I use premium gas. When I start my vehicle, it is fine, but after about 5 minutes of driving it is just not running right, it goes from smooth to rough. The misfire diagnostic came from a code reader a friend of mine owns. I had a complete oil change, a full service done only a couple of days ago, so all is good on that front. Puzzled.
My car has 144000 miles.
My car has an automatic transmission.
Misfires are caused by lack of fuel or the ignition system isn't providing the needed spark. At least these are the most common single cylinder failures. It is possible, all be it remotely, that you have a compression problem, but this would reveal itself when cold as well. Compression problems don't come and go.
The good news is you know which cylinder the problem is at. My first question to you is, how many miles ago were your spark plugs replaced? Most cars don't go beyond 60,000 miles. Some do, but this only means you will experience a failure with a spark plug instead of replacing them before they fail. Without knowing what was done with your full service, this very's quite a bit based on many factors, I cannot say for certain what to do next. If your ignition system was serviced along with your full service, I would take the car back to who did the work. This will likely fall under a warranty issue with them.
Injectors can fail and clog. If you have an injector problem in cylinder number eight, it can be cleaned with the appropriate injection cleaning machine. The machines work much better than dumping something into the fuel tank. The injector is an electric component that under goes much stress. It may have electrically failed.
A thorough diagnostic test should be performed by a qualified technician. I recommend the following service from YourMechanic; engine is misfiring inspection which can be performed by an expert at your home or office.
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