Before anything, I have looked around, but to no avail, I couldn't find anything that actually helped.

For a while, I have had misfires. It was very few back in the day, but as time went on, they have gotten worse and worse. So bad, that it was multiple cylinders misfiring. Dealership said I needed new valve covers, that they were leaking. So I replaced driver's side, and it went back to being pretty decent (not perfect). After a while, the misfires/sputtering came back kind of bad, so I took it back, and they narrowed it down to only cylinder 1 misfiring, and told me the passenger valve cover was also leaking, and that needed to be replaced, and quite possibly the ignition coil as well.

I decided to switch out the ignition coil on my own, went to an auto parts store, got an OEM replacement for 90 bucks or whatever, and switched it out. Car seemed fine for about a day or two, then it was back to messing up.

Weird thing is, when I turn my A/C off, the sputtering gets less, and the car doesn't seem to shake as much. I have no idea if this is related, or what's going on.

The picture included is a hose I disconnected to get the ignition coil off. I thought the first time I did it, it didn't have any oil around it, but I recently took my coil out to see if there was oil down in there near the spark plug, and I didn't really see any. Is there supposed to be oil around/in this hose? I have no idea what it's for :\

Thank you for your help. Would love to get my Lady back up and running on the streets

Edit: 2004 Base Model 350z, right at 105,000 miles on it. Just changed out spark plugs within last 3-5k miles.

 

Well, I hate to generalize something like this and guess but your problems remind me of my misfires this year. Like everyone I have leaking valve covers but never replaced them and don't plan on it. The process of finding a misfire can be endless and I have 6 full notebook pages over 2-3 months before I fixed everything and is way too much to try and share. The amount of things I googled and printed out on misfire diagnosis alone is probably a few dozen pages, pertaining to checking live OBD readings for O2 sensors, taking vacuum readings, etc, etc. There is a lot of helpful info out there and it takes TIME to go through everything until you get it.

As mine got worse it became a little easier to pinpoint but not by much. I had random misfires then once in awhile I had a specific cylinder. Then I would swap ignition coils and document everything then see if the same cylinder misfired or the problem went to the next cylinder. That way you can tell if it is the coil pack that is the problem.
What I ended up doing is replacing one coil pack and two plug wire boots. Since you have had oil in there and some successful work done already, the boots are cheap. If you want to take a small gamble replace all 6 boots. They are easy to do on your own.

The two boots that I replaced were so oil soaked for so many years the oil kept bleeding out of them on my work bench for several weeks after I removed them. They were shot!

Oh, by the way if you didn't know it is very important to make sure you don't drive the car too hard or too long if it is misfiring because you can overheat your catalytic convertors easily. I could always smell mine getting warm before the check engine light would flash to warn you that they were too hot. If the light flashes pull over and shut the car off. This can happen even if the sputtering is barely detectable to you so be careful. You don't want that added expense. For some reason for me everytime I had a misfire that bad, turning the car off and restarting it allowed it to run perfect for awhile anyway. Don't know why that seemed to make the misfire go away for a day or two or even a week at random.

Good luck. It will be a pain. If you can afford it try the boots and you may get lucky and not have to worry about monitoring your car all of the time while you are driving it.

I also seem to remember one of the coil pack connectors didn't seat all of the way and found it much later. I have to go back to my notes and see if it was causing any problems. Connecting it properly didn't solve everything but I think it did change some of my computer readings. Point is since you replaced plugs go over all connections 10 times. I even had someone else double check mine and we never saw it. I think it didn't click on all the way and was kind of loose? Check and double check...

 

Where you took the hose off is just your PCV valve. It could be causing misfires also if it is badly worn out from what I read. There is a spring inside. It's only $15. I replaced mine early in my diagnosis. I had one from NAPA that broke off the second time I took the hose off. The Nissan one I got SEEMS tougher.
It probably won't change anything for you but it could be related. Look into changing it. Cheap enough to try and check off the list of possible causes.

 

The thing that stands out the most to me is that you had the oil leak fixed but nobody knows the condition of your coil packs and rubber boots that have been sitting in oil and firing in a bad environment for who knows how long. It makes the most sense to bet on the boots next if you want to guess.

 

The valve cover that I replaced was driver side, and I believe they got the misfires taken care of on that side. Now the one that is supposedly leaking is passenger side, so they don't know the condition of those at all.

I had plans to just replace both the valve cover, and all 6 coil packs, since they will already have a large portion of the work done already when doing the valve cover. Figured I'd just tackle both things at once.

I'll make sure to check on the blinking Check Engine light, big thanks for that!

I'm taking the Z to a local shop that specializes in Z's on Monday. The owner/son raced them back in the day, and still does a lot of work on them. Figured they'd be better on my wallet than a dealership since I get a discount from my local Z club membership.