If you just got a new tune... I agree it's likely the culprit.
Disclaimer... I'm not a tuner... The following is based on my experiences with my own vehicle, and some experimentation with a couple different kinds of tuning software (Sniper, Ford Motorsport EPEC, SCT) ...
A good tune should have your STFT and LTFT within 5% or so. If you are using an SCT device this number is in decimal form.... Here's a quick synopsis:
1.0 is perfect... Meaning there is no trimming taking place.
. 85 means that 15% fuel is being trimmed out from the programmed value.... The car is measuring rich by 15% and the computer is taking that much out to try and keep the 14.7 : 1 stoichiometric ratio.
1.10 would mean the computer is adding 10% fueling... Car is measuring lean by 10% from the programmed value so it's putting that much more fuel in.
Low numbers = rich
High numbers = lean
Also the STFT and LTFT are cumulative overall... Meaning that if short term is at .85 and long term is at 1.10 then the net trim is 5% fuel is being pulled out... (15% out but 10% added = net of 5% out) Not bad... Probably runs fine most of the time... But..... The car will probably have some other issue along the RPM range like being rich or lean at a certain RPM... Or stumbling / hesitating while reving up (transient fueling issue)... Etc. The tuner should be trying to get both STFT and LTFT as close to 1.0 as possible.
The short term trim is dynamic and changes rapidly in response to real-time input from sensors. The long term trim is calculated by the computer and is more stable and can be thought of as predictive or anticipated fueling... Meaning that based on all the input from sensors, and the commanded / desired values in the tune, the computer LTFT is predicting what trim is necessary to keep the car running optimally in the near future... Like for when the engine may be revved up, or down...
The key in the above paragraph is the sensors readings and commanded values statements. Out of whack STFT and LTFT, which still land the net overall trim in the acceptable level (+/- 5%) will allow the car to run fine... But still indicate a tune that still needs revision (likely fueling or MAF maps.. And maybe timing maps) or there is a sensor problem (O2, MAF) or maybe a vacuum leak.... A vacuum leak throws all kinds of things off and the car can't really be tuned right until the leak is fixed.
A high LTFT number but with a close to perfect STFT number is a good indicator of a vacuum leak... Here's why... The STFT will be close to 1.0 because it's accurately measuring what is coming across the MAF and adjusting the fueling accordingly ... But... The vacuum leak is letting in air that isn't being measured at the MAF... So when the O2 sensors read that leaner mixture coming out the exhaust they tell the computer the fuel mixture is lean, and thus, is going to need to be richer as soon as the motor revs up and increases load... So LTFT value goes up.
If STFT is dead on, or close... But LTFT is way off... You likely have a vacuum leak.
If LTFT is dead on, but STFT is way off then you likely have a tune problem (MAF transfer function, fuel table(s), timing, etc.)
Hope this helps you with seeing what your car is doing... And also holding your tuner accountable if you see something off.
I'll be standing by for someone who knows more than me to call me out (forum pitfall).
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