2013 Hyundai Elantra - Engine Misfiring/Bad Idling
2013 Hyundai Elantra Limited, with 164k miles, 1.8 L 4-cyl engine.
I was told by the previous car owner that it was misfiring, and needed replacement spark plugs. Without really checking, I got four new plugs. I pulled out the old plugs, and they were all iridium. All looked fine except 1 which was covered in black carbon build-up. I replaced them anyway.
10 miles later, the car is misfiring again, especially near idle. Especially as the car warms up or idles a while.
It will idle at 900 on cold-start, down to 750rpms, then once it warms up more, start dropping as low as 400rpms, up to 1100 and down and up again, repeatedly, while misfiring. Also misfires (putt-putt effect) from around 750rpms up to ~2,000 rpms until it gets above 2,000 and then seems to smooth out.
The engine seems to shake and the car vibrates pretty strongly, but it comes and goes and doesn't always vibrate.
A final possible factor: The car was sitting for around 6 to 8 weeks, until the owner sold it to me. It had 4 gallons of old gas in it. I filled it with 8 more gallons of new gas.
I have also recently replaced oil with 5W20 synthetic, and done a brake job on all four tires.
Hope this isn't too much information. Thanks in advance for any help. :)