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P0304 misfire problems. HELP!


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Hi guys, first time poster here, long time backyard wrencher. This is my first subaru and I'm real excited. A 98 legacy GT wagon. I have a p0304 code with a new oem ecu, fresh tune up, new injector (on misfire cyl) new cat, gaskets resealed (head machined) passes compression test and passed intake vaccum test. She is in rough shape but i'm pretty dedicated to bringing her back. if you think you know how whats up please let me know. thanks! 

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Is this 0304 a constant miss, or is it mostly running?  If it is constant I would pull the plug right after a run and see if it is wet with fuel as it should be if spark is the issue.  If it is dry the issue is probably that injector.  If you didn't pay about $100+ for the injector, I would start there, major problem with counterfeit and junk injectors going around.  You could check this by swapping two injectors and see if the problem follows.  Of course you have to be real careful to not roll an o-ring during all of this.  I would also inspect the injector harness for the health of the pins.  Also, and easiest so start here, you can use a stethescope to listen for each injector clicking as it cycles to see if the #4 sounds different from the others.

 

For the spark side of things you can use a timing light on each plug wire to see if the impulse is happening on each of them.  You can use a multi meter to check the resistance of the sections of the coil, but being a wasted spark system if the coil was bad you should get a miss on two cylinders, not just one.  Same with the ignitor driving the coil.  I've seen it where you don't get a connection where the plug wire goes on to the coil, especially if you are using dielectric grease.  What happens usually is that the boot traps some air as it goes on and it then pushes the wire back off the post and no connection.  I carefully use a tiny tool like a jeweler's screw driver or a bit of weed whip line laid along the post while I push the wire on to leave a path for the air to escape.  When the wire is seated then you pull out the tool and Robert is your Father's brother.

 

I think that should get things started and then based on the results we can try some other steps if needed.

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