I have a weird CEL issue, I always get the code P0304 (Cylinder 4 Misfire). So what I do is re-set it and it is good for a day until it comes back on, when it comes back on the car idles rough and you can tell that it is a misfire, however here is where I am perplexed. So long as I do not reset the CEL the car will eventually after warming up begin to run fine and go back to normal for months and months at a time, but as soon as I decide to reset the CEL shortly thereafter it will go through the process of running rough and misfiring again. Once it finishes the process so long as the CEL is on it will continue to run fine.

 

I feel like my car is haunted, it some how knows when I have reset the CEL light and when I do it will turn it back on again and as long as I leave it alone (i.e. not reset the CEL) it will run fine, or maybe I am going crazy? Any input is appreciated:)

If the CEL is illuminating, and a misfire code is set, then there was a misfire wether you felt it or not.

 

If the CEL is already illuminated, and there is a misfire that you can't feel you will never know, the CEL is already on.

 

Post year and model of your car.

 

Start with the basics. Spark plugs and wires. OE parts.

Sorry forot the basics, I have a 1996, Automatic tranny, wagon. I will replace the wires this weekend and see what happens. Thanks

 

If the CEL is illuminating, and a misfire code is set, then there was a misfire wether you felt it or not.

 

If the CEL is already illuminated, and there is a misfire that you can't feel you will never know, the CEL is already on.

 

Post year and model of your car.

 

Start with the basics. Spark plugs and wires. OE parts.

Outback?

 

Or just Lecagy L series?

 

Milage?

I will replace the wires this weekend and see what happens.

Suggested parts would be the NGK V-power plug for this vehicle, and OEM plug wires.

Yes, definitely go with OEM for plugs and wires. It will be a bit pricey, so you may want to shop around (I recommend http://www.1stsubaruparts.com).

 

In the meantime, you can try swapping a couple of these items around and see if the problem follows. E.G. take one spark plug wire and swap it for the other one, and swap the spark plug too. If the problem moves, you can be pretty darn sure it was one of those two items. To further diagnose, just move one of them or the other.

 

What you don't want to do is just start throwing a bunch of parts at it because it can get (needlessly) expensive.

 

Lots of things can cause misfires, but you want to start with the cheap/easy things first.

 

A good example of mine diagnosing a misfire code ended up being on a Land Rover Discovery Series II, but the concept is still the same. I had a consistent PO305 that would come and go. I tried the wires, plugs, and new coil packs to no avail. I was told it was sticky valves, so I tried the seafoam clensing technique, and that didn't help. I was then told some other valve issue, a dropped cylinder sleeve, or a piston or headgasket problem, so I pulled the heads. Upon removing the head in question, the rocker bolt holding the valvetrain on for this particular cylinder wasn't even tight! I pulled the heads for nothing, but I had no way of knowing what was going on until I got that far into it. I threw a bunch of parts at it before hand without any luck, and ended up finding the real problem (though it was a rather unusual problem).

Sounds like a good old fashioned tune up (OE wires and OE recomended plugs) will fix the problem.

 

 

nipper

So long as I do not reset the CEL the car will eventually after warming up begin to run fine and go back to normal for months and months at a time, but as soon as I decide to reset the CEL shortly thereafter it will go through the process of running rough and misfiring again. Once it finishes the process so long as the CEL is on it will continue to run fine.

 

I suspect the ECU is going through a learning process after the reset and eventually gets things to run smooth, but it takes a number of run cycles to do that.