For about the last 5 months I have been running 89 octane gas, just due to cost (I know, I know, not that much difference in cost..) anyway, car has been running great. Earlier this week I filled up with 93 as gas prices have come down (91 is not available in my area). Since then, I have been having some driveability issues that I am attributing to the higher octane. Hard starting, stumbling, etc. Just wondering if the computer is having to "relearn" some things with a higher octane gas and this will eventually sort itself out. And is it not good practice to go back and forth between 89 and 93 octane, i.e. maybe I am "confusing" the car a bit. 94 Z28, LT1, M6. Thanks.

 

It might have just been bad gas. I've always run 93 and have never had trouble with it. Did you happen to by it from a Sheetz station?

And mixing 89 and 93 will bring the octane down a little to something in between the two.

 

Thanks. I didn't mean mixing 89 and 93 in the same tank, I meant running a few tanks of 89 then running 93, back to 89, etc. I have a hard time going over the $4/gallon threshold. If I can get 93 for under $4, I will get it, otherwise it's 89. Yesterday evening I thought it would never start. Never threw a code or anything, just turned over like forever before it finally caught.

 

I know what you mean. It's hard for me to pay $3.99 a gallon for 93, but I push myself through it.

Like I said, may have just been bad gas. Even more likely if the station just had it's tanks filled up. Could try some Stabil or Lucas that has the ethanol treatment in it or some dry gas. What's the ethanol rating where you live?

 

Around 10% I believe. I need to find a place that has ethanol free gas. I have used that website that kists them but those places are usually either closed or not there any longer. I'm guessing the website has old info or only mom and pop stations have this type of gas and the big chains are eating them up.

 

better check the fuel filter to see if theres water in it or if its clogged up. does sound like bad gas. i used reg in mine before the build and it ran better than with super.

 

I'm jumping on the bad batch of gas bandwagon too. And no, using higher octane is not going to cause runnability issues. Neither is going back and forth with different grades, except for having slightly diminished performance when using lower octane, as the engine's knock sensor will kick in and retard the timing.

 

yup, sounds like stale gas, or water. Try changing the fuel filter and on the next fill up use branded gas, and maybe a treatment of Lucas. I have been using 89 octane because of the price of 93. and my engine doesnt act any differently, most likely because the anti knock sensors compensate for it. I use 93 when i can. when was your fuel filter changed. maybe you fuel pressure is low too, but that wouldnt cause the sudden change.

 

You beat me to it

octane is the ability to resist detonation. The knock sensor is there to retard timing if it detects "pinging". You do lose performance by saving $2 per week running 89. A couple of things to ponder; Ford displays on their web site the power ratings for the Coyote when using cheap gas vs good gas. Go have a look at the difference. (18 HP 2013 Ford Mustang | View Full Specification Library | Ford.com )

Also, an engine running with retarded timing tends to run hotter. Today's modern cooling systems adapt for that because who (vehicle mfr) wants to tell a consumer that they must use good gas vs the cheap stuff?

 

Whew! I thought I was imagining things when I went to regular because of price, and the car ran fine!
Cannot lug the car as much, but in MD the price diff is 30-40/gal for Supreme, and I am a working dad.. that is $5-6 a tank, and times ARE that tough.