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GM refueling events

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Found this very interesting, but equally scary article about GM refueling events.


http://www.gm-trucks.com/forums/topic/78169-e-85-refueling-events/page-1 After post #20 it gets off-topic.


#1 thing I learned when changing ethanol % is you need to burn thru a substantial amount of gas before shutting off your vehicle. I typically fill up across the street on my way home from work in EV mode (ICE not running), then park in the garage and plug in the charger.


To refuel correctly:



These are the steps we take to do the initial relearn event:

  • Drain the fuel tank.
  • A complete key cycle needs to occur when the fuel tank is drained to ensure the ECM/PCM recognizes a refueling event.
  • Key ON, engine OFF with drained fuel tank.
  • Key OFF, add at least 11 L (3 gal) of non-blended gasoline to the fuel tank.
  • Perform the Fuel Composition Reset using a scan tool.


Ok time for good news bad news. GM has released revised calibrations for some FlexFuel (E85 compatible) vehicles that will accept the introduction of gasoline/ethanol blended fuels with ANY ethanol concentration that falls between 0 and 85%. What does that mean to you, many of these vehicles have a recall to cover this new calibration and part of that recall is checking the content and adjusting it as necessary, all covered under the recall. To find out if your vehicle is covered contact your dealer with your VIN and current miles on the vehicle. You will still need to go by the refueling guidelines after the reflash it is just so the vehicle will accept fuel between 0-85% where before the reflash it would only recognize fuels between 0-10% or 85-90%. 

Bad news, if it does not fall under the recall and testing shows that the learned and actual content is not withinn spec it will not be covered under warranty. The warranty is to cover manufacturer defects, this is not a defect. There is a section in the manual on how to refuel your vehicle, it isn't GM's fault if you didn't read your manual to know how your vehicle works. The purpose of my original post was to emphasize the importance and to shed some light to the how's and why's of the system.



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Considering I've had to force literally a thousand of these events, I'll clarify what the engine computer is actually looking for. Their 3 gallons and 7 miles is a guideline and only the first is used to calculate when a refill event occurred.


The vehicle looks for a certain change in volume between being on & off for over 60 seconds. When the tank is mostly empty, it's around 1.32 gal (5L) in our 10-16 gallon fuel tank test vehicles. When fuller, it looks for closer to 1.75. Personally, we always add 5 gallons (due to container size) during every refill event.


As for the 7 miles. The value the computer uses is actually based on several other factors to include: volume of fuel consumed, MAF (air flow) within a certain range but above idle and <~75% load, # of times the oxygen sensor sees a lean/rich switch over, plus about a dozen others. For our tuning, we have access to hundreds of tables that all change the compensations for ethanol based on the blend rate and temperatures. I have to hand it to GM, they did a pretty slick job with it. But Ford did the algorithm better. Haven't gotten to test Chryslers.... yet

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Thanks John,


It's nice to have this stuff verified and in writing.


Can I assume the longer you can prolong the "relearn" event, the lower your LT trim will be?


I wonder how Ford does this. They are the only ones I know of that had a flex hybrid. Of course it was just a fleet of test vehicles, but still was official.

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I tried, but I can't find anything online that tells me how to force a refuel event that didn't require the GM Tech 2 tool (~$400).

Edited by TD

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