fuel trims with gm maf

[obx tsi]

When viewing ltft on my palm it displays two different numbers, one when you are applying gas and one at no throttle or idle. Am I assuming that I need both numbers at 0. Right now they are at like 2 with gas and 7 at idle. Stft seems to be ok. It also seems to run rich under a load, judging by smell and my digital narrow band o2. I guess the question is why two different ltft numbers and what should I adjust to get it closer to 0. All I have to tune with is the maft right now. Thanks for any advice.

 

[Kevin Jewer]

Those numbers are close enough to move on to bigger and better things. Anything within 5% is fine IMO. That being said, there are 3 different long term trims. The palm type loggers can display the "current" trimbeing used at that instant. It might even be all they can do, it's been like 10 years since I used one. At any rate, LTFT Lo is idle, LTFT Mid is cruise, and LTFT High is tough to hit since you usually tip into open loop at that point. You are seeing Lo and Mid. The Lo trim will adjust while idling only, and the Mid trim will adjust while cruising only. Make sure to give the ECU a good 5-10 minutes in that (steady) state after making changes, it adjusts LTFT slowly (dumping STFT value into it over time to zero STFT). Hopefully that answers your question.

 

[obx tsi]

Thanks for the reply. What do you mean bigger and better things? At this point I just need it to not be running rich and sucking to much gas. I think I understand what you were explaning about the fuel trims. Thanks.

 

[Kevin Jewer]

If the O2 sensor is working properly, and fuel trims are not maxed out (12.5% IIRC for LTs, and something similar for STs), your fuel economy is out of your hands and should be typical. By moving on to bigger and better things, I mean you don't need to spend any more time arsing about with fuel trim tuning if you don't want to, and can work on other things. I did miss your question about what to adjust to get it closer to 0. If the numbers are positive, you need to add fuel on the translator, and if they are negative, you need to take fuel away. In this case you want to add 5% fuel with the control that affects LTFT Mid (that would put it at ~2% like the Lo). What exactly you have to adjust on the translator depends on rev/model, and it's been far too many years since I've used one to be very helpful. That information is almost guaranteed to already be on this board however, or on the MAFt forums or in the MAFT documentation.