Help me understand long term and short term fuel trims!

ok i understand that a + trim means its adding fuel(running lean) and - reducing fuel(running rich). is ltft the more important parameter to monitor?Ive read the atr help file and it says ideally u want 0 but + - 8% is acceptable. As an example, if i was seing my ltft fluctuate from -8.0 to +8.0 is that what they mean by 8%. Ive been paying most attention to stft and for brief moments ive seen them go into +14.8!! thats backing off going down a hill to maintain speed then light throttle once down hill to keep speed up, then they shoot up to.+14.8 for a few secs

thanks in advance mitch

Just monitor LTFT. The short term ones will feed into the long term trims, hence over time the LTFT will go down, and so too short term trims. All short term trims do is help meet the nominated AFRs in the tune.

And yes you want to be withing absolute 8%(plus or minus 8%)

so under all conditions idle, part throttle and wot your ltft ideally needs to be -8.0 to +8.0?

Idle don't worry bout too much, part throttle only. Under WOT the long teem fuel trims will be 0 as it will have transitioned from closed loop to open loop

WOT doesn't matter...
it's basically the ECU making calculations... Your maf cal says that at x volts on the MAF sensor, y air is flowing through, so the car puts in the relevant amount of fuel.

What ltft and stft show is how much fuel the ECU is adding/subtracting based on the reading from O2 sensor...

If your targeted AFR ratio is 14.7 and your O2 sensor reads 14.5 the ecu will remove fuel and generate a -ve stft.

Have a stft long enough, and it begins to affect the ltft readings

+/- 8 is a good range to aim for :)

Fuel trim from what I can understand so far is accurate when the actual and desired ratio are within .010 of each other, this ensures that the ltft you are seeing isn't effected by fuel enrichment or de acceleration. It's important to calibrate the Maf based on this to achieve 5% ltft or better, by getting the fuel trim as good as possible it's then easier to monitor any problems further down the track effecting fuel/air systems.

cheers guys, much appreciated, im starting to get a handle on it now, so would i be ryte in saying ltft are the ones to watch? however if the stft are in too big of a range wont this affect the ltft after awhile?

Thats why you need to run the car for a tank of fuel, through all throttle ranges and varying driving conditions to let the LTFT build up before doing a MAF calibration.

Gotcha:keg:

Ok went for a drive tonite monitored my ltft, at idle and deceleration i get -3.9, under light throttle i get mainly -0.8 or -1.6 however at light throttle at a certain throttle position i get -8.6 and if i gently apply more gas it will go back to -1.6. also i dont know if its worth mentioning but i never went wot but in 2nd and 3rd gear, but coming into boost a little i got 0.05 of knock retard is this bad?

What are you trying to do? Are you trying to calibrate your maf?

i havent tryed to calibrate it yet, there just results i got looking at my ltft, just want to knownif there in same regions, if so i wont bother doing a maf cal

Im not sure what software you use but cant imagine the process being to different, to get clear fuel trim information you need to have steady maf voltage and relative throttle position for a sec or more (for each voltage cell of maf calibration) its at that condition the fuel trim % is what your looking for, in the the case of using Versatune that % is then put into a work sheet where the maths is all done for you to give you the multiplier needed to correct your current g/s for that voltage.