Suburban hard starting when hot

I have a 99 2500 suburban with the 7.4L vortec engine . I have own it since it was new and im having starting problems with it . The problem is the engine starts right up like it should when its cold -like sitting overnight , But if I drive it and get it warmed up if I turn it off and let it sit for a few hours it cranks over for about 5 seconds before it fires up , when it cranking it sounds like it want to start but struggles,if I give it some throttle it seems to help it start . If you go on short trips it starts right up after sitting for 20 minutes or so . Its only when it sits for 3 hours or so I have the problem . I watched the exhaust when it happens and when it fires up it seems to be very rich black smoke from the pipe so a second then clears. I think it is a vapor lock problem or something to do with the Arizona gasoline we have here . I have heard the gas we have here in the summer can cause problems like this but this is the only car that does it for me . I am a mechanic so I have checked all the normal things , I did replace the fuel pump with a new Delphi unit and no change at all . Fuel pressure hold steady over night . There are no codes , I hooked up my OBD II scanner and all sensors are working as they should , Any ideas ?

Sounds like fuel injectors are leaking. Put a fuel pressure guage on the fuel line and check the pressure. shut the engine off and watch the guage. It should hold steady. If not there is a leak. Preasure regulators also like to leak. Regulator most likley.

Did you run some throttle body cleaner through your engine? I normaly would run some throttle body cleaner through the engine while its running, you gotta give it some gas while your doing it to keep the engine alive, otherwise it'll stall. That should get rid of the black smoke. Works almost everytime I have the black smoke problem.

I did clean the throttle body last year when I replaced the intake gaskets due to coolant leaking from the gaskets , I have ran injector cleaner in the fuel tank but have never done anything else . I will put my fuel pressure gauge back on and check the pressure when it happens again . I think it might be a leaking injector but im not sure .

I hooked up my fuel pressure gauge last night and running is 53psi , engine not running but pump on for 2 sec is 65psi . engine off pressure 55psi . I did a 3 hour test on the holding pressure in the rails , started out at 55 psi , after 15 min pressure dropped to 48 psi , after 1 hour pressure started climbing back up to 51 psi for the remainder of the time pressure was steady at 55 psi , I think the pressure climb was due to the arizona heat of 112 yesterday , sub was in the sun so fuel probably was expanding . I still think the injectors are dripping fuel into the intake - I read it is a common problem with the injectors in the 7.4 l . Would the regulator be able to leak fuel into the in intake ? I was reading on this forum about ford/bosch injectors replacements , anyone used these and where is the best price on them ? anyone have the ford / bosch part numbers ? Thanks .

You fuel pressure is to low.
Specs on it is 55/62 lbs of pressure
Here is some guide lines of fuel pressure checking.

Hard to start cold.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
Later V6 and V8 gm engines should have 55-66 fuel pressure.
Cold start should be 55/62 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 55/62 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak.
If pressure does not come up with return line pinched or blocked off.
Go to fuel filter and hook a gauge direct to a line at fuel filter.
Pressure should come up to 75 or more and will hold after fuel pump is turned off.
If it leaks down there after fuel pump is turned off it is LEAKING BACK THREW PUMP CHECK VALVE OR PUMP MODULE LINES.
Post back your fuel pressure readings.
MT

It sounds like your injectors are leaking. I would stick with the GM injectors.