6spd coupe cranks but wont start, when it cranks the headlights get super dim and the rpm's on the combination meter don't move at all. it has spark, and there's gas in the fuel rail. the last time i tried starting it i recorded the battery voltage and it started at like 12.7v then when i cranked it, it dropped down to 8.4v then it jumped around between 8.4v through 9.8v and after i stopped cranking it it went up to around 11.8 v. not sure whats wrong

 

Sounds like the battery is getting weak, take it down and have it load bank tested. However since it DID CRANK but the rpm's didn't move at all that sounds like a failed crank sensor. Replace with an OEM or Hitachi sensor, don't bother with any other brands because they usually don't work or don't last long.

If it cranked SUPER slow then it might just be the battery.

 

does it make a difference if i can see the rpm's when i hook up to the obd2 port?