Is there a process for "teaching" the correct adaptive fuel trim values to an LR3 after clearing them? I looked in the service manual and TSBs, but didn't find anything.

Background:
After doing a 105k mi service with new OEM plugs and coils on my 4.4L LR3 I was getting Bank 1 and Bank 2 lean error codes. I did the usual process of throwing parts at it to fix a vacuum leak (throttle body gasket, PCV valve, oil filler tube, cleaned the MAF sensor, etc) and when nothing worked, eventually used the Gap IID tool to clear adaptive fuel trim values and the codes have not come back.. BUT: my fuel economy is trashed. *maybe* 10mpg, with a tailwind. Before my 105k service I was averaging 17mpg, and after the service (with the lean codes, mind you) it was closer to 19. Needless to say, I miss my old 17mpg, especially with 91 octane at over $6/gallon in California right now. Wondering if I am missing a step and need to do a certain procedure after clearing adaptations (idle for 10 min from cold, etc.)

 

Random thoughts:
What are the live values for short term and long term fuel trends? This may point you to the O2 sensors or it may be that one in a thousand times that it is actually the cat that is bad.
How old are the O2 sensors? What brand are they (on some cars it makes a difference if they are not OEM brand) You could change the right side upstream sensor (easiest to get to), reset and try again.

My MPG have always improved after an adaptive reset within 1 tank of fuel. I know this sounds absurd, but how many miles since your last oil change. I sometimes notice a decrease in mpg during the last 800-1,000 miles ( I change every 3,750) Also if I am late, I get the VANOS making noise during idle. When it ticks it is telling me to change the oil

May not fix your problem directly, but these are things to try.

Hope this helps..... Let us know how it goes.

Jeff

 

sounds like the MAF might be dead or dying. Looking at what you replaced/fixed, the MAF being out of whack will kill your economy if it's sending false readings to the computer. what do the live MAF values/voltage look like?

 

@Blackngold77 The MAF was replaced when I did plugs and coils, but then everything idled super rough and threw codes. I cleaned the original MAF with proper MAF cleaner spray, and switched back to it. Ran fine after. Going for a long drive tomorrow and will watch MAF values.
@Rufflyer I'll watch the Fuel Trim on a ~100mi drive tomorrow and report back. Not sure what brand O2 sensors, but I would guess OEM (Bosch?) I'm the second owner and the first seems to have only had dealer service. If the cat were going bad, wouldn't I expect to have a big drop in power too? It's driving great, not stinky or anything, just guzzling.

 

Well, I think it resolved itself with longer highway driving at speed. I was getting 19.5 mpg averaged across the 75 mile return leg of mixed 2-lane highway@ 40-50mph and freeway @ 75-80mph. As long as I don't have to stop an start too much, the LR3 really does respectably! Too bad I live in Los Angeles, where free-flowing roads are a rare joy!

@Rufflyer Just the same, I would appreciate any clarity on whether these numbers are good or bad, or how to figure out what normal should look like.

At idle, cold:
Long term fuel trim (bank 1/2): -5/-5
Short term fuel trim (bank 1/2): +6/+7
Air flow meter sensor (mv): 1362
Mass air flow (g/s): 5.7
O2 equivalence ratio (bank 1/2): 0.981/0.980

At idle, at operating temperature:
Long term fuel trim (bank 1/2): -4/-3
Short term fuel trim (bank 1/2): +5/+5
Air flow meter sensor (mv): 1416
Mass air flow (g/s): 6.5
O2 equivalence ratio (bank 1/2): 0.980/0.981

At ~70mph, at operating temperature:
Long term fuel trim (bank 1/2): +5/+5
Short term fuel trim (bank 1/2): 0/+2
Air flow meter sensor (mv): 2446
Mass air flow (g/s): 55.2
O2 equivalence ratio (bank 1/2): 0.981/0.980

At ~40mph, at operating temperature:
Long term fuel trim (bank 1/2): +9/+9
Short term fuel trim (bank 1/2): -2/-2
Air flow meter sensor (mv): 2553
Mass air flow (g/s): 39.3
O2 equivalence ratio (bank 1/2): 0.977/0.973

At idle, at operating temperature after long drive:
Long term fuel trim (bank 1/2): -3/-3
Short term fuel trim (bank 1/2): -3/-3
Air flow meter sensor (mv): 1240
Mass air flow (g/s): 4.3
O2 equivalence ratio (bank 1/2): 0.984/0.980

 

Your fuel trims look great. Total fuel trims (sum of long and short) under 10 + or - are good. Maf g/s at temp and idle should be around the same as the number of liters your engine has (at least for engines larger than I think 3 L), so good there too.

 

two months to the day and the CEL is back on. :/ Same code: bank 1 lean.

If it makes a difference, it got super hot here this week. Should I keep resetting fuel trim, or is there a next step to diagnose something? Also.. how worried should I be about driving with one bank running lean?

 

If it makes a difference, it got super hot here this week. Hot weather gives you less dense air, so it would want to start leaning out to compensate not add fuel to make up for too much air.
Should I keep resetting fuel trim, No give it a chance to establish some long term fuel trends before you reset adaptations again.
or is there a next step to diagnose something? Try cleaning the MAF again.
Also.. how worried should I be about driving with one bank running lean?

Remember, the ECU throws the code if it tries to enrich beyond 25 (I could be wrong on the number) to compensate for too much un metered air. If the short term fuel is too high for too long, the code is triggered.

Hope this helps some

Jeff

 

Running lean can kill your cat....