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...

 

[jfoj]

A few things for everyone pulling and/or monitoring data with regard to MAF readings. Unfortunately ALL the App/Tools when set to Imperial measusements the MAF reading will be displayed in lbs/min, which is correct from the standpoint of Imperial measurement, HOWEVER, it is no where near granular enough to provide useful data. I have tried to get some of the App developers to DEFAULT MAF readings to g/s (grams/second) even if IMPERIAL measurements are selected without any luck.

SO PLEASE look deep within the App/Tool you are using and select the MAF reading to be displayed in g/s rather than lbs/min. Most tools allow independent adjustement of data for a specfic sensor.

Back to the origial post.

A few questions.

1. How many miles on the truck?

2. How many miles on the current oil?

3. Has any make up oil been added since the last oil change?

4. Do you make a lot of shorter trips less than 30 minutes at a time?

5. Do you use the Remote Start feature often to allow the engine to warm up before you drive?

6. What city/state do you live in? Trying to determine generate Winter temps you experience.

Back to the issue of oil level and question of fuel smell.

First thing to do is pull the dipstick and make sure you have oil to wipe off and smell the towel/rag for traces of fuel. Smelling the oil is a good way to determine if there may be an abundance of fuel in the oil.

Another situation is when the oil is saturated with fuel, sometimes the fuel trims can be negative under some conditions.

With regard to oil level on these engines, it is difficult to determine the proper oil level with the disptick at times. The oil will expand once warmed up, but I would not expect it to expand a full quart. Sometimes you may need to spin the dispstick in the tube when it is fully inserted to get a good oil coating on the dipstick.

I also think the oil fill level with filter change at 8 quarts is under stated. It has become clear from a number of members on this and other forums that 8 quarts with filter will only show up about 1/2 way in the hash marks on the dipstick. It has been reported that these engines take between 8.5-8.7 quarts with oil change. I run 9 full quarts in my engine when I change oil. Not been a problem to this point.

I would also recommend you check the oil level with the dipstick every 2nd fill up and smell the the oil. Once you get in this habit you will get a "good nose" for the oil smell.

If you are not changing the oil yourself, ask for the shop/dealer to provide you with 1 or 2 extra quarts of oil. This way you can top the oil of correctly after the shop puts 8 qts in the engines and you have the same oil for make up oil if you need to add any between oil changes.

HIGHLY recommend that you change the oil at the 50% OLM indication rather than running all the way to 0% OLM indication. Oil is way cheaper than steel!

As for for overall Long Term Fuel Trim values, you need to get check them again once you get the oil changed and then need to monitor for a while. Then as a community if other members can gather data, we can then determine what is considered as "Normal" or what "Nominal" values are. There are a lot more variables on the newer engines with variable cam timing, AFM/DRM and so forth, so sometimes values may seem out of the range when in fact they may be normal.