I have a 2014 Toyota Corolla with 36k miles and last year it had a bit of rough idle. It only happened right after engine start up and it would only last for about 3 seconds. Engine is at around ~1000 rpm and then it will be rough when it goes down to ~700 rpm but after 3 seconds it will go back to ~1000 rpm and be smooth again.
I didn’t pay that much attention to it and after some basic maintenance/oil change recently the problem went away.
I also changed the brand of fuel I use for that car a few months before the oil change. The old brand of fuel has been used on that car for 5+ years and I thought it was okay.
How do I know if the problem was fixed by the oil change or the new brand of fuel? Was that rough idle the effect of bad gas?
The problem went away, so it’s no big deal now.
But if you need to know if it was the fuel or not, bring your car down to low fuel, and add 1/4 tank of fuel from the old company you used.
Monitor how the car behaves until you’re low fuel again. If you have any issues, immediately fill your gas tank with high quality fuel.
Bad gas can cause issues if the octane rating is lower than what it should be, and could cause detonation. If you add premium (93 or 91), you can counteract the really low octane bad gas with high quality gas, and it should prevent damage.