Warm Starting Issue
2003 Z06 with the factory fuel system unchanged except for 96lb Bosch injectors. The car has an iron block and a UPP twin turbo kit.
Also running a UPP secondary fuel system which consists of a separate line off the fuel tank, to a AEM 380 pump (set up on a Hobbs switch) which feeds to an Aeromotive fuel pressure regulator. The regulator then feeds through a check valve and on to the front drivers side of the factory fuel rails (Schrader valve is removed of course). The bottom of the regulator returns to the line coming out of the tank (between the fuel filter and the tank)
I have a fuel pressure gauge on the regulator itself and another on the factory fuel rail. Other helpful things to know would be an optima red top battery relocated to the trunk. 100% 0 gauge copper wire running from the battery up to the front and connected to the two factory connections (red wires that used to go to the positive terminal when the battery was in the factory location). The battery is grounded with a 0 gauge wire in the rear.

Issue:
When the car is cold, it starts just fine. Cranking is fast and it fires right up, 99% of the time. Before cranking, fuel pressure on the factory side of the rail is 0psi. Soon as I turn the key to ON, pressure is back up to 52 psi.
After driving for a bit and everything is up to temperature, the car turns over/cranks just fine but struggles to fire. I have tried cycling the key to ON 3-5 times before trying to start with little luck. After driving around and letting the car sit for about an hour, fuel pressure on the factory side of the rail drops to about 16-18psi. If it sits long enough, it will go down to 0 psi. Soon as I turn the key to ON, pressure is back up to around 52psi. I have noticed that battery voltage when cranking drops down to 8 on the gauge cluster (8 is as low as the gauge reads). If I put the throttle to the floor, it will eventually start, so this leads me to think it’s a fueling issue.

Possible causes I can think of:
Starter solenoid is getting too hot?
- If the solenoid was getting too hot, I would expect it to crank slower which is doesn’t. I also don’t see how this would explain why when I put the throttle to the floor, it fires, unless it’s simply because I’m flooding it to fire.
Fuel getting too hot in the factory lines/rails and experiencing fuel vapor lock?
- I haven’t really heard of this in fuel injected vehicles. I guess I could see that even though the fuel pressure gauge shows good pressure, it doesn’t necessarily mean its liquid fuel. It could be vapor in the lines and still show good fuel pressure.
Battery has a bad cell?
- Doesn’t make sense since it fires fine when cold, voltage is 12.1-12.5 with no load, and drops to 10.5 when cranking
- I need to check the voltage on the warm cranking to see if there is a difference.
Bad check valve in the factory fuel line?
- Seems possible but I have full pressure return to the rail once I turn the key to ON, just before attempting to start so that makes me think this isn’t an issue
- I could try to install a check valve in the factory fuel lines?
The Tune?
- I have had 2 tuners look at the issue and neither have been able to figure it out; all seems normal in the tune for cranking fuel

Any ideas?

 

Had same issue with my DD ...it was a very dirty throttle body.