LS2 E38 hard start when hot-fuel pump not priming?

[rtagg]

I have an '07 Corvette LS2 with an E38 ECU in a Fox SVO Mustang. It starts great when cold or even when hot if I start it again within a few minutes of the initial shut down. If it sits for longer periods (20-30 minutes), it will crank for at least 10 seconds before firing. If I apply any throttle while cranking it gets worse, so I?m assuming that the heat soak is evaporating the residual wall fuel, and it's cranking lean, needing fuel. I have an Autometer fuel pressure gauge on an Aeromotive regulator under the hood, and when I turn the key on, if I can hear the fuel pump priming (I have it set for 4 seconds) I get 58PSI on the gauge. If I shut it down and go to restart it right away or even a few minutes later, I hear the pump priming and the gauge goes right back up to 58 and the engine starts fine. After sitting for 20 minutes or so, when I turn the key on, I don't hear the pump, and the pressure gauge will creep up to 10 or so. If I crank it then, the gauge doesn?t move until startup 10-12 seconds later and the gauge then goes up to 58. I?ve tried two different sending units with the gauge and it does the same thing each time, so I think the gauge is correct.

Shouldn?t the ECU command the pump to prime each time the key is turned on? If not, is there a table or option to adjust it?

Thanks,
Randy

[blindsquirrel]

What fuel pump? And why is it not holding pressure when shut off?

When it does the long-crank/no-prime thing, can you get it to prime if you keep cycling the key (without cranking)? Have you checked to see if the relay is activating when you get the no-prime condition?

[rtagg]

The fuel pump is an in tank Walbro 255 LPH high pressure. With my old setup (Ford 4cyl turbo and AEM infinity ECU) the pump would prime whenever the key was turned on. I'm using the same regulator as before, so the only real changes to the fuel delivery system have been the ECU, the wiring harness which is new, and the higher operating pressure of the LS (58PSI as opposed to 40PSI). I don't have any idea why it isn't holding pressure, but since it isn't starting rich after the long cranking period, I pretty much ruled out leaking injector(s). The best I can figure is that the pressure is either bleeding off through the regulator or back through the pump. I can swap out the fuel pump relay tomorrow and see if that has any effect.

[blindsquirrel]

The pump should have an internal check valve, and the regulator should have the return line shut down tight anytime pressure is below the set pressure, engine running or not. The pressure is going somewhere, if it's not the injectors and it's not pissing on the ground it has to be either the pump or the reg. That may not be related to it not doing the prime at key-on, but, it should still be holding enough pressure after only 20 minutes to fire even without a pump prime.

Replacing the relay won't do anything. What I'd want to know is if the ECM is really not commanding the pump prime only during this one specific set of circumstances, or if it is commanding it and the pump just doesn't do anything. Checking for power at the relay output and/or ground trigger to the relay from the ECM would give a better idea of what's happening (or not happening) when it's screwing up.