major power loss

[flipjbm]

hey guys, just yesterday i finished putting some prothane motor mounts, 2g exhaust manifold, big 16g, and some 550cc injectors on my car, well when i got done i made sure everything electrical was hooked up....when i went to start my car it felt like the battery was dead, it wouldnt crank at all, so i jumped it off but when the jumper cables where on the battery and hooked up to my friends car it still felt like it was dead even after 5 mins of sitting....well somehow it eventually got power and started, it sat there for a while idling and i was gonna go for a quick spin and as soon as i hit the brake the car dies as if it lost all of its power, well we tried to jump it again and it wouldnt start again

today i got around to investigating it a little more, when the car is off with no key in the ignition it reads 11.9-12V which i figure is good considering its a 12V battery, but as soon as i put the key in and turn it one click it jumps down to 4V, you can hear my fuel pump just dying in the back b/c theres no power, but when i take the key out it jumps back up to 11.8-12V, ive gone through the steps of checking all the under the hood fuses and the in car fuses, ive checked my grounds, i even disconnected my amp and cd player...the only aftermarket electronics i have hooked up right now are my fuel pump, greddy turbo timer and the bulb for my boost gauge

i went out today and checked all of my grounds, its almost perfect voltage on everything, i took the alternator to autozone and they said it failed the test, so i got a new one just to make sure that wasnt it, i still get the same problem

im really stumped on this one, what could be draining that much power? anyone have any ideas for me to check?

thanks in advance

 

[SlowdsmGST]

Your batter has a bad cell. Sounds stupid but you average batter hold 14. Volts and when you crank it goes to 10v if it goes below 8 it will not start. I work at Autozone as a manager and its a common thing when a car just sits. I would try to get a batter test done. Normaly if the Alternator goes bad its a good idea to replace the batter as well.

 

[TalonESIT]

my moms car had the same exact problem and it does sound like a battery issue, enough juice for acc. but not enough amps to start the car, almost positive that it will be fine after you change your battery

 

[prplhz96]

I agree, it does sound like a battery issue. Now you have to find the cause for the dead battery, could be just a "bad" battery or a bad alternator even. I would start with a new battery and go from there.