Yesterday I took some time to investigate an interesting issue that my Jeep had developed over the past week and a half or three tanks of fuel.
Immediately after fueling my fuel trims usually go high negative. This is a design trait of many operating systems in order to reset the adaptives as well as fine tuning of the fuel trims to evaluate fuel composition and quality, especially on flex fuel vehicles.
I had some slight issues with my LTFT (Long Term Fuel Trims) last winter that I was fine tuning and after tweaking my tune, specifically my air flow calculations, back in February the 6.2 L94 engine has been running flawless in my 2011 Jeep. From single digit winter driving in February to triple digit heat over the summer and from 2k feet elevations all the way up to 13k plus feet in Ouray it has performed flawlessly and adjusted fuel trims perfectly.
Last week when I fueled up on the way home from work I noticed that it stayed high negative on my LTFT's longer than normal. I also noticed that during the week I got a CEL several times and each time I was idling at a stop light for an extended period of time. I could watch my LTFT go to -30.4 before setting the rich code (P0172 & P0175).
Yesterday after breakfast I had some time to myself so I decided I was going to check a couple of items on the Jeep. First being the air filter, which I generally clean at each LOF service but this one had a recent trip to Moab on it so I thought it could be possible that it may be dirty even though I put my Outerwear on it for the week of the rally. After removing my air filter I noted that from the outside it appeared a bit dirty but with a light inside it was obviously not plugged or even that bad for that matter. I installed my backup filter anyways. I usually keep a cleaned/oiled filter in a ziplock baggy that I carry in the coach for backup and also switch between the two during a LOF service @ approx. 4.5k miles.
Also while I was at it I though I'd pull the intake duct off and clean the throttle body. During our trip to Ouray in August I worked the snot out of the ole' girl and thought maybe I got a little bit of oil vapor from the PCV system that could have gummed up the throttle body and affecting the nearly closed position at idle which could be why it was running rich at idle. And since it was already out I also sprayed the MAF sensor with cleaner and let dry. The good news is that the MAF sensor was clean, the air filter wasn't that dirty and there was hardly any residue in the throttle body. I also ran my hand inside of the intake duct and could feel no grit or anything indicating that the air filter is performing its duties as designed. I was especially concerned due to the amount of dirt/dust that we had encountered in Moab last month as it was extremely dry and dusty. Luckily I was the trail leader on several of the days so it was minimal but the other days I was back in the dust as tail-gunner, although I held back a bit further to stay clear of as much dust as possible.
I decided to hook up my HP Tuners scanner to the Jeep and go for a drive as it does a great job of giving visual cues that are easier to watch while driving than my Snap-on Modis Ultra scanner. Plus I could load a few more PID's onto the screen for viewing.
I noticed that just idling in the shop I could tell it was running rich from the smell and the idle was a bit off to me. During my drive I noted that the O2 sensors were switching perfectly and very quick just like they should have been. I also noted that the MAF sensor appeared to be reporting properly as was the MAP sensor, each of which can cause my issue if not reporting correctly. What caught my attention was that while the upstream O2 sensors were switching rapidly my STFT (Short Term Fuel Trim) numbers were pretty much spot on at +/- <5 which is near perfect, usually around +/- 1.5 or less, yet my LTFT was still at -27 to around -29.6 while driving. This had me scratching my head because the LTFT is supposed to be a mean or average of the STFT but the STFT was pretty much at zero and hadn't even left the single digits.
I pulled over to study my data and that's when I saw it. I noticed that my ethanol fuel content was at 45.1%. That was erroneous as we don't have E85 around here and most everything is only max of 10% ethanol. I thought maybe I got a bad batch of fuel. I went home and did some reading to discover how the GM operating system determined the ethanol content percentage and it has to do with the O2 sensor reporting and fuel trims immediately after a fillup. This made perfect sense as my issue started immediately after a fillup a little over a week ago.
If you look at this screen shot of my HP Tuners data log you can see where I've circled both upstream O2 sensors as they are switching perfectly and the area to the left with my STFT and LTFT numbers that are in a yellow box.
I went back to the shop and pulled out my Snap-on scanner and reset both the ethanol content adaptive as well as reset my fuel trims. The ethanol percentage went to 3% which is the factory default so I'm certain after the the next fueling event it will go to around 9-11% which is where it is usually at but I will confirm after that next fillup. The wife and I went for a drive nice drive afterwards and everything appears to be reporting properly now but that was definitely a learning curve as I have not experienced that scenario before on my Jeep, or any vehicle for that matter.
Sorry this was a bit lengthy but I just thought I would share in case others notice an issue with fuel trims after a fueling event.
