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2007 Mazda 3 - code P0301 - incredibly rough idle and acceleration

We recently bought a used Mazda 3, ~85k miles currently. Before yesterday I hadn’t noticed any issues. Out of nowhere, while giving it some gas, the engine starts stuttering and/or hesitating. Letting up on the gas to just cruise at a constant speed didn’t seem to raise any issues. At idle, stuttering/sputtering/hesitating exists. These symptoms seem to only last until the car is fully warmed up, at which point they disappear completely! This happened yesterday, and I have only driven it twice. Once was to a local parts store where they diagnose check-engine-light codes. They came back with P0301 - Cylinder 1 misfire. Among other things, listed possibilities included spark plugs so I decided to pop mine out and take a look. Opening up cylinder 1’s plug, when I took out the ignition coil (which sits directly atop each plug), the thing leaked what I’m assuming is oil as I pulled it out. Just a drop, but that doesn’t strike me as normal behavior. The bottom-most end of the coil assembly (the black plastic/rubber thing that you stick on the plug) was white & crusty. I took out another one to compare, and 1) No liquid drippings, and 2) It looked squeaky clean.

So at this point my best guess is that I need to replace that coil. But I’m no mechanic, and really have no experience in this realm of spark plugs and ignition coils, and am searching for a second opinion on whether the things I’ve mentioned all point to that faulty coil.

Yes - you should replace that coil. This will most likely put the car back to running well.

But them take the old coil and the car to a shop and ask them if they might be able to figure out why this happened - unless you can figure that out. I can’t tell exactly what you’re describing about the condition of the coil. But it sounds like it is a symptom of yet some other problem.

Replace the coil AND the plug on cylinder 1. See if it runs OK. Then I’d replace the other 3 plugs and examine the ignition coils. If any of them are “crusty” as was #1 coil replace it too.

Then you need to figure out how the contaimination got into the cylinder head. Perhaps the coil on #1 had a bad seal, or wasn’t installed properly. If there is oil and contaiminants coming from the base near where the plug screws in then there may be a need for a more significant repair. Looking at the old coil can give you evidence if the contaiminant came from the top of the coil and dripped down, or from the bottom coming up from the old plug.

Alright - initially thought that the oil was coming out of the coil itself. I took out the spark plug (which otherwise looks “OK”), and there is some oil around the threads. Makes me think about replacing the valve cover gasket, though I would assume that there’d be oil in the other spark plug tubes as well and I don’t see any. Are there gaskets around the individual spark plug tubes? Is there a proper name for them? I can’t seem to find any reference to them on the usual auto parts websites.

Compared to pulling the head, a new spark plug isn’t too expensive. The old plug may have had antisieze compound on it, or perhaps wasn’t torqued in tight enough. For now just replace the plug(s) and coil(s) that are affected and check them in 20K miles. If there is a problem, in time you’ll know. Don’t go looking for trouble. If there is a real problem in time you’ll know, trouble will find you.

the white crusty stuff it a product high voltage arching. most likely caused by carbon tracks in the spark plug boot. the carbon tracks could by caused by condensation in the boot, possibly from a poor at the spark plug/boot/coil pack. if the spark plugs are installed from a top of the valve cover, the oil could be seeping through the valve cover spark plug gasket.

Let’s assume for a minute that a small amount of oil is leaking in from somewhere, either a leaky valve cover gasket or something along those lines. What sort of damage can occur, other than destroying the occasional spark plug and/or coil? Replacing the valve cover gasket doesn’t look like it would be too difficult a job, and the gasket itself isn’t all that expensive.

It’s quite common to have an oil leak from the valve cover gasket set (tube seal) on only one cylinder. Just change the valve cover gasket set.

I would strongly advise curing the valve cover gasket leak because you do not want to get into the habit of running an engine when it’s performing very poorly. It’s not just a performance and fuel mileage issue; it’s also a matter of this being rough on the catalytic converter, O2 sensors, etc.

OK - replaced the coil, plugs, and gaskets. Car seems to be acting normal. Of course, after something like this happening, every little “noise” I hear makes my heart skip a beat, but I think things are good.

The Check-Engine-Light is still on, solid. It’s been about a day since I replaced the parts. Should this light be off by now? Does it do this on its own, or do I need to take it into a shop and have them take care of it? I’d do the “unplug the battery” thing, but I’m afraid of A) The stereo requiring a code to activate. B) The keyless entry to unprogram itself. C) Something else funky.

Don’t disconnect the battery unless you know for certain that it won’t make a mess. These days it can make big messes (as you seem to be aware).

If everything is straight, the computer will turn the light off eventually - it normally takes a few drive cycles. Somewhere deep in the technical info on the vehicle you could probably find the exact conditions that will turn it on/off for different codes.

But what I would do is go to one of the auto parts stores. Have them pull the codes - to make sure you don’t have any new or different one - and ask the person using the reader to clear the codes once you know what they are. That’s 2 birds w/ one stone. (Or just keep driving).