Hi Scotty,

I've been greatly enjoying your YouTube videos for years now. I wrote you once about this problem but couldn't find the reply if it was made in your videos. I bought this 2012 Mustang, 3.7 v-6, 6 speed automatic from a local credit union, repo'd, and ~68,000 miles on it. It currently has ~112,500 miles showing on it.  It ran perfect and strong and I've loved it since buying it. I'd purchased a 4 year/48,000 mile bumper to bumper warranty at the time.  When I first got it, it had a brand new battery in it but it would go dead if I happened to not drive it for a couple of days. I took it to the local dealership after having to jump it too many times and then recharging the battery. They found the alternator output to be on the low side and installed a new alternator, $550 , but thankfully the insurance covered it 100%.  I believe the alternator also had a bsd diode allowing the battery to drain.  Shortly afterwards I began noticing fuel mileage dropping off significantly.  Where it had consistently gotten around 32 mpg it dropped to 25-26 had a fuel smell and began 'lugging' the engine' when you'd step on the gas to have it downshift and pass or speed up. Let me say now that the engine idles perfectly and runs great always ( other than the lugging and downshifting issues ), and doesn't show any trouble codes.  It did set a couple of trouble codes at first,  both upstream O2 sensors showing problems and another I can't recall ( it was a secondary code related to the sensors and was resolved when they were replaced ). I had the O2 sensors replaced by a reputable older mechanic I know whom I  trust.  After he'd replaced them a few days later it reset the code for the upstream sensors again.  They were replaced and a new set installed again ( Autozone stood behind their product and replaced them).  I installed a new charcoal fuel vapor canister and emission sensor/valve under the trunk. Replaced the throttle body with a new one. Put in new proper Iridium plugs, properly gapped, changed all emission sensors atop the engine, had new catalytic converters put on it by my mechanic friend.  Heck,  I don't know what all exactly I have replaced  (or haven't) so far.  Thankfully my insurance coverage would reimburse for the vast majority of the costs.  I've been to the local dealership far too many times to count.  Always told "we flashed the computer", that should do it.   It's eventually gotten so bad that I've been having to manually downshift when I go to pass or speed up.  Otherwise the engine lugs until I just put my foot way into it and finally get it to downshift.  I've taken it to other dealers also and one finally said "yes I can feel what you're describing but don't know what could be causing it.  It may be the lock-up torque converter".  Of course my 4 year warranty expired in August of this year.  I'd replaced the factory air filter at first with a K&N factory replacement, then installed a cold air intake with a new oiled air filter.  This helped some but yet again the mileage began getting less and less and the lugging worse. It seems to not be getting enough air, causing it to lug.  Several weeks ago I noticed it was driving much better and then noticed the rubber mount attached to the air filter had ripped open allowing much greater/less restrictive intake of air into the throttle body.  I've also replaced the MAF sensor which seems to have actually made it worse.  I've always completely grounded the system at any time I've replaced anything and let the system re-learn but to no avail.  My mileage fluctuates between 19 to maybe 23 mpg currently.  Of course I feel that since my warranty coverage has expired if I should take back to the dealership they will likely 'magically ' be able to determine what is the cause and could finally repair it, now that I would be paying for it after over 3 years of them doing nothing for me while it was covered.  Are there any ideas that you could give me of what I might look at that could possibly be the cause?  I've thought all along that the constant jumping and such when I first got the car could have harmed the ECM or such, and might possibly be the cause.  My mechanic friend seemed to think the same after he'd checked and diagnosed the entire car over with his expensive computer system.   I'd often thought to myself " Boy, I sure wish that Scotty Kilmer was nearby to me where I could let him take a look".  I'm located in Northwest Georgia, not too far from President Trump's rally last week in Rome.   I have children and grandchildren in middle Tennessee that we frequently visit.  But regardless,  I would be extremely grateful if you could give me some ideas if what might be causing this.  Thanks Scotty and God bless always. 

p.s.  I may have forgotten some if what has been done on this and should I recall further items or services I'll update you.