ChemEng

I started my car today after being parked for over a week. Outside temperature was around -6C so it wasn't very cold. The car started and was idealing rough. Check engine light was flashing and VSC light was on too. I stopped the car after about 5 minutes and hopped on my other car to run some errands. I was surprised because about two years ago (less than 5000 miles ago (I know I don't drive much!)) I replace all spark plugs and ignition coils. Why would I have a misfire if the fuel injectors and spark plugs are almost new. When I got home, I started my RX again and it started fine with no check engine light or VSC light. It sounded normal no rough idealing or shaking. I hooked up my Blue Drive scanner and it read a P0304 code which is a code for misfire in cylinder #4. I drove the car for about 15 minutes and it was fine. Does anyone know what could be the reason for this misfire? And why did it only appear once? When I changed the spark last time, I believe I had a misfire and it didn't go away until I changed them.

Oro

The most likely cause is of course a bad plug or coil. There are bad ones out there, particularly if OE was not used (not sure here). There is a low but predictable failure rate on any electrical part. Thoroughly check out those two before moving on to lower-probability things like an injector, etc. Fortunately, it’s easy to do.

Swap the #4 coil over to #2. Wait and see if it re-occurs and if so, whether it stays at cyl #4, or tracks to #2. that will help you diagnose it. Hopefully you kept a few of the old plugs and coils, they can also be helpful for diagnosis or emergency use. I keep two of each in the trunk of each car. If it appears to be the coil, swap in one of the old ones and forget about it. If the plug is indicated, you can put in an old one for the short term, but then eventually get a new replacement.

On the upside, if it turns into a bigger problem, we all know an outstanding mechanic in Alberta!

ETA: oh, and to answer your “why one time?” Question: parts expand as they heat up. Many small electrical discontinuities show up when cold at first, then go away as the engine warms or the weather changes. They will show up more consistently as they worsen. No absolute guarantee this is at play here, but the pattern is not unusual.

ChemEng

Thanks Oro for the quick response. That's what I'm planning to do... basically wait until the check engine light comes on again and proceed with the test to roll out the ignition coil. I remember that I bought the best plugs and coils RockAuto had to offer last time I replaced them. And yes, I kept the OEM plugs and coils in anticipation for something like this . I remember only one of them went bad which was in the inner bank where you have to remove a gazillion things to reach it. That's why I replaced them all. The previous owner did replace the original OEM ones, so most the ones I have are supposedly still good.

I'm not sure who you were referring to when you said there is an outstanding mechanic here in Alberta. If there is a mechanic from Alberta that is well known in this forum, I would be very interested to know who he is.

Oro

I would go ahead and swap 4 to 2 (or 6, doesn’t matter). That’s the front side of the engine so easy to access. Then if it happens again you have your feedback right away. Coincidentally, I did this a month or six weeks ago on the 10 GS in my sig line - also was #4 that misfired. Has not re-occurred yet. I’ve been meaning to buy a spare pair of coils for it as that car, I DO not have any extra and clearly need to get some.

@lex2k is a long-term, very helpful, and active professional member of the forum. He’s in Alberta but I don’t recall precisely where. That’s the reference I was making.

LeX2K

If the original coils are not cracked or show signs of heat damage I would put them back outside of the one that you say went bad. At least start with the front bank. But before you do that, go with what Oro suggested you don't want to inject another variable just yet. I have yet to find any coils on Rockauto and the like that are as good as OEM. Did you also buy the plugs from Rockauto? How many Kilometers on the car? Be careful driving with the CEL flashing that is the computer telling you to not run the engine until the fault is repaired.

Did you actually replace the fuel injectors?

I'm in Calgary but Alberta is a big place, almost as large as Texas.

ChemEng

Thanks LeX2K for chiming in
I have been busy lately and didn't get a chance to switch the coil packs. I know it shouldn't take 5 minutes... will try to do that this weekend. I seriously doubt the coil pack to have gone bad already after only 5000 miles.
Yes, I bought the spark plugs and coil packs from Rockauto. I got a reputable shop here in Calgary to install them for me. I have around 165,000 miles on the odometer now. CEL hasn't come back on since and the car drives as smoothly as ever for now.

Oro

I honestly wouldn’t be surprised at an aftermarket coil pack going bad. An old rule of electrical components was, “They fail fast, or they don’t fail.” Meaning a fault would usually show up quickly, or the part would last it’s normal life, no middle ground. This is less common these days with better materials and production techniques, but still relevant.

ChemEng

I swapped the coil pack from cyl#4 to cyl#2 this weekend. No signs of wear except for tiny spots of rust on both coil packs (their casing seems to be metal... I don't recall the original ones having metal casing.. could be wrong though). Now we'll just wait for that CEL to come on again.