Many you just solved it. The knock sensor is sensing. The ECM is rolling back the spark to stop the knock. Low octane is made worse by possible carbon accumulation. Put a bottle of fuel injector cleaner and "Top Tier" 93 in it, and take it for a nice long drive.

There's some fix-it-in-can stuff that's spoda help burn off carbod deposts but IDK about that.

 

Carbon build up while routinely using 91? The additives in 91 are pretty much the same as canned stuff.
I know some people think "Top Tier" means something more than a licensed trademark. I am skeptical, II have seen fuel trucks fill up at the terminal racks. toptiergas.com

 

The detergency has nothing to do with octane rating. If your engine can run on regular, higher octain alone will do absolutely nothing to keep your engine cleaner. (Although, some brands will happily let you THINK there's a difference other than octain, there typically isn't.)

The Feds have standards for detergent properties in fuel, in order to keep engines running better, in order to make the air cleaner. Top Tier thinks the standards aren't good enough and car makers agree. Top Tier promises better detergency than the Federal minimum.

I don't know know for sure how consistently adherent participating brands are to the Top Tier standards. I also expect that the bargain stations sometimes end up selling fuel that meets Top Tier standards.I do feel both of our dailies run better since we switched to Top Tier.

 

I guess the tanker driver might be able to control the additive level when they load. The gasoline all comes from the same pipeline, and there is a tank full of additive and a tank full of alcohol at the farm.
Different grades and products are separated in the pipeline but, any additives are added at the tank farm loading rack.

 

You are correct Donbrew, I drove tankers for many years full time and part time for all of the "Branded" oil company's.Driver can not control additives while loading, all computer controlled. There are MANY additive tanks at each loading rack. :)