Flex-fuel logic 101 – Do’s and don’ts.
1. The flex-fuel logic uses the fuel level input from the fuel tank as a trigger to tell the system to relearn. It needs to see an increase in fuel level of more than 10% to trigger the system. So if you're going to switch fuels, make sure that you're adding an adequate amount of fuel to trigger the logic. Good rule of thumb, add at least 1/2 tank.
2. DO NOT leave your vehicle running while refueling. Personally, I'm not sure why anyone would do this, but to each their own. In the case of switching fuels, you MUST shut off the vehicle. Failing to do so can cause the trigger mentioned in tip #1, above, to not set properly, and thus the flex-fuel logic will not learn. Then you'll have either lean or rich DTC codes, depending on which fuel you're switching to or from. THIS ALONE has been the most common cause of the emails we been receiving. DO NOT LEAVE YOUR VEHICLE RUNNING WHEN REFULING! Got it? Good.... let's move on.
3. Whenever you switch fuels (and you shut off the engine when refueling, right?), you should do your best to drive the vehicle immediately afterwards to allow the flex-fuel logic to learn the new ethanol content. Ideally 15-20 mins should be plenty. In "theory" the system should relearn properly, eventually, even if the vehicle is shut off prior to completing the relearn. However, in reality, it causes issues.
Example: customer refuels at a gas station 2 minutes away from his house or work. He then drives to his destination, shuts off the vehicle, and leaves it parked for several hours. He comes back out, vehicle is now cold, and starts up the vehicle. Well, since the flex-fuel logic has not yet learned the ethanol content, the vehicle is not going to reference the proper tables for cold start, idle, etc. Also, there is a few seconds of delay from the time the engine is cranked until the O2 sensors "turn on" and begin correcting fueling. If, for example, you just filled up with E85, and didn't let the vehicle relearn, fueling is going to be very lean. The engine is going to sputter and lope and run rough and possibly even stall. You can even have check engine lights for things like misfires or system too lean, etc. The engine will eventually smooth out once the O2 sensors start correcting fueling, but of course the owner of the vehicle has already freaked out... called us 10 times, sent 7 Facebook messages, 15 texts, and posted in 5 different Facebook groups asking if his engine is blown.... I'm exaggerating for comedic effect, but you get the idea. The point is that all of the drama could easily be avoided if the owner understood how the flex-fuel logic works and gave it the proper time to work.
So, now everyone understands how the flex-fuel logic works. Spread the word. Educate others. Happy motoring!
Ken Osborne ~ Oz Tuning
