LTFT too high

[jlmanatee]

I'm chasing problems as indicated by my erratic and too-high LTFT readings on my 2006 W24 8.1L engine w/6-speed Allison transmission. I've got about 65k miles on the odometer. At cruising speed of 60 mph and 2000 RPM, my right and left banks vary between +14 and +20. If I put my foot into it and it drops down to 4th gear and 3000 RPM it jumps up to 22-25. That part is probably normal.

I recently put new plugs (gapped to .045) and plug wires from Ultra RV on to fix a very bad power loss, backfiring problem. The old plugs were very "old" looking with a huge gap over .060. That seems to have fixed my power problems very well.

So now, even though I'm pretty happy with the power and MPG (I got 8 mpg yesterday with no wind, slightly rolling hills and towing a Jeep Wrangler) I'm still worried about these fuel trim readings.

I cleaned the MAF but that has not seemed to help. The O2 sensors seem to be putting out voltage and the fuel pressure is within specs while parked. I'm usually not one to throw parts at a problem but a new MAF and O2 sensors are relatively cheap so I ordered them from Rock Auto yesterday. I guess my next concern should be to keep an eye on my fuel pressure at the rail while I'm driving to see if that is an issue.

Incidentally I should mention that my STFT readings vary between +/- 4 which seems normal from what I've read about fuel trim.

Any and all comments are much appreciated.

[bigtex]

What are stft and ltft at idle?

[jlmanatee]

Fuel Trim Info 8/29/20
2006 8.1L Workhorse W24
65k miles
(all data while parked)
coolant temp: 195 F

RPM:605 - idle
STFT1:+/-2 0 avg
STFT2:+/-2 0 avg
LTFT1: +13.28
LTFT2: +6.25
MAF:8.2 gm/sec

RPM:1000
STFT1: +/-2 +0.85 avg
STFT2: 0 to +2 0 avg
LTFT1: +9.38
LTFT2: +6.25
MAF: 12 gm/sec

RPM:2000
STFT1: -3 to +2 0 avg
STFT2: -2 to +4 +1.5 avg
LTFT1: +14.06
LTFT2: +14.84
MAF: 26 gm/sec

RPM:3000
STFT1: +/-3 0 avg
STFT2: -1.5 to +2 +0.5 avg
LTFT1: +19.53
LTFT2: +19.53
MAF: 38 gm/sec

[bigtex]

Check fuel psi. Most likely it will be the maf but I would rule fuel psi out first.A bad 02 sensor typically will not show the stft and ltft’s you have.

[CJ7365]

I think you are on the right track to replace the MAF and O2 sensors, obviously I would change out the MAF first and then check the changes.

I had somewhat High LTFT both sides not as high as yours (+10-15), and cleaned the MAF, with no help, installed new MAF, now I have great readings maybe sometimes +or - 5 LTFT and + or - 2 STFT

Your STFT look good, at least to me.

My LTFT NEVER goes up that high when pedal to the metal

If the new MAF does not help, at least you can start trouble shooting vacuum/fuel pressure etc... problems

[larryjb2]

Here's a good explanation of fuel trims if someone is curious.
https://www.mechanic.com.au/news/und...rm-fuel-trims1

[jlmanatee]

I installed the new MAF and O2 sensors. The sensors were a bit of a chore but came out with heat from a mapp gas torch. I also added a fuel pressure gauge to the dash, connected to the fuel rail. I then reset the PCM by disconnecting the neg. battery cable for 15 min. I brought the engine up to about 180 degrees. I started to take my readings when the engine suddenly went from closed loop back to open loop. It had stopped taking signals from the O2 sensors. This scared me for a bit, but a restart of the engine put it back into closed loop and my readings got better. It was like the PCM decided it was time to re-calibrate?
Fuel pressure was 59 psi throughout the testing.

Idle
SF1 -2.4 to 0
SF2 -2.4 to +.78
LF1 +7.8
LF2 +6.25
MAF 8.25

1000 RPM
SF1 -1.75 to +2.5
SF2 +/- .78
LF1 +8.59
LF2 +7.0
MAF 13

2000 RPM
SF1 -2 to +3
SF2 -3 to +3.5
LF1 +14.8
LF2 +16.4
MAF 25

3000 RPM
SF1 +/- 4
SF2 +/- 3
LF1 +19.53
LF2 +14
MAF 42

I think now it will be a matter of driving it for awhile IRL and rechecking the readings. I read somewhere that it can take up to 1000 miles of driving before all the readings settle down.

[xc-mark]

Its possible the fuel rail has crap in it. I know the 6.0 motors didnt run a fuel filter and that sometimes would clog up the screens in the injectors . I would also suggent giving it a good dose of Tectron fuel cleaner. you can get it a walmart

on the 6.0 L truck the fuel supply enter the rail on the drivers side and returned on the passenger side rail tot he tank. So the drivers side rail would get and fine particles stuck in it.

[jim58]

Originally Posted by jlmanatee

I installed the new MAF and O2 sensors. The sensors were a bit of a chore but came out with heat from a mapp gas torch. I also added a fuel pressure gauge to the dash, connected to the fuel rail. I then reset the PCM by disconnecting the neg. battery cable for 15 min. I brought the engine up to about 180 degrees. I started to take my readings when the engine suddenly went from closed loop back to open loop. It had stopped taking signals from the O2 sensors. This scared me for a bit, but a restart of the engine put it back into closed loop and my readings got better. It was like the PCM decided it was time to re-calibrate?
Fuel pressure was 59 psi throughout the testing.

Idle
SF1 -2.4 to 0
SF2 -2.4 to +.78
LF1 +7.8
LF2 +6.25
MAF 8.25

1000 RPM
SF1 -1.75 to +2.5
SF2 +/- .78
LF1 +8.59
LF2 +7.0
MAF 13

2000 RPM
SF1 -2 to +3
SF2 -3 to +3.5
LF1 +14.8
LF2 +16.4
MAF 25

3000 RPM
SF1 +/- 4
SF2 +/- 3
LF1 +19.53
LF2 +14
MAF 42

I think now it will be a matter of driving it for awhile IRL and rechecking the readings. I read somewhere that it can take up to 1000 miles of driving before all the readings settle down.

ST's look good. Have you tried disconnecting the battery and see if the LT trims will clear? If they do clear you'll be starting from scratch rather than waiting 3 or 4 tanks for the LT's to readjust.


Jim58

[Sanchez_315er]

Did you reset the ECM after the plugs and wires?