Thank you for the suggestion. I don’t know anything about how a spring is related to this situation. Please explain.
I asked AI about the different additive choices. Here is what it said. Does anyone agree or disagree with any part of this?
What that means for your three options, all of which work by being added to the fuel tank or oil:
- Sea Foam — Mostly a tank additive when used the “normal” way (poured into gas), so it won’t reach intake valve deposits. It can be effective against carbon if used as an intake/throttle-body treatment (sucked through a vacuum line into the intake tract while the engine runs), which does physically contact the valves. But that’s a service, not something you just pour in the tank.
- Marvel Mystery Oil — A mild upper-cylinder lubricant/fuel additive. Same limitation: added to fuel or oil, it never reaches the back of the intake valves in a GDI engine. Not going to prevent valve carbon buildup.
- Lucas Upper Cylinder Lubricant — Same category, same limitation for this specific problem.
Bottom line: None of these, used as a simple fuel-tank additive, will meaningfully prevent intake valve carbon buildup on a GDI engine, because they don’t reach the valves. If you want to stay ahead of the problem, better options are:
- Use Top Tier detergent gasoline consistently (has stronger, standardized detergent packages).
- Keep up with oil changes and make sure the PCV valve/system is functioning properly, since oil vapor is the main source of the deposits.
- Periodically have a real intake valve/throttle-body cleaning service done (Sea Foam or a similar product applied through the intake, not the tank) every 20–30k miles as preventive maintenance, rather than relying on a bottle poured into the gas tank.
If you do want a fuel-tank additive for general injector/combustion chamber cleanliness, Sea Foam is the most reputable of the three for that purpose — it just won’t be doing anything for the intake valve carbon that caused your misfires.
You’re using a Top Tier gas from Costco, I’d skip any other fuel tank additives and have the valves cleaned as needed. @Tester 's original recommendation (the spray Sea Foam used in intake, as opposed to the liquid added to the gas tank) could be good if you’re comfortable with the procedure.
A weak or broken spring can allow the valve to bounce, which reduces the compression and also allows some of the unburned fuel to escape, resulting in a misfire.
“If you hear hoofbeats in the distance, it’s safe to assume it’s horses and not zebras”
Start with the basics, air, fuel and spark.
- The other cylinders are fine so it’s a safe guess that the air is OK.
- Again, the other cylinders are fine so if it’s a one cylinder fuel problem it’s probably the injector.
- Likewise, one cylinder so it’s probably either the plug, the wires or the coil pack
My experience with Hyundai and most other brands has been that at 10 years and/or 100,000 miles all these parts are showing signs of age or wear so this is just preventative maintenance and if you can DIY it’s cheaper than a breakdown and tow,
No need to look for zebras 
How difficult or expensive is this to diagnose? And if it needs to be fixed, how expensive is that?.
I have a question about this. It’s not a question about the validity of your response, it’s just a question for my understanding of the situation. Question: After the carbon cleaning, the car ran fine for 5 months. Why would the same cylinder have the problem again? Of course there’s a one in four chance that it would be that cylinder randomly. But my guess is that it’s not random. Also, why did I get the misfire, but after I cleared the code the misfire hasn’t come back?
Thank you. I don’t think I originally noticed the word “intake” or realized the significance of it.
That can only be answered by a shop where ever you are. You probably need to find a shop that will drop the diagnostic fee if they get to do the work. Finding out the real problem will help you decide if the vehicle is worth the repair cost or just needs replaced.
It could be cylinder #2 has a higher flow rate of PCV and EGR gases than the other three. And that valve builds more deposits faster . Especially if the engine has high mileage and has started using oil.
Tester
If the other suggestions don’t work, the valve cover would need to be removed and the springs on the number 2 cylinder inspected. A broken spring should be obvious but you can’t tell a weak spring by looking at it. The valve springs are not expensive, easier just to replace and the valve cover replaced and sealed. So it’s mostly labor and you won’t know if it’s fixed until you try.
Intermittent misfire that occurs every 5 months? That might be difficult to demonstrate to your mechanic, in the meantime you could spend a lot of money on additives.
A broken valve spring can cause an intermittent misfire, but will usually misfire routinely, like every day.
Replacing valve springs is a major engine repair that can cost $4,000.
So many questions and so easy an answer
- Start cheap - If a $5 can of Miracle in a Bottle “fixes” the problem great but don’t expect a permanent fix.
- Real Fix - If the $5 fix worked for 5 months it’s a sign that your problem was marginal but it didn’t actually fix anything. Instead consider that it’s a warning that somethings about to fail, because of wear or time, at the worst possible moment.
- Cost - If you’re a DIY you can find the cost at something like Rock Auto or otherwise you have the time to find a reputable independent mechanic.
It’s your choice, either replace the worn out parts now and have another 10 years of problem free driving or save your money to pay for the tow back from your missed job interview or the date with that incredibly cute Suzy. 
Has anybody bothered performing other tests?
Maybe the misfire is due to another problem, such as low compression?
Since the misfire keeps returning on cylinder 2, I’d focus on finding the underlying cause rather than repeatedly cleaning the carbon. I’d have the mechanic check the cylinder 2 plug/coil, injector, compression, and intake/valve deposits. The recent fluid changes and fuel additive are probably unrelated, but I’d mention them to the mechanic.
I agree Repulsive. I would like to know what the #2 looks like when you pull it after the misfire returns. Normally a plug from a well running engine is light tan in color. If your burning oil or running rich it will be black. I do like the idea of swapping the coil with another cylinder. Good luck.