Battery drain

So every time I go to start my car my battery drains by 50% I just in stalled a new starter, alternator is good fuses are good, I just installed a new IAC sensor, as well. I need to figure out what's killing my battery. Battery is brand new. Seems like I'm not getting enough volts but don't know where to look at this point. I know I have a failed injector seal that's been dumping gas into cylinder 4. So it's struggling to spark and get the rpms up in order to run. At the same time my volts drop instantly. Any help or leads would be appreciated.