Ok, I've got a 2007 Mustang GT, 5spd manual. Current mods: C&L CAI, Corsa Exhaust, Motoblue UDPs, and a 91 octane tune from Brenspeed.
In 2nd gear at 5,000rpm and in 3rd through 5th at 4,000rpm. The car starts to backfire really bad, and the car stops making power. I took it to the dealer. They told me it was an air to fuel problem, and that it should stop if i put the stock intake back on (this was before the tune). I put it back on, and it still does it. They look at it and replace the spark plugs. It works fine for a week, then it does it again. I thought it could also be a timing issue, so I got the tune. I programmed the car with it, and the car still backfires. I deprogram, and take it back to the dealer. This time they say it was a problem with the MAF sensor. So they replace it. I get it back, and it still backfires with stock tune and stock intake. Any thoughts on what could be the problem? Btw, the car only has a little over 8,500 miles on it.

 

Also, the check engine light flashes if u hold the rpm at those ranges. It does not stay solid.

 

Could be a coil pack or even the mass air flow sensor going bad...if you can get the check engine light to stay on an obd II scanner should be able to identify the problem...

 

I thought a flashing CEL was caused by a misfire or multiple misfires. Check the plugs and packs.

 

its not the MAF sensor, they already changed that

 

try checking your coil packs or plugs. i had the same problem. kept fouling the #1 cyl. plug. i guess it was just bad gas cuz after 3 plugs it never happened again

 

Wait so did you run the intake without a tune for awhile?

 

yeah, but i took it off after the backfiring problems. its been in twice since i took it off

 

$hit! hoped everything is alright.

 

I had that problem. And everytime it was the spark plugs. One time it was a loose plug. Another time it was that the plugs weren't gapped right.