isen't there some sort of relearn procedure that most people neglect to do when switching between e85 and regular? i think i read that on this forum some where
I am going to start a little thread on E-85 compatible vehicle refueling and hope on of the admin's will pinn it. The refuel info should be in your owners manual but I'm going to explain why. Improper fueling events cause hard starts, missfires, rich or lean codes and in extreme conditions can cause a no start. As a technician I have seen a bunch of problems created by improper fueling. If a tech hasn't been burned by it a few times checking your content may be last on his list creating comebacks and unhappy customers. This applies to all GM E-85 compatible vehicles built 2005 to present and a few built before that, they all use the same refueling procedure.
The first GM vehicles that were E-85 compatible used a fuel content sensor that measured fuel content as it was being used. This was a very efficent way to measure content but the sensor also needed to be a computer to do some major calculating on the fly. As a result they were about $400 a piece. GM decided to go the way of most other manufacturers to cut costs and do away with the sensor and go to a learn algorithm. It is very efficent as long as refueling is done properly. I wish we would make salespeople go over the procedure to head problems off at the pass but that's another story alltogeather.
First let me (try) to explain a little about what the vehicle is doing. When refueling has occoured the PCM or ECM depending on vehicle basically says "we have new fuel" and for the next few miles (distances vary but I'll give you the recommended later) it starts adjusting fuel injector rates and spark timing and watches what happens based on a few of the sensors and uses some crazy math to figure out what is being burned. Lets say you have half a tank of E-10 and add 1\4 tank of E-85, the vehicle sees that since the last drive cycle there has been a 25% increase in fuel volume. It starts varying, watching and calculating and eventually will come up with a tank content of something like E-40 (I know someone is checking my math, don't bother it's just an example I know its not accurate). The vehicle is not measuring what is in the tank it is measuring the difference of what it knew was there and what it calculates was put in. If it gets one calculation wrong every fill after that will be exponentially wrong as well. If it thinks it's burning E-10 and it actually is burning E-40, since the volitility is different there will be problems.
On to refueling events. In order for the PCM to watch for one and then adjust for it some things MUST happen. First the key MUST be off. This is a good practice for all vehicles not just E-85 compatible. With the key on evap testing may happen and obviously with the cap off the vehicle will see a large leak, as I'm sure many of you know this gives you a check engine lamp on a vehicle that may be perfectly fine. I'ts especially critical on E-85 vehicles because of the prementioned way the PCM is doing calculations. I know the kids like the radio on and you want to listen to the rest of that song or the ball game, so do I, but you shouldn't. Next the fuel level change must be 3 gallons or more, I recommend more just to take slightly wrong fuel level sensors out of the equation (that gives me another idea, why using top tier fuel protects that sensitive level sensor). Finally The vehicle must be given enough time to make the calculations, our data says and I quote "usually by the time 7 miles of operating temperature driving has been completed" so it's not the same every time it's just a guideline. Basically don't go to the gas station a block away while the vehicle is cold and go to the store 2 blocks from the station the new fuel will not be learned and it will not pick up where it left off, once the learn drive cycle is done it is done. The vehicle needs to be in closed loop for any adjustments to be made and closed loop doesn't happen in the first few minutes of any given drive cycle. Once in closed loop it will take a few minutes for all of the calculations and fine tuning to complete.
You may be asking why not just calculate it all the time and do away with the refueling events? Simple answer, it's not efficent, fuel mileage would suffer and it takes quite a toll on processing or so we are told, sounds viable to me and I couldn't change it if it were not so why bother asking why. If the calculated content is wrong it goes something like this. Vehicle comes in and the complaint makes me think fuel quality or content might be an issue. I use the Tech-2 to look at what the PCM has calculated it to be, then take a fuel sample and visually check for water and impurities then I check the actual content. If it is more than 15% off then the tank MUST be dropped down, emptied and dried out and an initial refuel event needs to be done. If that fuel is not E-10 or E-85 it must be disposed of. Using a blend like E-40 for a refuel will not work, you just lost 25 gallons of gas. So to sum it up refuel correctly.
- TURN THE KEY OFF
- USE MORE THAN 3 GALLONS
- DRIVE 7 MILES OR MORE
This all is not an issue if you never ever from the first mile ever use anything different but if you use something different one time proper refueling needs to be done from that point on.
