2021 Yukon XL 5.3L Running Rich - Fuel Trim Questions

[Stewmatic]

Hey all, been digging into some quirks with my 2021 Yukon XL (5.3L, 2.5 months old, still under warranty). Noticed it’s down a quart of oil since I got it, but after warming up, the dipstick reads full weird, right? Using a BlueDriver, I’ve been logging fuel trims:

- Highway (66 mph, Tiptronic 9th, AFM off): Long-term trims steady at -9% to -10% on both banks.
- City driving: Seems richer, haven’t logged exact numbers yet, but I’ll check. Somewhere in the -12-15
- Bonus: Smelled fuel at startup today, faded after running.

Coolant temp’s steady at 200°F, so that’s fine. Thinking MAF, O2 sensors, or maybe a leaky injector since it’s rich across both banks. Oil’s synthetic (prob 0W-20), planning a change Tuesday. Any thoughts on what to tell the dealer? AFM/DFM a factor?

 

[Fless]

Agree with a possible leaky or over-metering injector. Also check the MAF reading with a scanner; at hot idle it should read right near the engine displacement (5.3) in g/sec.

 

[Stewmatic]

This is what I got, based on some calculations the airflow looks about right. Not sure about that fuel pressure range 36-48?

 

[iamlegion]

I wouldn’t waste 5 minutes of my personal spare time diagnosing a truck under warranty, just my $.02. Take it to the dealer and see if they’ll take a look for a small diagnostic fee - if there’s a problem the warranty should cover it and if not you traded $150 for peace of mind and not chasing ghosts.

 

[blanchard7684]

View attachment 452696
This is what I got, based on some calculations the airflow looks about right. Not sure about that fuel pressure range 36-48?

This is at idle right?

I get 0.6 lb/min air flow at idle.

From a recent data log (2024 Suburban 5.3)

At 45 mph on flat level ground I get 2.3-2.7 lb/min (air flow). Throttle Pos is 23-29%. RPM is 1350. This is an AFR of 13.7-14.4. DFM was active.
At 60 mph on flat level ground I get 3.4-3.9 lb/min(air flow). Throttle Pos is 30-34%. RPM is 1400 rpm. This is an AFR of 14.4-16.0. This was in L9 with DFM deactivated. This was in a small range of a whole data log where I was going over small inclines at a good frequency. Outside of the flat sections this thing goes very rich and corresponds to large spikes in calculated load.

One thing I've observed is at low rpm in the 45-60 mph range, it takes only the slightest incline for the calculated load to spike and AFR to drop well into fuel enrichment. Throttle position responds as well. It seems like the transmission mapping at this speed is trying to do everything to avoid a downshift. (At higher speeds it seems to be the opposite...every gust of wind and a slight incline I get a downshift).

Another observation: with DFM engaged, the AFR is frequently well under 14.7 but the fuel rate drops, and also the rpms jump around alot (more than 50 rpm). So the cylinders that are carrying the load run very rich, but the fuel consumption goes down. It isn't a significant drop in fuel rate though...

I wouldn't be too worried about a fuel trim of -9, -10%. The 5.3 has a big task of pushing around 6000 lb brick. Over several data logs in a mix of driving, I'm seeing sub 14.7 AFR 80% of the time. Even at highway speeds of 75 mph in L9 at 1800-1900 rpm.

I think the fuel dilution you are experiencing is from short trips where the oil doesn't get hot enough to appreciably vapor off fuel. oil change intervals beyond 5000 miles can cause this as well.

I would change the oil and reevaluate the fuel trims. It may take a bit to see a response.

Oil viscosity and grade of oil is another hot topic for another thread...