Flex Fuel problem?

[2007chevy5.3l]

Okay, so my truck has the flex fuel option and I know it uses a calculation to figure out what ethanol content is in the fuel by a sensor, correct?
Well I seem to have some crazy issues still with my truck. I havent had any time to do some logging and catch it on a log but what is happening is my AFR are still going crazy lean for some odd reason, mostly at idle or nearing idle. Example- Im sitting at a light idling and the thing is running fine, AFR ranging from 13.8-14.7 and all of a sudden it goes to 15.8-16.2 and the engine stumbles and labors to stay lit. Does this for a few seconds then goes back to the 14s and is fine. Also notice this happening after I let off the throttle slowing down for a stop, with no throttle and brake applied the AFRs hit 16s for a few seconds and then starts running normal again nearing the stop.. May I add, the times that I notice this problem being really bad is after I fill up.. So it is leading me to think its having a hard time calculating the correct ethonal content in the fuel...?

In my tune the stoich is set for 0%-14.68, 6.2%-14.32, 12.5%-13.97 which is stock settings. In the flex fuel parameters the option is enabled, the sensor type is calculated, the default % is at 0.0% (If the vehicle is fitted with a flex fuel sensor and it is deemed to be faulty (eg P0178 or P0179 set) this is the default ethanol percentage that will be used.) And the sensor diagnostic is disabled.

My question is, can I basically turn off the flex fuel and make it run just off the stoich AFR, kinda like non flex fuel engines and set the default stoich at 14.32 since our fuels contain +/- 10% ethanol, And see if my issue goes away?


Hope I make sense and you guys can understand what im saying and want to do.

Thanks for any help!

[2007chevy5.3l]

Disabled the Flex Fuel and still have problems... Gonna keep looking

[slows10]

Is this an open loop speed density tune?

[2007chevy5.3l]

Nope. Closed loop.

[slows10]

So long tube headers, no cat and no muffler? O2 sensors may have a problem with that.

[2007chevy5.3l]

O2 voltages seem to be in check.
To make things more interesting my buddys truck is doing similar things. His is virtually stock..

[slows10]

So the stock o2 shows lean when the problem happens and you are confirming with the wideband. Post alog showing the wideband and narrowbands.

[2007chevy5.3l]

Yeah ill post a log when i get home here in a few minutes.

[slows10]

Does it do it almost consistently at every light, Or just once in a while?

[2007chevy5.3l]

It wont do it everytime. Thats why i thought it was flex fuel related