Help Troubleshooting Fuel Trims

[GregBunney]

Hello I’ve been reading every dang forum I could find about rough idles and fuel trims and can not figure this out. I’ve got a 2003 5.3 Yukon, gas, and with the colder weather it started to idle rough and recently die while idling. Thought it would be a vacuum leak coming from BMWs and I checked with carb cleaner and can’t find any leaks that way. This lead me to think maybe fuel issues? I’ve put in a new Delphi MAF a-couple days ago and this made it idle slightly better, but now it dies at idle and didn’t before. Now I’m leaning towards O2 sensors but I’m struggling to pin point my issue here. Also only codes I get (got) were fuel trims bank 1 & 2 along with the MAF code from before I swapped it. Getting really bad MPG too. (10.7 and I know everyone says how you drive blah blah blah, I’ve been trying as HARD as I can to get my mpg up and couldn’t pass 11 until the new MAF and now I’m sitting at 11.7) any advice would be great! TIA

 

[GregBunney]

Hello I’ve been reading every dang forum I could find about rough idles and fuel trims and can not figure this out. I’ve got a 2003 5.3 Yukon, gas, and with the colder weather it started to idle rough and recently die while idling. Thought it would be a vacuum leak coming from BMWs and I checked with carb cleaner and can’t find any leaks that way. This lead me to think maybe fuel issues? I’ve put in a new Delphi MAF a-couple days ago and this made it idle slightly better, but now it dies at idle and didn’t before. Now I’m leaning towards O2 sensors but I’m struggling to pin point my issue here. Also only codes I get (got) were fuel trims bank 1 & 2 along with the MAF code from before I swapped it. Getting really bad MPG too. (10.7 and I know everyone says how you drive blah blah blah, I’ve been trying as HARD as I can to get my mpg up and couldn’t pass 11 until the new MAF and now I’m sitting at 11.7) any advice would be great! TIA View attachment 449502View attachment 449503

Photos are also freeze frames of when the fuel codes popped up.

 

[Fless]

Your short term fuel trims are okay; will your scanner allow you to reset the long-term fuel trims?

But before you do that look under Powertrain live data (#2, I think) and see what the alcohol content is reading. Are you running any E85 or anything above E15 for fuel? If not, and the alcohol content doesn't match the ethanol % of the fuel it has, reset that, too. That may also reset the LTFTs.

 

[GregBunney]

Your short term fuel trims are okay; will your scanner allow you to reset the long-term fuel trims?

But before you do that look under Powertrain live data (#2, I think) and see what the alcohol content is reading. Are you running any E85 or anything above E15 for fuel? If not, and the alcohol content doesn't match the ethanol % of the fuel it has, reset that, too. That may also reset the LTFTs.

I’m not running E85 just 87 from Costco normally. I happened to look yesterday and I want to say it was around 7% but I’ll double check today and see if the scanner will let me reset the LTFT.

 

[Fless]

Sounds like the alcohol % is tracking the actual fuel, so that's good.

 

[GregBunney]

Sounds like the alcohol % is tracking the actual fuel, so that's good.

Refueled today and have a 4.5 hour road trip today. Scanner still says 8% for alcohol but I don’t believe my scanner (launch 123x) can reset fuel trims. LTFT still reading at 25% after driving for about an hour. Side note I put in a Delphi MAF 2 days ago and it threw another MAF code today? Bad unit or what?

 

[Fless]

LTFTs can take a while to adjust.

What MAF code?

MAF live data is easiest to check if your scanner is set to metric units (Imperial requires a conversion of the reading). If possible, at hot idle, read what the MAF is reporting in g/sec; it should be similar in value to the engine displacement in liters; e.g., around 5.3. I'm not sure what the variance can be, but maybe +/- 1 gram is a guess on my part.