Fuel pressure drops to zero after engine shut-off

Faulty fuel pump return valve causing fuel to drain back to fuel tank , connect a inline fuel pressure gauge start engine , note pressure stop engine pressure could drop a few psi then hold , if pressure drops close to zero replace valve or pump .

Some models have a fuel pressure service port on the injection fuel rail. Test pressure running should be around 36 psi shut engine off if drops to zero then pressure regulator is leaking into the return line.

Pumping the gas pedal and turning the key on and off would not have any affect on the problem. And the fuel pump runs as long as you are cranking the engine, the fuel pressure regulator allows the extra pressure back into the tank.
The excess fuel came thru the injectors or the pressure regulator and probably would have even if you had just turned the key and nothing else.
The injectors are turned on by the computer based on several things including coolant temp and air temp. The fuel mixture should have been very rich if the air temp was 35 below zero, but it should have leaned out once the engine warmed up. The longer you crank an engine in any weather and it does not start, the more fuel is dumped into the cylinders.
Its possible you had one or more injectors that were stuck - froze open when you first cranked the engine.
I hope you did not damage the catalytic converter or the O2 sensors.

You probably should perform a fuel pressure test to see if the fuel pressure is bleeding off of the fuel rails when the engine is shut off. If it is, you could have a bad fuel pressure regulator or a bad check valve in the fuel pump module (which will require replacement of the module).

One way to test for this condition without any tools is like this:
cycle the key ON, wait about two seconds, then OFF, wait about 2 more seconds, and back ON again. Repeat his 3 or 4 times before turning the key to the CRANK position. If the engine starts immediately every time you do this, then the hard starting is definately caused by the fuel pressure bleeding off.

veral but my first thought was a leaky injector or fuel pressure regulator.
here is some detailed info, here goes.......
During engine operation, the fuel system is maintaining the required fuel pressure to meet the demand of the engine. When the engine is shut off , the fuel system should be at or near the same pressure level. The rate at which the fuel pressure drops can be an indication of an internal system leak (assuming no external leaking occurs) you don't have this as it would take only hrs to have a start problem.
Two most common areas of concern are the fuel injector nozzles and the pressure regulator. If the regulator valve has a poor seal, fuel will leak past the valve seat area and, since the pump is no longer running, pressure will quickly drop off or sometimes slowly. The causes for this could be a worn valve, weak spring or defective diaphragm. An internally leaking regulator can cause long crank times as it takes the pump longer to build pressure in the system.
The other potential problem area could be the fuel injector nozzles. The nozzle may be leaking due to a deposit build up on the nozzle or a worn out internal valve seat. In this case, fuel leaks directly into the engine's cylinder. Depending on the number of injectors on the engine, this could be a real problem if more than one injector is leaking fuel into the cylinders. This is a common cause of long crank times and hard starts after a short engine shut down.
One leaking and takes days to leak down.
Personally I'd start with replacing the fuel pressure regulator. Finding a leaky injector..try this. turn the key to the on position wait 5 minutes look at the injectors see if there is one that looks wet or has deposits around it.
You can also do a sniff test (not while running!) sometimes you can get a smell of gas from the leaking injector. (if the wind isn't blowing) Actually try this first before replacing fuel pressure regulator. If it starts up quicker after the 5 minute key on one of the above is the answer.
hope i didn't confuse you.
Best of luck.
Stay warm tis -5f here.

Key on engine off pressure on your truck should be 60-66lbs. A fuel pump replacement is definitely in order, bad thing is it is a whole assembly.

You should hear the whining coming from the pump when the key is turned on.If you suspect he fuel pump check fuel pressure if possible or try hitting the bottom of the tank and see if it starts then.The Vortec 4.3 requires a minimum of 54lbs of fuel pressure to start.It will run on less once it is running.Will it start with an alternative fuel such as throttle body cleaner?

you may need to get the fuel pump tested for proper voltage and pressure

If the engine has to be turned over a number of times before starting it may be fuel pump related, more overly the check-valve within the fuel pump. If this has failed (or leaks back) the fuel system will need to come to pressure before the engine will start. I'd check your fuel line pressure when running you should see @ 40PSI or so; once shut down the system should hold pressure; if it drops off to zero then you know where the issues lyes.
I hope this may have helped.
Tom

I am not really familiar with the setup there but I'd be looking for a second relay to kick the pump on when cranking.