2018 Mustang GT Long Term Fuel Trim

[bschoon55]

Hey Guys,

2018 Mustang GT here, 6-Speed Manual with 20k miles on it. For quite awhile I have thought this car doesn't run quite right, it sounds terrible with light throttle application at low speeds. I've taken the car to two Ford Dealers.... one of them I went to 3x and the lead mastertech told me to go somewhere else and at another dealer they failed to follow through with their promises of looking at my car.

It's had a total of 4 trips to the dealer and every time they will not put the car on a lift, hook up a scan tool... NOTHING. I bought myself a scan tool and started running live data on the car. No codes, no faults.... BUT on the live data, I noticed my long term fuel trim is always pretty deep into the negative side.

On idle my Long Term Fuel Trim 1 is always at -3.9% and Long Term Fuel Trim 2 is at -6.25%.... when I was driving around Long Term Fuel Trim 1 got to -10.1% and Long Term Fuel Trim 2 is at -12.5%.

I'm not sure if that's too far into the negative or not, but it seems pretty far down to me. What do you think? What should I do from here?

(The Short Term Fuel Trim btw stayed right at about 0 the entire time).

Thanks for reading everyone!

Sponsored

 

[ice445]

Ideally it shouldn't be more than +/-5%. Have them update the PCM to the latest revision if they won't do anything else.

 

[Jackson1320]

Hold up a second. The long terms are-5 to-12? And the short terms are 0. Your short terms should be moving almost always a the long term should start moving after reaching operating temperature. The s/t and the l/t together should be +/-5% no more. The long term desides what is needed from the s/t so odd. Was it purchased new/used is it tuned. Have you tried o2sensors. Something is off

 

[Cobra Jet]

I was also wondering if the car was or is tuned.

What is not making sense (to me) is that the Service Centers are not "diagnosing".

OP:
Is there a reason they are giving you as to why they won't scan it or place it on a rack to diagnose?

 

[General Geoff]

Dealers like everyone else are short staffed and they just don't have the manpower to take care of everyone. Is it right? No. But that's just the way it is right now.

 

[bschoon55]

Good evening everyone, sorry for the lack of responses by me... I wasn't getting notifications and never came back. The excuse I've gotten from dealers is basically, if there are no lights on the dash... they aren't looking at anything.

After some more time with my scan tool...... on idle my long term fuel trim is -4% to -6%, while driving it goes from -10% to -14%.


My short term trim on idle is at 0%... but once I start driving or put my foot on the gas pedal, it goes ALL over the place from +5% to -5% and the number spins so fast either way it's hard to keep track of.

My scan tool has also been saying there have been numerous misfires as well, BUT I have never had a check engine light on my dash and I have ZERO codes or faults on my scan tool either. I have run the car in accessory mode and the check engine light does work. In fact, my scan tool reports that Ford has the car setup to where I will not get a code for the fuel trims unless it goes +35% or more or -35% or less! Crazy.

The car is completely stock with the exception of just a small MBRP Axleback Exhaust. I bought the car used, but was promised nothing was changed. I have noticed when letting off the gas or downshifting, the backfire has picked up quite a bit.

The car sounds super cool, but I know she isn't running right. The timing is off with something or we're about to break who knows what. I complained to a dealer enough that they are going to hook up a scan tool to my car on Tuesday. I plan to go to the dealer with my scan tool plugged in and I'm going to show them my numbers before turning the car over to them.

Something is wrong, I just don't know what.

 

[bschoon55]

Also

I was also wondering if the car was or is tuned.

What is not making sense (to me) is that the Service Centers are not "diagnosing".

OP:
Is there a reason they are giving you as to why they won't scan it or place it on a rack to diagnose?

You've really made me wonder something though... I bought it used from a Ford Dealer with only 7k miles on it... they wouldn't sell it as a CPO which was interesting, it was just as is......... Would you say it would've been possible for the original owner to pull his tune and everything he modded on the car and put it back to "stock" and tell the dealer it's completely stock......... they put it on their used car lot, I buy it two weeks later and that's what it is?

I mean there's a SLIGHT chance something could be messed up from a tune I never knew about and it might be one recalibration away from being ok? Makes you wonder if that's possible.

 

[Jackson1320]

Well you’re stft are perfect and your LTFT are not to far off. The stft and LTFT should add together to make +5% to -5% so -3stft and +4ltft =+1

 

[bschoon55]

Got back from the dealer. They hooked it up to the Ford IDS system and it has a broken purge valve that is stuck closed. They told me they could get it fixed within an hour and they had the part in-stock, then they realized about 10 min. later they didn't have the part so now I have to come back another day when they have the part.

I'm nearly 100% positive this thing has been messed up for at least 4-5 months already.

 

[PaddyPrix]

Got back from the dealer. They hooked it up to the Ford IDS system and it has a broken purge valve that is stuck closed. They told me they could get it fixed within an hour and they had the part in-stock, then they realized about 10 min. later they didn't have the part so now I have to come back another day when they have the part.

I'm nearly 100% positive this thing has been messed up for at least 4-5 months already.

Good on you for being persistent.

For what it's worth, if you have a laptop (and it can be a super cheap shitty windows one, or a 10 year old one) grabbing a USB<>OBD-II cable, and running something free like ForScan is the best bang for buck on the planet. Something like that you could put your LTFT+STFT up on a live stream, go take it for a quick drive, and then pull off and save or just replay that to catch it, and perhaps all the other related sensors. Having all that data makes it that much easier to tell them exactly what it is and have them fix the part, or at least know what's up before they try to sell you a bunch of crap if you're out of warranty.

 

[Toretto]

fingers crossed for you that this will solve the lingering issues you've been having

 

[Cobra Jet]

Got back from the dealer. They hooked it up to the Ford IDS system and it has a broken purge valve that is stuck closed. They told me they could get it fixed within an hour and they had the part in-stock, then they realized about 10 min. later they didn't have the part so now I have to come back another day when they have the part.

I'm nearly 100% positive this thing has been messed up for at least 4-5 months already.

Well good to hear and now you know what the issue is from a diagnostics perspective. You probably could source the part at another local Ford shop or online and do it yourself, but it's better to get it done under warranty to have a record of the fix.

To answer your other question about buying a previously owned Mustang that may have been modified (tuned) - and the owner removing to return to stock...

Yes, it most likely happens more times than anyone knows. The problem is, it can cause issues with the next owner as well as any Servicing Dealership when that car has to go in for a repair. The same is true when prior owners change As Built data using Forscan. It's becoming a problem and Ford has issued a few SSM's to the franchisee about it.

Hardware modifications (exhaust, wheels, suspension, blowers, etc) are easy to spot. The software modifications (loaded tunes, As Built mods, incorrect removal of tunes or corruption of BCM modules) is harder to detect on used S550's more so if a fault isn't immediately present.

 

[bschoon55]

I called around to a couple more Ford dealers and did find someone with the part I needed. They installed it yesterday and the car runs the exact same. Hooked up my scan tool and the fuel trims are just as bad as they were before... so the Purge Valve did nothing.

Two dealers hooked it up to Ford IDS and said Purge Valve otherwise the car is healthy and it turns out the purge valve did nothing. Oh well... I guess?

All I'm doing is wasting time and getting mad, so I guess I'll just leave the car alone and maybe it'll just run like this and it is what it is.

 

[Toretto]

Have you ever tried doing a KAM reset?

It's worth a shot and you can do it yourself....

1) Disconnect positive and negative battery cables from battery
2) Connect the positive and negative cables together (use gator clip or booster cables to keep them together) - make sure your battery is covered so none of these cables make contact. Leave this for 15 minutes so that the car can go into sleep cycle.
3) Reconnect the cables to the battery, lock the vehicle, and walk away with the key for 15 minutes
4) start the car, revving it every 30 minutes until car idled for 5 minutes
5) Drive the car, slowly ramping up on speed/intensity for 15 minutes

The above effectively resets all the modules in your car, discharging the capacitors, resetting all the modules and sleep cycle. No harm in trying.

 

[LouisiAnimal]

I’m having this same issue. Random misfire on startup and misfire on 1. can feel an mis about 1800 rpm. Ltft -20 STFT stays around 0. Tuner said my injectors might me stuck in open. Got em cleaned, Cleaned injectors, new coil pack and plugs, purge valve seems to be working, changed upstream o2 on that bank. Idk what else to do from here. IMRC locked out.
17 gt w/18 ported mani, catless headers, full exhaust, t56 swap.

Sponsored