Higher Octane Fuel Hurts Transmission?

[caddyshackgolf18]

For the past 2 fuel runs I have used 89 octane instead of the 87 that I normally use in my 17 5.3 Silverado 6 speed auto. The shifting of the transmission has gone down the toilet the last 2 fillups. I am almost out of the second fill and going to go back to 87. I normally drive pretty smoothly and know that these have a learning ability to the transmission. I've had some rough shifting that has occurred off and on but has gone away as I've calmed back on my driving habits. However, this feels different. It feels like the shifts are just off. They aren't well timed. When flooring it in first to second it just doesn't shift in time. It is a slow shift into 2nd and you feel the lag before it catches the next gear. The truck does feel underpowered. 

 

I know that higher octane fuel is better for higher compression engines because they combust later. I am wondering if the higher octane fuel is killing the power of my engine and throwing off the shift points? The only mod I have is the Airaid tube (still have regular filter). I don't hear any vacuum leaks coming from the engine. 

 

Any thoughts? I was planning on unplugging battery after I fill up again with 87 to see if it gets back on track.

[Black02Silverado]

Higher octane has nothing to do with the way your transmission shifts.  I would just run 89 if were me.  Talking to several tuners, they state that these engines benefit from the higher octane and wouldn't go lower than 89.  Makes sense with the higher compression ratio that we have over the old 5.3's.

 

If the shifting gets worse then you have other issues, basically something wrong with the transmission.

[Boardmember]

I’ve run various tier 1 octane grades for multiple successive tanks.   And I had an 8 speed that had indigestion the entire time I owned it.

 

Zero correlation. 

[Grumpy Bear]

I seriously don't understand the thought process...

[Rob Okray]

Causation does not indicate correlation.

[caddyshackgolf18]

Thanks for the replies. Unless I got a bad tank of gas I’m going to pull the battery off and see if that fixes it. If so it’s a computer thing. If not I’ll take it in to get looked at. 

 

[Rob Okray]

I had ice cream the night before I was diagnosed with a brain tumor. Therefore, ice-cream causes brain tumors. While a true story, not a true scientific conclusion.

[swathdiver]

Sorry to hear of your plight Rob. 

 

Low octane fuels generally speaking cost more to use.  They deliver lower fuel economy and lacking the additives that benefit the fuel system and valvetrain, do not contribute to an engine's longevity.  You want to see a difference in performance, engine idle, mileage, etc. run 91/93 and never look back.  And if you have a yellow gas cap, don't deprive your motor the pure enjoyment of running E85.  If cheap enough, it costs less per mile to run than gas.

[Mileguru]

1 hour ago, Rob Okray said:

Causation does not indicate correlation.

Other way around.

 

55 minutes ago, Rob Okray said:

I had ice cream the night before I was diagnosed with a brain tumor. Therefore, ice-cream causes brain tumors. While a true story, not a true scientific conclusion.

Correct, correlation does not imply causation.

 

While we're on the topic of high school statistics class, do you think there is a lurking variable causing the poorer shifts and it's not actually attributable to the higher octane?...or maybe this is the placebo effect (in reverse)?

 

To be serious though, were any other mods done lately? Was the 87 and 89 gas from the same gas station? Significant ambient temperature difference lately? Warm up time the same before analyzing the shifts?

[Rob Okray]

1 hour ago, Mileguru said:

Other way around.

 

Correct, correlation does not imply causation.

 

While we're on the topic of high school statistics class, do you think there is a lurking variable causing the poorer shifts and it's not actually attributable to the higher octane?...or maybe this is the placebo effect (in reverse)?

 

To be serious though, were any other mods done lately? Was the 87 and 89 gas from the same gas station? Significant ambient temperature difference lately? Warm up time the same before analyzing the shifts?

I blame the tumor. Poor shifts. Going to blame that on the confuser, or the sensors giving bad info to the confuser.

[Rob Okray]

I find it hard to fathom that any of the newest gen trucks have enough abuse for sticking shuttle valves or out of adjustment bands etc.

[Rob Okray]

Even my 07 with 108k and my 91 with 230k were smooth as glass with shifting. Both auto and both on the original trans.

[Rob Okray]

Heck even the 79 was good. I know that one had no confuser.

[HondaHawkGT]

50 minutes ago, Rob Okray said:

I find it hard to fathom that any of the newest gen trucks have enough abuse for sticking shuttle valves or out of adjustment bands etc.

Thankfully there are no bands in the 6 or 8 speed transmissions. 

[Rob Okray]

12 hours ago, HondaHawkGT said:

Thankfully there are no bands in the 6 or 8 speed transmissions. 

There ya go. Bob's yer uncle.