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.

 

[bodabdan]

I would verify that ECT, IAT sensors are showing reasonable numbers.
IAC might be leaking through.
Verify that there are no exhaust leaks or intake duct leaks.
I'm guessing your trim is that high trying to make up for super cold temps or the O2 sensor is showing lean. At least that's what the ECU is thinking.
Those would be my first checks.

 

[TJ Baker]

LTFT still reading at 25% after driving for about an hour.

Are you able to read which long term fuel trim cells are at that 25%

 

[Marky Dissod]

LTFT still reading at 25% after driving for about an hour.

Are you able to read which long term fuel trim cells are at that 25%?

At 25%, you have a leak, on whatever side says 25%.

 

[TJ Baker]

At 25%, you have a leak, on whatever side says 25%.

Maybe so, but an hour of driving in one fuel trim cell is not what you are going to read at idle later in a different fuel trim cell.

I think knowing what trim cells are higher and what trim cells are lower can sometimes help to narrow down the possibilities.

 

[GregBunney]

At 25%, you have a leak, on whatever side says 25%.

Reading 25% both sides for some reason. When I’m back home I’ll double check with carb cleaner and probably just to the intake gaskets.

 

[Fless]

You might think about trying a smoke machine before doing the intake gaskets. You can also use a propane torch (unlit) instead of carb cleaner.

If you have vacuum brakes pinch off or plug the hose connection to the brake booster to see if it makes a difference. Inspect all vacuum hoses and connections. The PCV hose connection on the driver's side is prone to split and leak.

 

[mhaywoodcz]

I ran into a similar issue years back. Not the same but had high fuel trim issues. My long term trims were high after a 2+ hour road trip. After the MAF and MAP sensor replacement without changes, I found it to be a fuel pump issue. The previous owner had had the pump replaced but they didn't crimp the new electrical wires together. They twisted the ends together and taped them up. After the replacement and proper crimping of the wires, better trims and no issues since.
Again maybe not the same but just thinking it could be a pump issue.

 

[Marky Dissod]

LTFT still reading at 25% after driving for about an hour.

Are you able to read which long term fuel trim cells are at that 25%?

At 25%, you have a leak, on whatever side says 25%.

Reading 25% both sides for some reason.

When my LTFTs were off by 25%, it was an intake manifold leak.

 

[GregBunney]

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.

After driving acouple hours and looking I got another MAF code. Brand new Delphi unit reading at 3.4g/s. Annoying. I’ll change that again when I’m home in a-couple days and check for vacuum leaks again. I just think it’s weird it’d have a vacuum leak both banks and I couldn’t find anything the first go through

 

[GregBunney]

I ran into a similar issue years back. Not the same but had high fuel trim issues. My long term trims were high after a 2+ hour road trip. After the MAF and MAP sensor replacement without changes, I found it to be a fuel pump issue. The previous owner had had the pump replaced but they didn't crimp the new electrical wires together. They twisted the ends together and taped them up. After the replacement and proper crimping of the wires, better trims and no issues since.
Again maybe not the same but just thinking it could be a pump issue.

I was beginning to think about the fuel pump, just don’t have a pressure tester I could use to see what it’s at. I’ve noticed that everything in my rig the PO. Touched was NOT done right and that’s coming from a young guy in his 20’s. Like why tf is my transfer case torqued to 100 with thread locker? lol. Anyway I’ll try to get my hands on a fuel pressure test kit.

 

[GregBunney]

You might think about trying a smoke machine before doing the intake gaskets. You can also use a propane torch (unlit) instead of carb cleaner.

If you have vacuum brakes pinch off or plug the hose connection to the brake booster to see if it makes a difference. Inspect all vacuum hoses and connections. The PCV hose connection on the driver's side is prone to split and leak.

Thank you for this advice I’ll go through and check this all soon! Appreciate it!

 

[Fless]

I was beginning to think about the fuel pump, just don’t have a pressure tester I could use to see what it’s at. .... Anyway I’ll try to get my hands on a fuel pressure test kit.

Many auto parts stores will "rent" them for a credit card deposit that is refunded when the kit is returned.

 

[GregBunney]

Are you able to read which long term fuel trim cells are at that 25%

I don’t really understand fuel cells yet but I saw 5,7, and 20 a lot while diving yesterday when I checked.