[torqueless]

Every 30K miles or so ny 2002 wrx gives a P030x code and I have always been able to fix it by replacing the plugs, until now.

This time after replacing the plugs, the car still gives the P0302 code, along with vibration and loss of power. Actually it alternates between this error condition and driving fine at random.

Does the fact that it sometimes drives fine eliminates some possible causes mentioned in the misfire code sticky thread?

I attached photos of plug #2. Does it tell anything useful?

 

[McBill]

Is the misfire always all cylinders or random cylinders or is it always cylinder 2? Your spark plug looks fairly normal.

 

[rqjoe]

I agree that plug looks good. Can't say much about the misfire except to ask if you changed out the wires also? Wires "flop" and even get wet if they have hairline cracks; that "may" explain randomness.......

 

[torqueless]

If I take the car to a subaru mechanic, is it possible they have a better diagnostic tool (perhaps a subaru-specific ODB 2 scanner) to pinpoint the problem, or are they just going to go through the list of possible causes in the misfire sticky thread?

The error is always P0302.

If I leave the spark plug connected to the coil and hanging outside the engine and start the car, would that be a good way to see if there is spark?

 

[rqjoe]

no. plug must be grounded. And if it is a random thing, you will just see it spark and never know when it misses.
Is 0302 for a specific plug? If so, swap wires and see if it changes.

 

[torqueless]

I swapped the #2 and #4 coil, hoping the error would change from P0302 (cylinder 2 misfire) to P0304 (cylinder 4 misfire). Surprisingly, the car runs perfectly fine since then.

I now believe the real problem is poor contact between the coils and the wires. I recall when disconnecting the coils, the connector detaches way too easily after pushing the release tabs. I think the process of swapping coils caused the poor contacts to become good again, at least temporarily.

 

[rqjoe]

Cool.
Yeah, as things get old there are all sorts of new potential problem causes one would never guess with a new car.
Glad you seem to have figured it out.