Cold morning, crank, wants to start, but won’t
 

1999 blazer with a 2003 motor. 275k miles. Sunday, it was 55f outside, I hit the remote start and it was cranking for awhile and fired up(2-3 seconds) Tuesday comes 42f and it keeps cranking, I go outside freezing myself and it cranks over and fires 1 or 2 Times but won’t start up, voltage is still 12.3 or so, I keep trying and trying and nothing, I cycle the key 3 times to build pressure in the fuel lines and nothing happens, I hear the pump running, no security light. The starter was just replaced, as well as the timing chain. The only way I got it to start up is from a jumpstart, the voltage jumped to 14 and I turned the key and it started up like it was at operating temperature and idled at 1400 rpm for a couple minutes. I don’t have a knock sensor yet because the 99 sensor doesn’t fit the 2003 motor bolt hole, also the crankshaft sensor os giving a code. Never had this happen before, but the motor was swapped this summer.

Voltage during cranking?

Ground strap maybe?

Ok
 

dips just below the 12v mark and I’ll check all my grounds but just odd that when it’s getting a jump it starts right up, you might be up to something there