or their best theories as to what it might be.

 

Thanks!

Parker

What kind of coil did you put on it and what plugs did you use in the engine

Hi, according to the invoice the plugs were NGK and the coil was a Standard Motor Products UF-538 Ignition Coil. I've heard to avoid Bosch plugs.

Ok so 2 things, subarus are finicky with there ignition system, the OEM coils are made by diamond. I'd start there. 2 with that many miles I'd also do a cylinder leakage test and drop the exhaust to see if any of the valve guides have fallen. They are known for that as well.

Like you said, your car is unique, so you need to check your fuel system and spark plugs also.

 

Start by changing a coil pack and spark plug with one from a good cylinder. See if the misfire moves.

 

If not, swap an injector and see if the misfire moves.