Switching from higher to lower octane

[Skippy21]

I was using a Brenspeed 93 octane tune (great tune) for about two years straight. I moved to a city with heavy traffic my mpg went down to 15 and I decided to switch to 87 octane to save money since I was just sitting in traffic most of the time anyways. I used the "stock" SCT XCal 3 87 octane Fuel Economy tune. After driving less than 30 miles the check engine light came on. My vehicle is 2008 GT with about 76, 000 miles (the vehicle is very well mechanically maintained).

I’m going to go home tonight and teach myself how to use the XCal to read the code, but has anyone had similar experiences? Is it “bad” to switch from higher to lower octane gas even when you use a tuner? I understand there is the chance that it is a coincidence that I changed the gas and the light came on but I figure with all the regular maintenance I do (myself most of the time) that this is unlikely.

Thanks all for your help and thoughts.

 

[jpplaw]

Are you running a CAI? If so, is the "stock" SCT tune set for it? If it isn't that will cause a CEL. The MAF will be out of whack.

Any other mods?

 

[DarkFireGT]

I agree with the above. If you have any mods, and the canned SCT tune is set up for stock, then that's why you have the light. Either way, if I were you, I'd still go and order a 87 or 89 tune from Brenspeed. You'll get better mileage than the canned tune, plus more power and better throttle response. The canned tunes are terrible in my experience.

 

[Skippy21]

Roger that. I have magnaflow x-pipe, pypes mid-muffler cat back exhaust, Steeda 90mm CAI and UDP and a few other upgrades not related to a tune. After seeing the code that came up (P0420) I agree it must be the tune file. The 93 tune was running perfectly fine. My mileage is still at 15 mpg with apprx 65 miles driven but its still cheaper than 93 at 15 mpg. So I guess I'll hit up Brenspeed for another tune rather do that than see if replacing O2 sensors fix it..... Thanks for the help!

 

[DarkFireGT]

Yeah you definitely can't get by with the canned SCT tune for all of that.

 

[jpplaw]

yep, you definitely need the new tune. It's needed because of the steeda CAI.

 

[Sparty92]

Why such crappy gas mileage with a 93 tune? I don't have a CAI/Tune or anything other than mufflers. I'm hoping my MPG won't go down like that.

LOL

 

[mustangdave]

Why such crappy gas mileage with a 93 tune? I don't have a CAI/Tune or anything other than mufflers. I'm hoping my MPG won't go down like that.

LOL

well, he said he was sitting in city traffic, theref ore the reduced mileage. If you sit in a traffic jam at a dead stop, you are getting 0 m.p.g.

 

[DarkFireGT]

Before I put the the charge motion control valve delete plates, I was getting about 23 city (hitting red lights every 6-8 blocks but no sitting in traffic really). As my car sits right now, I average 19 in the city, 25-27 on the interstate (75mph) and... over 32 on the highway (55)! I run a 93 tune from Bamachips.

I definitely do see a big decrease in construction areas where it's stop and go, so I can see why you're having trouble. Honestly, if I had an 89 tune, I'd probably be running that now. I never go to the track anymore and it's not enough difference to notice on the street. I just have been too lazy to order a new tune.

 

[GOLDENPONY]

I am running a Brenspeed 87 octane performance tune. I have a K&N Aircharger CAI and Corsa exhaust and the car runs great. I used to run a Brenspeed 93 octane tune and don't see a whole lot of difference in the 87 octane tune, except my gas bill is lower.