P0303 & P0304 misfire codes ONLY between 2.5k-3.5k RPM on throttle decelerating

I recently bought a 98 LGT with 149k miles with 20k on a rebuilt motor (after timing belt slip). Topic title states my issue. P0303 & P0304 misfire codes ONLY between 2.5k-3.5k RPM while slightly on throttle but decelerating. Car does not feel at all like its misfiring and runs great besides the flashing CEL. Only seems to happen after engine is fully warmed up.

I've have:
- Replaced spark plugs (NGK)
- Replaced plug wires (Denso)
- Insulated plug wires near injector wires
- Replaced upstream o2 sensor (Bosch)
- Replaced crank position sensor (OEM)
- Swapped injectors 1 with 3 and 2 with 4
- Had a shop check TPS
- Checked compression - 180 across all cylinders
- Checked EGR valve and ruled out control solenoid



I'm really at a stalemate as to what to try next. I was hesitant to do coil as when I've had coils go out in the past they either work or don't, not just in certain RPM ranges, and definitely feel like a misfire, while there is no noticeable misfires with the Legacy besides the code. I've heard some other things to try but just in relation to persistent P0303 & P0304 codes, I haven't read anyone that narrowed it down to this RPM and condition range. I'm halfway tempted to try an ECU from another EJ25 like a 99+ LGT.

Can someone please chime in with some insight or things to try next? Any help is much appreciated.

Check your intake manifold for leaks

Checked it today with carb cleaner.. no dice..

Any other thoughts? I might try to find a used coil and heard some weird things about a neutral safety switch for some reason doing the trick. Gotta do some more research.

maybe the coil or the igniter but i would think they would not be speed sensitive. subaru engines use a ''wasted spark'' ignition. when #3 fires, so does #4. the spark going to the other cylinder is wasted since there is no fuel /air mixture under compression. so a 3 / 4 misfire fits wit that.

does your crank sprocket wobble when at idle? one poster on another forum swears that a loose crank bolt and damaged crank key way can cause misfires. the fix being to remove the bolt, pulley and sprocket? clean it all up and reset it correctly. using a washer on the bolt so it will not bottom out and so you can tighten it correctly.

this sounds like a stretch to me but he swears by it, claiming to have fixed several engines with misfires after nothing else worked.

so look at the crank pulley at idle. if it wobbles the bolt is loose. but even if it doesn't wobble it could still be off. i would probably try a used, known good, coil and igniter before removing the crank pulley, it's less work.

ps: i wouldn't think the neutral safety switch would be speed sensitive either.

I did the "dollar bill" test on my exhaust and valves are looking more and more likely... going to check compression tomorrow.

Thanks for the info!

Just tested compression.. 180 psi across all cylinders.

Checked EGR valve, seems ok.
With engine off I pushed the valve up and plugged vacuum line and it stayed in place, which would rule out diaphragm leak. With engine on at idle manually activating the valve causes engine to hesiatate.
On a short drive I waited until warm and I got the flashing CEL, pulled over and clamped the vacuum hose. I was still able to replicate the CEL with EGR disabled so that would rule out the solenoid having it open too long or the valve sticking on release.


@johnegg: Crank pulley does not wobble at idle or on throttle

I might cave in tomorrow and buy a used coil, I have a feeling I will just be throwing money down the drain with that though.

I have these same codes but my CEL is constant and flashes. So I am thinking it is my coil. I just has the HG done so I hope they put everything together correctly. After reading your list of fixable items I am not so confident.

plugs and wires first. what kind of each?

mine had the same problem and a new ignition module fixed it

Originally Posted by dylanwaible

mine had the same problem and a new ignition module fixed it

did your ignition module have corrosion like this

I'm going to go check my ignition module right now, lol.

As far as your neutral safety switch, I have a CEL code for my inhibitor switch on my first gen SS, which is the same code for both the neutral safety switch and the inhibitor switch. I have a 4EAT however. I have a little bug where it feels like there is a misfire or fuel enrichment problem until the car warms up after about three minutes and it goes away. I talked to a very knowledgeable subie tech who I trust and he feels that a neutral safety switch or an inhibitor switch would in theory not cause such a problem. The neutral safety switch is just telling your ECU where your shifter is in space, in his opinion. He gave the caveat that he could be wrong. But I have read a few threads where someone with a 5MT had a similar problem to yours when their neutral safety switch went kaput.

I have a spare inhibitor switch, but since I have to remove the down pipe to switch it out, I haven't replaced it yet, although I now think it will not fix the problem. I am leaning towards a vacuum line leak since the problem has come and gone each time I find and replace a broken vacuum line. I also think that in my case when the engine is cold the ECU tried to either enrich or lean the fuel mixture to compensate for something, giving me the funky misfire feeling. Then as the engine warms up everything goes back to normal.

Can you tell what side it is coming from? My problem is always on the passenger side.

Mine was just a bit more corroded. not much worse than that though. when the CEL was blinking i was literally getting power only from cylinders 1&2 but when the CEL stayed on solid it was fine. Mine also only did it when fully warmed up.

I just checked my ignition coil and it is the inverse of the above pic. Mine looked very good with no corrosion whatsoever.

Originally Posted by dylanwaible

Mine was just a bit more corroded. not much worse than that though. when the CEL was blinking i was literally getting power only from cylinders 1&2 but when the CEL stayed on solid it was fine. Mine also only did it when fully warmed up.

thanks for the reply, sounds like a new coil might be in order. did you replace your spark plug wires also?