My 99 gmc 454 starts good in cool weather, but hard when it's warm or engine is warm.. when it does starts it's almost like its flooded.... any answers to what i need to do?

How to Start a Car in Freezing Cold Winter Weather

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wikiHowFreezing cold weather can take a toll on car batteries. ... It's much more difficult to start a cold car with a mostly-empty tank, so do yourself a favor and gas up ...

Why is it so hard to start a car in the winter? - HowStuffWorks

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To start a car in winter, your engine has to work extra hard. Learn the top 3 reasons your car is hard to start in cold winter weather.

Debunking 5 Cold-Weather Car Myths - Popular Mechanics

Wrong oil. Check the engine temp and thermo. Make sure oil is compatible with service. eg. your driving style and the weather. Castrol is pretty good oil. I used it in air cooled motocycle engine to solve shifting problem in hot weather. All lifters do not get the same cooling. That one is prob running a bit warm. good luck with that.

Not enough heat sink under the ignition control module inside the distributor or the module itself is faulty.

1. Air filter clogged
2. Fault with the electrical systems and or fuel.
3. Fuel not reaching the injectors.

These are some of the problems that would cause this.

And did this happen right after they replaced the injectors, or recently?

Check the fuel pressure regulator. Pull the vac line after it is warm and see if it is leaking gas into the vac line.

can be lifter going bad,rocker arm loose (once oil warms and thins out) can also be a minor exhaust gasket leak

Very good chance that it's bad fuel injectors and/or fuel pressure regulator. Very common with the 96-2000 L29 motor (Vortec 7.4L)
Replace with better injectors, the OEM ones are NOT WORTH cleaning. Go to http://www.fullsizechevy.com/forums/general-discussion/technical-maintenance/213662-96-99-454-vortec-owners.html
for more info...

It sounds like your carburetor is flooding, check to see if the choke is set too tight.
to do this, when the engine is cold, remove the air cleaner assembly, push the gas pedal one time to the floor. in the carburetor throat is a flap, this flap closes as engine cools so the engine receives more fuel for cold weather starts, the flap is at the top of the carb. this flap should have about a 1/8" gap between the throat wall and the flap when it is fully closed, if if is completly closed it will give the engine to much fuel and cause it to flood. if you need to adjust just follow the linkage that the flap is attached to, and it will lead you to the choke housing, this is where you can adjust it.
once this is done start the engine and let it warm up, as it warms up this choke flap should open completly up in a vertical position.
if it doesnt, then you probably have lost power to the choke housing or your heat tube that comes from the exhaust manifold to the choke housing is bent, off or missing. most likely yours will be electric.
i hope this helps, good luck

Will the engine start on its own after the engine is warmed up?
Let me know.
Regards,

Check fuel pressure, and regulator. A leaking regulator gives a warm engine more fuel [ it does not like it] a cold motor does. same as a flooded motor. Good luck