High STFTs at IDLE

[ralphyvisuals]

I have a 2016 Mustang Gen2
2018 Manifold *locked out*
PMAS Fenderwell CAI
Catless LTH
93 Tuned by PBD

Car has been running weird for weeks now I noticed when I'm at lower rpm (1200-2200 ish) car would sometimes buck and starts surging and AFR would spike to 15-16 on the dash (especially when car is still cold) I had to let go of the gas until it settles down and it would drive normal again after few seconds. I also noticed a delay of power when you go WOT and it doesn't pull as hard as it used to. I've sent logs to my tuner and he advised me to check for possible "vacuum leaks" because my STFTs are way too high at idle (10-17%) but goes down to normal at highway speeds (usually 3-5%) and WOT

At first I tried to diagnose the problem myself I've cleaned the MAF plenty of times, made sure clamps on the CAI are tight and secure, etc.

I decided to take it to 2 different performance shops that specialize mustangs for smoke test/diag to see if they could find the problem.

1st shop I went to on my way there I accidentally stalled the car at a red light and threw a check engine light and wrench light but as soon as I restarted the car the wrench light went away but the CEL stayed on, the shop checked the code and it was P0100 *Mass or Volume Air Flow A Circuit*. He smoke tested the car and found nothing and fixed the MAF connector he said the pins were bent and not aligned properly etc. and cleared the code.

2nd shop I went to they had the car for almost a day and did another smoke test/diag and found air leak around the injector o rings & at the back of the intake manifold.

So I went and buy brand new OEM intake manifold gaskets & injector o rings from the dealership and installed the parts myself, I took the intake manifold off and I made sure the intake ports were clean for the new gaskets and lubed the injector o rings.

After the install I noticed the STFTs (1-3% at idle) were perfect but then I remembered I disconnected the battery and it resets the fuel trims.
Took it for a test drive for couple of miles to check the STFTs and boom back to normal but this time they were lower than what they're used to be (5-10% at idle).

At this point I'm very lost and I don't know what to do next, I've also replaced all 4 o2 sensors and new oem evap purge valve.
What's next to replace? TB? MAF? Injectors? Fuel Pump?

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[ice445]

Short terms are hard to read because they change based on so many factors. What do the long terms stabilize at during a drive and at idle? And has the MAF ever been cleaned with the appropriate cleaner?

 

[mejohn50]

STFT is only half the story. Total fuel trims are STFT+LTFT unless LTFT is disabled. You can’t look at just one or the other. Also, it’s much easier to make sense of fuel trims if you log it and either do a custom math parameter in HPT where you add STFT and LTFT together or spit the data out into a CSV file and add them together in a spreadsheet.

LTFT are what the PCM thinks it needs to do based on some historical data and STFT are how much it’s correcting the fueling after the “initial” LTFT correction.

Example: if LTFT is -5% and STFT is 2% total fuel trims are -3%.

 

[ralphyvisuals]

Short terms are hard to read because they change based on so many factors. What do the long terms stabilize at during a drive and at idle? And has the MAF ever been cleaned with the appropriate cleaner?

My tuner turned off the LTFTs in the tune to see what's going on with the STFTs

I've used CRC MAF cleaner

 

[CarDad29]

If your LTFT is disabled and you're at 5-10% STFT at idle what's the problem?

Also keep in mind there are other variables that'll affect the trims daily (weather) and you've made other changes that could also be showing effect (fixed potential connection issue at MAF and swapped to a different manifold [I get it's the same but there are differences]). 5-10% trim isn't much.

 

[LOL WUT]

If LTFTs are disabled, anything 10% and under is generally acceptable at idle. Sure, closer to 0% would be great, but there is a lot that impacts your STFTs.

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