My LTFT's have been around -1.6% at idle for years now. I did a log trying to resolve another issue and noticed that they are at 10% on one bank and 15% on the other at idle. I have checked and eliminated the following:

spark plugs
plug wires
connections
vacuum leaks
maf sensor

I am replacing the o2 sensors as they have 114,000 miles on them. I do not get any codes and the tune is fine other than idle. I get a high count in the misfire index as soon as I step on the gas but it goes away instantly and then the LTFT's go to -1% normal driving.

What else can I check??

 

have you checked the fuel pressure?

 

OOps yes forgot to add that. I did check that and it is fine, 60 psi.

I am also running some fuel injector cleaner through the system now.

 

Check for a cracked exhaust manifold. That's was what happened to mine. The right manifold was cracked behind the shield, and my fuel trim was high like yours.
Replaced the manifold and all was good again.

 

Sure hope its not that, I just replaced it a few months ago!! Ill check that thanks!

 

Are you still running a MAF? And are the LTFT positive or negative? And are the O2 sensor voltages osciallating (switching) at idle? post up a log.

 

I bet it's your O2 sensors.

 

I do have a MAF and the values are 10 to 15% positive. I put new O2's in with no change. They do oscilate at idle.

Weird thing is yesterday everything cleared up and the LTFT's went back to 0%. Then today they are back to +15%.

I replaced ALL of the vacuum lines. Checked the manifold, exhaust and intake for leaks. I checked fuel pressure, replaced the fuel filter and ran seafoam through the fuel system.

I replaced the EGR gasket and cleaned the valve as well. How do I test to see if the EGR and MAF are functioniong as they should?

 

My MAF flows 5-6 gps at idle not 10-15 like yours is.
I have a 2000 2500HD with a 6.0L.

 

The 10-15 is the % that the LTFT's are adjusting, not maf flow. I will check my GPS flow at idle though.