My current fuel trims - idle, 2000rpm, 3000rpm all no load

[Mudsport96]

An additional thought.... You read on the internet that the STFT and LTFT are added to get the actual trim that will be applied. Yes and no. My STFTs bounce up and down rapidly about +/- 2% -> +/-3%. Let's call it a 6% swing. I don't think the ecu would swing the fuel delivery 6% every second or so. Doesn't make sense. Probably the STFT values are gathered over ? 50 readings and an average is determined then some part of that average is used to adjust the LTFT.

I am not sure how often samples are taken for short term trims. And the long terms are the average of the shorts, again not sure when they are moved from ST to the LT file. Are you in a relatively flat area or have some hills? Could be that you are on and off the gas enough that it enriches more often.

 

[2006Tahoe2WD]

I am not sure how often samples are taken for short term trims. And the long terms are the average of the shorts, again not sure when they are moved from ST to the LT file. Are you in a relatively flat area or have some hills? Could be that you are on and off the gas enough that it enriches more often.

I've watched the LT and ST going up a steady incline recently and both seem to behave themselves - no big moves. When running on the flat I try to hold the pedal steady and watch the airflow in g/s and see if I can make it also steady then watch the trims and see what is going on. As I mentioned all could be working fine.
Another data point would be what others are seeing on LTs and STs at steady cruise. You may have posted that.

 

[2006Tahoe2WD]

Yesterday I tried displaying the Commanded Evaporative Purge and couldn't get it to work. That is a standard listed PID but for some reason it is not listed in OBDLink. So my experiment to compare trims to the purge air amount isn't going to happen right now.

 

[2006Tahoe2WD]

Yesterday (Thursday) I bought 10 gallons of 100 Oct gas at the 76 station here. $10/gallon.
This was mixed with about 4 gallons of 87 Oct. The fuel trims lowered a bit but at steady-state highway the LTs are about the same. The 100 Oct might also I have ethanol, this is unknown.
I'm still working on getting the OBDLink MX+ to report the Commanded Evaporative Purge.

 

[2006Tahoe2WD]

After the tank with 100 Oct, I did not see much change (LTFTs) at cruise speed. I did see a bit of improvement at idle. Idle now running +2-4%. Thinking on this a bit more and my current guess is the injectors are somewhat dirty or at least not putting out what they should. E.g. at low flow the rate is okay but as the flow goes up the rate seems more restricted. The next step is to put in Techron and see what happens. I know my fuel pressure is good - I drove with a gauge taped to the window.

 

[Hollywood 2]

After the tank with 100 Oct, I did not see much change (LTFTs) at cruise speed. I did see a bit of improvement at idle. Idle now running +2-4%. Thinking on this a bit more and my current guess is the injectors are somewhat dirty or at least not putting out what they should. E.g. at low flow the rate is okay but as the flow goes up the rate seems more restricted. The next step is to put in Techron and see what happens. I know my fuel pressure is good - I drove with a gauge taped to the window.

Not sure if yours is flex fuel. I have a 2007 avalanche and it has been giving me fits with Stabilitrak needs service. Spent almost 600.00 for mechanics to find bad mass airflow sensor running borderline around 9-10grams intermittently. Normally around 5-7. Drove it for a few days and it did it again today. After a litlle research I reset my fuel trims and also alcohol sensor with an xtool d7 scanner I own and long term fuel that was stuck on -18% went back to 0/6/9 as it should. My short term trims are normal from -3/0/3/6 etc. Not sure if this helps but I'm pretty relieved at the moment and just to share the info with someone else.

 

[2006Tahoe2WD]

Not sure if yours is flex fuel. I have a 2007 avalanche and it has been giving me fits with Stabilitrak needs service. Spent almost 600.00 for mechanics to find bad mass airflow sensor running borderline around 9-10grams intermittently. Normally around 5-7. Drove it for a few days and it did it again today. After a litlle research I reset my fuel trims and also alcohol sensor with an xtool d7 scanner I own and long term fuel that was stuck on -18% went back to 0/6/9 as it should. My short term trims are normal from -3/0/3/6 etc. Not sure if this helps but I'm pretty relieved at the moment and just to share the info with someone else.

Thanks for the report. Every bit of info. helps others diagnose issues. My current thinking regarding my trims is that the injectors need cleaning. My Tahoe is not flex fuel so I shouldn't have issues associated with that. I want to buy another scanner, my Foxwell failed, they gave me my money back witch was good. I liked the Foxwell because it had Chevy two-way functions e.g. roll the window up and down. Most scanners don't have that sort of Chevy command available.