TL/DR version

Got a P0302 misfire code after finishing some other work (battery replacement). Pulled the #2 plug and saw it was pretty bad. Pulled the #3 and saw it was even worse. To add insult to injury, when I pulled the plug cap off #3, the small "rubber" boot came off and stayed down in the hole. Took a bit to fish it out and tore in up in the process. <sigh>. so I decided to pull the others out and the rubber boot came off #1 and had to fish that out as well and tore in the process. So I ended up buying the kit to include all 4 of the plug "wires" and a set of plugs. Put everything back together and it runs great again. See pics below.

Here's the rest of the story.

My son drove to see a friend for the weekend. She lives about 70 miles away. He gets to the area and stops at a 7-11, car ran fine. He comes back out and the car wouldn't start due to a depleted battery. Gets a jump and it fires up. Goes to his girl friend's. Needs to move the car later and it barely starts. Next morning it won't start. I suggest he get a jump from his girlfriends car but leave the jumpers connected for a few more minutes to charge his battery up a bit before heading back. He drives home with no issue. Throw my scanner on, no codes, live data looks good. My scanner has a battery and alternator test feature. Alt tests fine, battery is "marginal". I put a volt meter on it and test under various loads (head lights, AC, radio, etc.). At full load at idle, it would read just over 12v. Rev to 2000+ and get high 13's, almost 14. Drop back to idle and drops back into the 12s. The battery was replaced about 3 1/2 years ago but there's no sticker or other date indicator about when it was made (it's an AC Delco). Alright, maybe it's time for a new battery. Put one in and now everything reads fine under load. He goes to run a local errand and calls to report that the engine "stutters" a bit when first accelerating from a stop in Drive. It's only for a second and then runs fine. he gets back and I drive around the tract and get the same thing. Just a quick "hiccup" and it's fine but it's repeatable. No CEL. I check for codes and there are none. Double check battery stuff and it's still good. Pull the intake tube off to peek in the TB and it's pretty clean. Clean the MAF, double check a few other connectors. All looks good. Since it only happens once warmed up and only when first taking off, I suggested we push through it and see if it eventually throws a code.

Well the next day it threw the P0302 code. I pull the coil and the plug and it looked really roasted. So I pull them all. When I pulled the coil wire off #3, the little boot on the end of the cap came off and stayed in the hole. I used a long pick to try and lift it out. It did not want to come out. I worked my way around it and it started tearing apart but I managed to get it out. So now I can remove the #3 plug. It was even worse. So I pulled the 2nd coil (on #4), pull the plug and it's just as bad as #2. The little boot on #1 also came off and stayed in the hole. After a bit more effort and swearing, I managed to get all of the pieces out. Pull the plug and it's equally bad.

Now, we've put about 40K on the car since buying it in 2017 from a local dealer (had 114K at the time).. He told me that the previous owner had the car recently serviced. I think not. Anyway, I put the new plugs (did check the gaps!) and new wires with the existing coils. Starts right up, runs smooth, no more "stutter". My son took it for a longer drive and reported it's running OK now.

So here are the plugs:

#1, #2, #3. Note 1 & 3 have almost no electrode left. (these are platinum plugs). I measure the gaps and #3 measured almost .070"!! #2&4 were around .050".


1, 2, 3, and 4. (in order). The ceramic bases look terrible as well.


The boots from #1 and 3#


So how many miles do you think those plugs have on them. I find it odd that #1&3 look the same but different from #2&4. I'm guessing the PO had the issue with the boots sticking and only replaced #2&4. What do you think?

Fortunately, he took it for smog testing before he left last week and it passed. Now that I cleared the codes, it will take a while for the readiness monitors to complete. I could not get it smogged until they do.

 

Nice thread... I am saving it as reference for future posting information about the importance of sooner than what is thought for an ignition tune up on a ECU/PCM control Mazda.

 

Yeah, my bad for not doing more PM on it. My son lived in AZ for a few years (back now) so I had no access to it. Now he uses it every day so I have to deal with stuff as it comes up. Other things like oil changes, trans fluid, brakes, tires, etc., we can plan around.

 

Funny you and I have sons that are not as service savvy working on vehicles.. LOL