Gas grade? Gmt 900 2011 5.3

[mymaggiebell]

Does anyone know if the current GMT900 engines can re-adust themselves automatically on the fly, to changes in fuel octane rating - both switching to higher octane or going back to lower octane, without the need for us to pull fuses to the PCM or disconnect the battery to force the PCM to re-learn and reset to the changed octane?

 

This is a big deal on the Camaro forums. They call it "Pulling fuses numbers 5 and 20." Camaros will recognize low octane gas and reset the timing advance tables to a lesser (read- lower performance) version and then you begin suffering lower performance. But- they cannot reset themselves to make use of high octane without first pulling the fuses to reset the timing tables. To fix this you pull the 2 above numbered fuses (underhood) for an hour or so and fill up with highest octane gas, then next start up, the PCM will begin using the Hi-Perf timing tables.

 

I know Ryan or someone will have a better understanding and info than me so maybe you could translate for us Ryan. If we all want to begin using 89 oct mid range gas, will the PCM recognize it without problems? Or is there a truck equivalent to pulling numbers 5 & 20 fuses to force the reset?

i had emailed blackbear, as long while ago with regards to higher octane and even using e85 with a tune. when i did my data logging it was 91 octane. i am now using petro canada (ultra 94), as i was assured the truck would adjust quite nicely!! if that helps answer your question with regards to octane adjustment.

[pendragon]

SAY WHAT! now I feel like a real twit, I've been running my 2010 Denali for 200,000 klms without realizing that I've been starving the engine, I had no idea that the truck had 2 settings depending upon octane ratings, as per my discussion earlier in this thread I will be running my 2013 when I get it next week on straight 91-93 octane fuel from here on!!

[MaverickZ71]

Don't know about the 2 fuel maps, and from what I understand, the owner's manual recommends premium grade fuel for the 6.2L. (The owner's manual is your friend!)

 

There was a very detailed post on this forum a couple of years ago from a GM engineer about how the 'octane sensor' works on the GMT900 and other GM vehicles of that generation. I seem to remember he said the system realizes if you do a significant refueling event (12 gallons or more?), and only then the system will sense the overall octane rating of the fuel in the tank over the next 7 miles (?) or so of driving, and then reset the current calibration for the spark advance, VVT, etc. If you don't drive 7 miles, it just leaves the calibration as it was set previously, even if you went from a previous tank of regular to a full tank of premium. Prior to reading that post, I thought all of the new cars had an instantaneous/continuous type of octane sensor, but it was explained that this would add several hundred dollars to the cost of each vehicle, so the General didn't go that route.

[TJay74]

Maverick, as I posted earlier and I have been into the stock GM calibration several times over the years including the one on my 2010 Sierra, there is only 2 base timing maps the ECM references, one is the High Octane timing map and the other is the Low Octane timing map. They are purely referenced based off of knock or detonation. If the vehicle does not see detonation it uses the High Octane timing map until it drops down into the Low Octane timing map due to recorded events of detonation that exceed a limited threshold.

 

From there using several other modifier maps the ECM can use to modify the timing with advance/retard due to IAT temps, coolant temps and so on.

[Jennabear]

The issue with the alcohol sensors (or in the case of the GMT900s, no alcohol sensors), when one switches from regular gas, to E85, or vice versa, you should always run your vehicle a minimum of 10 miles after fill up (without turning the vehicle off). This allows for the computer to adjust accordingly for the change in fuel.

[Cowpie]

Now all of this would make some sense combined. Since E85 generally has an octane of 105, there would need to be a level of time (say 7-10 miles) that the ECM would need to adjust accordingly. Now add that it senses detonation events also, and adjusts, this all is now a lot clearer than it used to be for me. Great discussion.

[NEFisher11]

Well i ran 89 for a month in my 4.8l and my mileage only went to 15.9 at the end of the month. I didn't change my driving habbits at all so i am not sure it was worth it should i have tried a higher octane then 89?

[joshw1303]

Well i ran 89 for a month in my 4.8l and my mileage only went to 15.9 at the end of the month. I didn't change my driving habbits at all so i am not sure it was worth it should i have tried a higher octane then 89?

Umm ya its worth it, especially for the 5.3l with pos afm.

 

Well i ran 89 for a month in my 4.8l and my mileage only went to 15.9 at the end of the month. I didn't change my driving habbits at all so i am not sure it was worth it should i have tried a higher octane then 89?

Umm ya its worth it, especially for the 5.3l with pos afm.