Won't stay running

If the problem only happens when the engine is cold, a bad electric sensor is likely to blame. If the car stalls even when the engine is warm, the cause is likely a faulty electric fuel pump.

Why is it that my car starts but won't stay running? ' Reference.com

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Your post is lacking any detail. However, if you are stating that the engine starts after being cranked but dies out (wont stay running) there are 2 ways the fuel pump is activated. When the key is first turned on, there is a 5 second fuel pump run (the ECM energizes the fuel pump relay). After the engine starts, the fuel pump relay will only stay energized when the oil pressure switch closes keeping the fuel pump relay on (oil pressure is above 5-7 psi). If the oil pressure switch is faulty or there is no oil pressure, the engine will stall just after starting.

check your grounds. I find many easy fixes by cleaning all the grounds around the engine bay. I would also check the ground at the fuel pump connector that wiring is prone to corrosion and breakage

Unhook your battery again for about 15 minutes. After you re hook it up, let car sit with the key in the on position, but not running. Let it sit for about 5 minutes, and then turn key off, and start it. It may run rough for a few minutes as the sensors and systems are all resetting. If you try running it right away, or cold, it can confuse the regen process

Fuel injectors may be dirty.. or plugged. The car just sitting there may have caused the gas to go bad which may have given them a head start on plugging from the get go. Get some cleaner for it.
When the dash lights blink, is it unlike anything you've seen it do before?
check the detail :

http://www.xcardiag.com/search/?q=engine

Does it stall as soon as you let go of the key

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

check fuel pressure, should be 40 psi.
try 2 things

spray starting fluid in air intake to see if it will stay running. It's fue then.

Try turning key to on nd wait 5 seconds for system to build pressure and pump to shut off. Turn key off, wait for relay clcik noise or 10 seconds. then turn key to ON again and wait 5 seconds to build pressure again. Then start engine. If it starts better, you have a bad check valve in the fuel pump or a bad pressure regulator on the fuel rail (unlikely).

report back

Try resetting the ECM, turn the ignition key to the on position "engine not running" and pull the fuse to the ECM and place it back in. Leave the keys in that position for 30 seconds and then turn it off and on again for 5 seconds and start. this will reset the ECM to default. It may stall again but let it, just start it again and let idle for 3min and go foe a test drive. This should reset the way the engine and transmission performance, and should reset the idle issue on your Envoy. Good luck and keep me posted.

With the breif discription you are giving. It sounds like the fuel pressure is bleeding off. Next time you try and start it, just turn the key on, don't start it. wait with the key in the on postion for 5 seconds and then start it. If it starts right up and stays running. Then my bet would be the pump and/or regulator are bleeding off the fuel pressure.
Are there any codes in the pcm? (even if the check engine light isn't on, there could still be codes)
The 8.1 also like to have camshaft sensor problems, which could also cause your complaint.