Tuning Tech. Common Subaru Misfire Causes, Diagnosis, and Repair. Legacy, WRX, STI ej255 ej257

Diagnosing cylinder misfires can be a tricky thing especially when the main cause has recently been changed. Until you've verified something worth your own eyes talking someone else's word for it is how you chase your tail for hours. ALWAYS start with the basics then move out. 9 times out of 10 the cause was something very very simple. Enjoy.

[@PeterHernandez-lg2eh]

Spark plug, coils, injector, compression, leakdown, vacuum check all done now what's left

[@WT4T]

What's the issue you're having?

[@miriamvivo4279]

Po302 cylinder 2 misfire. Able to drive normally.if sudden acceleration get flashing cel then goes steady. Will keep at it. Gonna test crank and camshaft postion sensors for ohm resistence

[@miriamvivo4279]

Your time and expertise is appreciated. I will keep at it

[@WT4T]

Is the car completely stock? Tuned?

[@miriamvivo4279]

Po302 not sparkplug,ignition coil, fuel injector, vacuum leak, compression. . Did leak dowmn. What the heck could it be. 2006 subaru sti????

[@timeout9851]

I know this is an old thread , but unfortunately having misfire issues with 2022 wrx, it’s got a bunch of bolt on , 310whp, tuned.. It will do a standard pull no problem but I can make it misfire every time by accelerating hard to 6k then back of accelerator let revs come down to 4-5k then plant it agin it with miss like hitting rev limit every time. Changed plugs, tuner had it plugged in to lap top we drove around making it misfire, no codes come up, nothing comes up in logs, no fuel issues recorded , no cylinders showing issues, dropped boost, retarded timing, added fuel etc no changes still misfires.. sent the issue to COB waiting on a reply …….

[@WT4T]

What bypass valve (blowoff valve) do you have?

[@timeout9851]

@@WT4T it’s a turbo smart plumb back, if that’s slightly loose, could that cause misfires?

[@WT4T]

@timeout9851 100% which is why I asked. If it were my car I'd start by putting the stock valve back on just for testing. Next question, is the car setup with any "pops and bangs" or "burbles" on decel?

[@timeout9851]

@@WT4T I will change out tomorrow! No it’s not set up for pops, it has a catted Invidia j pipe with Invidia 400 cb so pops a little by design I guess. Tune was a straight forward safe tune and runs smoothly, just the misfire when I accelerate then back off and accelerate hard again.. as said I can do a pull changing through gears without backing off and it won’t misfire…

[@timeout9851]

@@WT4T also the misfire has just started happening a month or so after tune, I fitted the bov myself and at the time was concerned I didn’t tighten enough because it was tight for space and hard to get good purchase on screws on bov

[@briggs1717]

makes me wonder how people get the boot on the coil pack on backwards lol.

[@WT4T]

Second time I've seen that. Both times I was scratching my head.