I have a 2004 Gen 3 LS6 in my ‘68 Corvette. The car has been tuned and runs amazing. Everything works as it should but I can figure out why it takes priming the car twice for it to start.

If I turn the key and let the fuel pump fully prime, when I hit the starter she just cranks and cranks and sometimes it will give a pop or a back fire.

If I turn the key and let it prime, then turn it off and prime it again, when I hit the starter she fires right up.

I have a fuel pressure gauge and she primes to 58-60 psi each time.

Any ideas? It more annoying than anything.

 

Sounds like the FFR is draining back? What are you running for a FFR? I have a 98 Wrangler with a LS6 swap and 80 Camaro with a LS3 swap. They do the similar to yours if they sit for a couple weeks. But they never failed to start and run well. On both I'm running a C5 FFR.
I would put a fuel pressure tester on the fuel rail to see what is happening at the start.

 

Pretty sure the Fuel Pump is supposed to be running even when the Key is turned to Start.

Something is off somewhere, maybe a setting in the tune.

 

That could be it. I think I need to research that some more. Thanks

 

Just for troubleshooting what happens if you...
1. turn the key to ignition
2. wait 5 seconds for fuel pressure to build
3. turn the key to start

Two of my cars have aftermarket fuel pressure regulators, and they both require that pause at step 2.