Amen brother on the weirdness that is Audi (all German cars really).

The basics on long term fuel trim is that the ECU uses data from the MAF, intake air temp (IAT), coolant temp, and O2 sensors to try and keep the engine running clean and efficient. A high positive long term fuel trim suggests that the ECU thinks that the car is running lean, so it's adding more fuel to compensate. This could have many causes, but here's a few:

• Failing/clogged injectors that aren't delivering quite as much fuel as they used to. The ECU can compensate for this a bit, but as it gets worse, it will give you a CEL eventually.
• Failing/clogged fuel system. If there's a problem in the fuel system that's affecting fuel pressure or fuel flow, but not enough that you get a CEL or have other issues.
• Failing/clogged O2 sensors that are giving inaccurate readings. I've also seen an exhaust leak trick the O2 sensor and make it think it's running rich or lean.
• Failing/clogged MAF sensor that's reporting incorrect air flow.
• An IAT or coolant temp sensor that's reporting incorrect data.

Don't start just throwing parts at it till you figure out what exactly is going on. I'd give it a good dose of injector cleaner in the tank and a good "Italian tune-up" (drive the piss out of it for an hour or so). If you have a scanner, have a look at the readings from the sensors I mentioned and see if anything seems wonky.

 

I just noticed the same problem with my Mazda 3, 2016 with 2.0 Skyactiv. No CEL yet but the LTFT is always elevated. It is about +9 on idle and it goes up to +12 to + 17 when on load. The car run fine though. No hesitation or any noticeable performance issue. So far, the engine has managed to compensate the problem with higher LTFT.

I also cleaned the MAF and checked for leak visually but didn't find anything. I will run some more diagnostic work. Just wondering you may have figured out what's wrong with yours.

 

+LTFT means lean.. like a vacuum leak on the intake or a clogged fuel line/injector.
CK

 

Yeah, +ve LTFT means it's running lean; -ve LTFT, it's running rich.

 

Same issue for our 2015 Mazda 3. The LTFT is at 20.3% and sometimes throws a CEL P0171. The car shop already changed the MAF sensor twice, but the P0171 came back after a month. They recommended I go back to the dealership to check. If it is indeed a clogging fuel injector, would putting in higher octane gas help or make it worse? I plan to use some fuel injector cleaner as well. any advice is appreciated.