tut_tutZOOM

About three weeks ago my wife was coming to a stop in the Pro(engine warm) when the car started shaking excessively near idle. It continued this shaking for the remainder of the red light and stopped after she began accelerating. The CEL was thrown due to this. I tried several times to replicate the symptoms and get it to do the same for me and it would not. I wish I had immediately checked the code, but did not. A couple of days later, the same thing happened to my wife again, but this time the car continued to misfire on accelerating (pedal had do be pushed very hard for any acceleration). Several days go by with no further incidents, and I find the same misfire condition myself while driving my poor Pro (violent shaking at stop, very poor acceleration).

I knew this was not good, so took her in to AutoZone the first chance I had for a free code-reading. It was throwing the following code:
P0300 Random/Multiple Cylinder Misfire Detected
My plugs have about 30K on them. I purchased new plugs at this time from AutoZone, checked the plug gaps, installed them, and reset the engine computer (battery disconnected, hold down brake).

Idle immediately feels much smoother, and the check-engine light stays off. All is well for 2-3 days when the CEL comes on again for my wife. No shaking this time or poor acceleration, just the steady light. I returned immediately to AutoZone to get the code reread.
P0300 Random/Multiple Cylinder Misfire Detected

So I'm thinking maybe I didn't completely fix the misfire problem so decide to take a couple more cheap shots at it. I buy a new PCV valve and a bottle of fuel injector cleaner which I run through the gas. I want to test the coil packs rather than replace them outright, which many dealerships seem to do. I assume there are standard resistance values across the leads which I could test with my multimeter. If anyone knows these values or could find them from a good pack, I would appreciate the help. I would also consider cleaning the MAF sensor and EGR valve (since they seem to be popular causes of misfires for many on this site), but wouldnt I get a code for these if they were having problems?

GNO

I've had problems with the connectors on relatively new spark plug wires give me problems. There've been a few threads on people having problems with Sparkco wires.

Since the CEL did not narrow down to a specific cylinder, it may be a coil, since each coil takes care to two cylinders. But even then, the ECM should've been able to narrow it down to just two cylinders. Which leads me to. . . valvetrain. If your valve timing is off slightly, then random misfires may occur. 60k is a bit early, but not too early. I've heard many recommendations to change (or at least check) timing belts at 60k.

tut_tutZOOM

The coil packs perplex me. If a misfire went back and forth between the two cylinders of a coil pack would it look random to the ECM or would those two cylinders individually throw the CEL? Or perhaps both packs began failing at roughly the same time and all 4 cylinders are missing due to such failure. I'm not sure exactly how the coil pack does it's job, either. I don't think a coil pack problem would necessarily mean that a particular cylinder (or two) could NEVER fire, but I very well may be wrong.

Timing belt scares me and you very well could be right. I had been pushing the thought to the back of my mind for a while, but thanks for making it resurface. Valve timing seems a likely culprit especially since no other sensor would pick up the failure aside from the misfires, am I right?

Thanks again, GNO, for the help!

GNO

You're Welcome.

Sad thing is, despite the negative threads on Sparkco wires, I'm going to order a set soon. I had a set of Magnecor's on my other car and they crapped out after the first plug change. Between that bad experience and the higher cost, Magnecor is out of the question for me.

I'm at 75k miles so I'll do the timing belt around the same time. Since you have to remove the plugs for a timing belt change, I'll do it then to keep me from disturbing the plug wires shortly after an install. But since I'll be doing the timing belt, I might as well do a cam swap right? I'm sick like that. 7k into my other drivetrain so far and now I'm thinking of mod'n the P5 and my truck. I need serious help.

Anyway, back on topic. . . another thing to check out would be your fuel pressure. If it were a particular injector, again only on cylinder would be affected. If there's another problem with the fuel system. . . random misfire. No fuel no fire. I doubt that's the problem b/c the engine would chug moreso than an occasional misfire. It's still worth looking into, though.

To test your coils, you can use an inline spark tester. Last time I saw one, it only ran for about $5.

Damn, I just re-read your original post and it sounds more like a possible cat or O2 sensor problem. Cleaning the MAF and EGR wouldn't be a bad idea as well. Low cost and easy to do yourself. Does the aftermarket intake have an oil-impregnated air filter? Oftentimes, oil from the filter gets on the MAF, which will skew the readings.

How many miles on the car? O2 sensor manufacturers recommend changing every 50k miles. Cats usually last over 100k, but should last at least 50k.

Sorry to bring up demons and expensive (relatively) repairs. But I sincerely hope you isolate the problem. That's probably why everyone else avoided your topic. Too many variables - hard to isolate. On the other hand, a lot of what I've mentioned should be part of routine maintenance, something too many people neglect.

tut_tutZOOM

I'm at 60,000 miles. My CAI does have a K&N oil-impregnated filter so I cleaned the MAF this afternoon.

I'm going to look for one of those inline spark testers that you mention so I can test the coil packs. I think I'll put the original plug wires on too, just to rule that out.

Thanks for taking the time and thinking through my problem. I do realize that it could be any of a million different things, but it helps to have another mind processing all the symptoms to catch things I'd inevitably miss. Timing belt is towards the bottom of the list of things I want to do, and at 60,000 miles I would hope it's still good. Guess I can't rule it out yet though as the problem.

ps- Good luck on the cam swap! Sounds like fun...

tut_tutZOOM

Well, those who have posted problems about carbon build-up at the EGR valve and that region were right-on-the-money with my problem. I struggled with getting at the EGR myself, and since the clock was ticking and I needed the car for a long-distance Thanksgiving trip I took the car into my local dealer over the weekend.

They immediately assumed clogged intake manifold so did it for just over 1 C-note. I really wish I had been able to get in there and do it myself, but just didn't have the tools or the clearance to succeed. Well, I'm poorer, but no more CEL for me and I'm glad they fixed it very quickly and without any extra add-on fees/parts.

Thanks again for the help, GNO, and I hope that those with "random/multiple cylinder misfire" codes check for carbon buildup FIRST. The tech at Mazda told me they've been seeing lots of problems with this.

GNO

Nice to hear the problem is solved.

Since they made that immediate assumption, there may be a TSB out. I guess I should've suggested you look there first. Hind sight is 20/20.