P0300 Random Cylinder Misfire
 

Hey guys. Working on my in-laws 2002 Protege. It is chugging heavily at idle, nearly dying, but never actually dies. This only happens at idle. Driving at speed seems to have no issues. It also only happens after the engine is slightly warm, (1-2 minutes).

Here's what I've done so far:
CEL gives a P0300 (random cylinder misfire).
Replaced new spark plugs - no change, still chugging.
Replaced ignition coils - no change, still chugging.

At this point, I cleared the CEL, drove it around for a bit (but not long enough for the CEL to come back on). Pulled codes again, this time a "pending" code came through, P0301 (cylinder 1 misfiring).

Does pending mean that it hasn't had enough time to test it thoroughly so it only has noticed cylinder 1 misfiring right now, and it may soon register that others are misfiring as well? Or is this a good indication that the ignition coils (which have built in plug boots/wires on them) fixed the misfiring on 2 of the other cylinders and now it really is only cylinder 1?

So to clear it up a bit, see the picture (not the actual car in question). You can see that cylinders 1 and 3 have just the cables going on the spark plugs. Cylinders 2 and 4 have the ignition coils with spark plug boots right on them. So effectively, the spark plug boots have been changed on cylinders 2 and 4, but not 1 and 3.
http://carphotos.cardomain.com/ride_...0006_large.jpg

Do you think there's a good chance that putting new cables on 1 and 3 will clear it? I know, it's a cheap, easy fix, but they live far away, and I don't get over there very often, so I'm trying to troubleshoot as much as possible so I can go over there ready to fix it.

Any other ideas about this problem?

Yes and yes.
Pending means the computer is running a tally and as soon as the threshold value is met the light comes on.
I always replace the plugs and wires together.
When you replaced the plugs did you see any oil on the base of the plug? I consider the hex where the wrench engages the base. That would mean the valve cover gasket set is leaking and may cause a misfire.
How did that plug look in general?

It is 10 years old so there may be vacuum leaks starting. All rubber parts are suspect. But my guess is that you are on the right track with the wires.

When was the last time the air filter was replaced?

Have you tried cleaning the MAF?

Misfires can also be caused by poor exhaust flow.
Good diagnosis video:

I have a 1999 mazda protege with 200k miles. I have done new plugs, wires, coil packs but still have cyl 1 misfire. I also have swapped injectors from cyl 1 to 4 to see if the problem folllowed it did not.

It has new valve cover gasket. no evidence of oil fouling on the plug. and well I am at a loss. Anyone have any good ideas?

gallan: How about a compression test?

I really hate to say it, but I already gave you real good advice. I had a misfire on cylinder 1 on my mazda 6. After changing plugs, ign coil, injector and fuel pump I still got a misfire on cylinder 1.
It was poor exhaust flow. I re-designed the exhaust and had a new cat installed.