Increasing positive LTFT on idle
 

I'm trying to troubleshoot an LTFT behavior only at idle where LTFT tends to fluctuate a lot, around 3% to 9%. This happened when I recently changed from a 2013 decatted header to a 2017 decatted header.

Previously it used to remain about 2% to 3%, and the rest of the LTFTs in other RPM ranges are about 1% to 3% unchanged. Now I understand this isn't 'much' by any means but I'm trying to understand the possibilities/reasons on the change as it hadn't happened before.

What I've done to diagnose so far:

1. No audible air leaks at exhaust and intake. Car is NA, unclamped hoses at intake look good and tight.
2. O2 sensors are screwed tight. Voltages are at 2.2V at front and 0.7V at rear, will need to reconfirm.
3. ECU reset does not change anything (for the heck of it, why would it). Tune is unchanged on Wayne's tune with the same MAF scaling.
4. Intake tubing past MAF is OK, no cracks and clamps are tight.
5. MAP is reporting approximately 33kpa (~0.33 bar) on idle.

Anyone with any ideas? Thanks!

One thing I've seen with the Cobb OTS map on my WRX is at idle, I have positive LTFTs at idle constantly. They'll vary up to about 12-13%, but are always at least 7%+. I've had the car smoke tested for vacuum/boost leaks, new front O2 sensor, new MAF sensor, car drives fine, etc.

The thing is, at idle, the percentage difference the ECU needs to add or remove is larger than when the car is moving and there's airflow through the intake at even 2k RPM. Generally speaking, seeing higher LTFTs are idle isn't super terrible, however a tune or tune revision (if you've already had it professionally tuned) can adjust fueling at idle to fix this. Just make sure all everything is 100% mechanically sound from a leak perspective - you obviously wouldn't want to tune around a clear leak somewhere when you should really fix it.

Cliff notes: It's probably fine and you don't need to super worry about it at idle. If it does max out and the ECU can't add anymore, it'll throw a code and it'll be at idle. Your fueling is likely ok in as you give it any throttle or load - concentrate on those numbers more. A professional tune can fix it if it's on OTS tune, or the tuner needs to spend more time adjusting fueling at idle if it's already protuned.

Thanks. I'm aware it's not a 'problem' by any means, as it is not affecting driving in any way - idle is stable and smooth, and LTFT is back to nominal values once out of idle. Since LTFT is naturally very sensitive during idle due to the very small amounts of air ingested, the LTFT movement is explainable however the only thing I can't figure is that it didn't happen before the header swap.

If I really needed to fix it, a slight adjustment to the MAF scaling would work but if the ECU could handle it then why not.

Assuming if I missed out a potential leak - likely the only area to take a deeper look would be between the exhaust ports and header? Also, the shop mechanic snapped a stud at the overpipe to header (spent half a day with an angle grinder and drill) but since that is after the O2 sensors there isn't much of a concern.