Hey everyone. Have an interesting puzzle here.

 

EJ25 in a '99 Outback. Its been running great, but a week or so ago the check engine light came on, and the codes said P0303 and P0304 (cylinder 3 and 4 misfire). My wife said the car lugged a bit at low RPMs right when it happened, but it was running fine after that as well as when I was checking it later that day.

 

Since the two codes came at the same time, I thought it might be the coil at fault. I didn't know how old the plugs were, so I figured it was time for a tuneup and gave it a new coil, new wires, and four new NGK plugs. (The old plugs looked fine.)

 

It ran well after that, just as well as before. But a day later the CEL came on again, this time for misfire codes for the OTHER pair of cylinders- P0301 and P0302 for cylinders 1 and 2. I was along when it happened, and the engine continued running seemingly normally. We monitored it for a few days, and it still idles smoothly, revs, and accelerates under load completely normally. The only observable abnormal thing it did is kick up to a fast idle a few times while we were stopped in traffic waiting for a train to pass. (AC/defrost was not on.)

 

I've done some searching, and found quite a number of things that could cause misfires... But the fact that my misfires were always in pairs, and that the pair moved to the other two cylinders after the coil/wire/plug change, has me stumped.

 

Any thoughts on what I might check next?

 

Thanks

Those pair's share one side of the wasted spark coil and one side of the ignitor. Otherwise they have nothing in common. So your potential problems are:

 

Coil

Ignitor

ECU

Wiring

 

GD