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Discussion Starter #1
I have an 05 Mustang gt. When it gets really cold, mostly when it gets to snowing temps, i can be just cruising at any speed, any rpm, and it will keep hesitating or stalling. it wont actually die but it will be like i suddenly let off the gas and hit it again or like i turn the key off and on again. one time it was so bad it kicked me into fail-safe mode. when it gets midly cold i can be cruising and it will sound and feel almost like its running out of gas. and it will always throw running lean codes if its cold. but i know it has to be something related to temp because when its warm it runs great. Any thoughts?
 

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theres a TSB on the early fuel pumps, in extended light cruise,(like >20 minutes on the highway) the fuel pump churns enough bubbles to suddenly lose pressure when the throttle is hit- mine does it like at tollbooths on the turnpike, gets kinda violent sometimes- barks tires/shuts down repeats a couple times... if I clutch, throttle, decel it wont do it- or as it does at least it dont startle you if its not engaged :)

if it does it just intermittently without >20 minutes steady state cruise, then I'd say put a new fuel filter on it- even if its not that old, one tank of gunky gas can clog them up...had fuel filters go 20 years, had some go 2 months...
 

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Discussion Starter #3 (Edited)
i saw that tsb too, which my car does do that even when its warm(though not very badly, just when im cruising and i punch it it hesitates for a second) so i know i do need to replace the fuel pump. but the problem is that it only does this when its cold. and the colder it is, the worse it is. and its not even when i let off the gas. i can be driving around town, accelerating or cruising, or have the cruise set on the interstate and it will just lurch/hick-up and the colder it is the more it does it. and i can reset the lean code when its warm and the next time it gets cold and i start having problems, the code comes back. its got to be some sensor. otherwise it would do it all the time. im just new to cars this high-tech so i wouldnt even know where to start. on an older car i would say replace the MAF sensor and/or the IAT sensor and your good but i dont know here.
 

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I dunno squat about these things, thats for sure, but- maybe one of the heated 02 sensors is failing- those have a lot of control over mixture, if the preheat part is screwed up might explain cold start trouble...in any event, if youre getting lean errors, be careful- if its truly running lean theres a real risk of destroying the engine in a hurry...

might also look closely at your airfilter box/intake tube-especially bellows area for loose fitting or cracks downstream of the MAF...if the MAF sees less air than is truly being inhaled, it will pull fuel accordingly and risk a lean meltdown... the way these newer car MAFs are so close to the airbox, a pinched or loose air filter gasket will really screw them up too.

again if its saying lean, I would figure it out asap even if you need to take it in...a warped head/gasket/burnt valve would make whatever it needs now seem cheap- a true lean condition in a little 300hp aluminum engine will definitely hurt it.

in sig, you mention full bolt-ons...are you running a tune? are rear 02's turned off? tried returning to stock (if cai is ok with stock)
do you have a codereader you can monitor with? mines shows fuel pressure- maybe have a passenger watch the pressure/02 values, etc while its acting up...could just be a fuel pump/o2 wire connection failing/thermally aggrivated or something silly like that...
 

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Discussion Starter #6
Just doing a follow up. Turns out it was two things. One, when I extended my o2 sensors for my longtubes I just spliced in wires(per included instructions)and the car didn't like that. I replaced them with actual plug in extensions and that fixed the bucking and stalling. Next, when my programmer crashed and sct "fixed" it, they actually screwed it up. Still waiting for them to fix it completely.

Lesson learned: extend your o2 sensors the right way. spend the $20 on extensions.
 

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i have the exact same issue.....bucks,hesitates when crusing on freeway when going from cold to hot.......closed to open loop! u said the reprograming and 02 fixed it? i had mine chiped but no o2 issue and no codes. only does it for about a mile after it starts then its fine. 05 mustang gt 31000mi. i did scan it and had no codes and all mode 6 data was good. one fuel pump was replaced under warrantee. when doing a koer test it said 02 and oil temp sensor were out of test range, not warm enough, ran it and repeated test and everything passed. ill focus on monitering o2 and taking the chip out. just wanted to know if that fixed your problem or if it returned?
 

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on the other post o2 do not control injector pulse width but they do determine short and long term fuel trim, the crank sensor controls injector on time
 
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