LTFT calculation factors, Really, and not just O2's?

[2008Silvy]

Because I've been fighting this lean bank 1 problem when on CNG, and I hooked up the ecu to read only the bank 2 O2 sensor for both banks, so it's the same, and it STILL reads lean.

So, what is it, fuel flow rate sensor, what is on a 2008 silverado LT 5.3L non flex fuel? I really need help on this one. The normal injectors are cut off (signal interrupted) and the CNG injectors take over when you switch to CNG, the CNG injectors follow the pulse width from the petrol injectors with a correction factor applied.

I just redid the whole system, pulled the head off again and re-did the whole top of the engine for nothing Dang sensors!

[06MonteSS]

if you're running lean, you're running lean.. switching O2 sensors from side to side isn't going to change that, heheh....

need to get tuned for that natural gas stuff man...

the stock GM tune, and the diablo tunes are made for normal gas, not cng or lpg... if you're going to run those as your fuel, then you will need to get a custom tune done specifically for those types of fuel...

[TheGrinch]

Thanks, but I didn't switch sensors, I totally unhooked the sensor on bank 1 so it's not even reading it. So now then, the O2 input to the ECM for bank 1 and 2 is identical, but it says bank 1 lean, so it's more to it than the O2 reading.

Can't tune for anything when it says lean, even though it keeps injecting more and more gas till it dies and obviously isn't really lean, and the O2 reading doesn't affect it. I'll probably try to have you take one of the sensors out when I figure out which one it is.

[06MonteSS]

Fuel Trim

Fuel trim refers to adjustments being made dynamically to the base fuel table to get the proper ratio of fuel to air. Short term fuel trim refers to adjustments being made in response to temporary conditions. Long term fuel trim is used to compensate for issues that seem to be present over a much longer period. Fuel trims are expressed in percentages; positive values indicate lean (add fuel) and negative values indicate rich (subtract fuel).

Fuel trim is generally calculated by using a wide set of data values, including front O2 sensors, intake air temperature/pressure (or the more elegent air mass sensor), engine (coolant) temp, anti-knock sensors, engine load, throttle position (and change in throttle position), and even battery voltage can effect fuel trim. Long term fuel trim generally should not exceed +- 10%.

[TheGrinch]

Evidently it finally logged me in on my other username lol.

There isn't any sensor for air for each engine bank is there? All MAP's and MAF's are just an overall? I just don't see anything that only affects one bank only, and since they have the same O2 sensor reading and that's out of the equation, which sensor could make one bank lean?

I think maybe the evap is staying open when I switch to CNG, and most of the air is just going to bank 1 for some reason at idle (bank 2 will go lean at some points). That would make sense. Still though, since both are hooked to the same O2 sensor now, the computer has to be reading something that only affects one bank to say that it's lean. Since it only happens when you switch to CNG, has to be something that happens when the fuel flow stops (recirculates) because nothings else is really changing. I've checked and double checked everything, swapped CNG injector rails, checked vacuum leaks, etc..

[06MonteSS]

see why I don't do tunes for that stuff now?? hahaha...

map and maf are one single reading/sensor...

and both banks are generally not the same... there can be a difference between bank 1 and 2 ltft's... they won't be exactly the same... and yes, it's possible for bank 1 to be just over the threshold and throw a lean code, while bank 2 is under that threshold and will not...

[2008Silvy]

Hey dude!! Check your PM's, figured it out, it actually reads the rear O2 sensors and calculates that into the LTFT and STFT's. Please send me tune with cats deleted so I can finally drive this thing!! lol.

Never have read that anywhere, always read that rear O2's are only for diagnosing the catalyst. By gosh I'm here to tell you it aint so And since CNG burns so much cleaner than petrol it says it's lean. Unhooked bank 1, and it went richer on bank 2. Unhooked bank 2, and finally the STFT's and LTFT's matched (since they're hooked up to the same pre-cat O2 sensor still.

Just sharing, hope it's helpful for someone in the future.