[CyprusCorner]

4 cylinder engines always rough idle, the idle will get worse as the engine mounts age. That is unrelated to engine knock. Many things can cause engine knock, but since you drive an accent, using regular fuel isn't one of them. Driving up a hill would be an expected scenario for an engine knock to occur. trying googling the causes of engine knock and troubleshoot from there.

Is only your A/C light flickering, or are other lights flickering as well?

Why did you recharge your A/C?

What was the error code when your check engine light started blinking again?

 

[CyprusCorner]

I don't know the error code because it's at the mechanic.
But he said ignition coil 2 is bad, which I agree with.

I had the shop refill the A/C because of the flickering A/C light. I assumed the refrigerant was empty. Now I know all the problems I'm having are likely related to this common ignition coil problem.

Again, anyone have recommendations/experience with aftermarket ignition coils?
At what point should I clean the fuel system?

A/C shouldn't need a refill unless you know that the refrigerant leaked out, the flickering light is not a sign that you're low on refrigerant.

As for fuel system cleaning, you shouldn't need one if you use top tier gas. Top tier gas and premier grade fuel are not the same thing. If you aren't using top tier, then you can use a techron fuel system cleaner or any other generic fuel system cleaner every 5-10K miles. The stuff hyundai sells is the techron fuel system cleaner with a hyundai label and a few extra dollars on the sale.