Ok so ive had the beloved 7 for about 3 months now and everything was good but one day as i was driving it the lights began flickering i made it home and checked the alternater and sure enough that sucked was dead. Replaced that and tries starting it up and ....it started but only for a few seconds then it began to die and would only run if i held the gas down but would backfire constatly and had absolutedly no power and when i tried to put it in gear would outright die. Suspecting the ecu i replaced it but the problems continue i am realy descuraged and could use any help...thx

 

After killing the 1st alternator, did you fully charge the battery before using the new alternator. If not, you could risk frying the replacement. Cars don't run well on dead batteries.

Since your alternator died, I'd go through the entire wiring harness. Check all fuses, relays,.....sometimes certain electronics get fried when an alternator quits.