Why won't my 1991 K2500 Silverado Won't start

My truck cranks over just fine but right after its done cranking it just dies. Its a TBI injected 5.7l 350 with only about 12,000 miles. The fuel pump is only 6 months old. I'm thinking its a sensor but I'm not sure
which one controls the fuel to the engine and creates the idle.
If i hold my foot down on the gas as I crank it, after the cranking cycle is finished it fires up just fine but this makes my auto-start useless and its too handy to lose in Alaska.
If anyone knows whats up tell me whats broke and I'll fix it.
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2nd to that (sorry about the title) I just spent the last 20 minutes going over my Chilton. If it is a sensor problem its down to only 4. I'm not all that sure the O2 sensor even applies here but my MAP; M.A. Flow; Idle Air Speed; TPS all seem to be candidates. But this is the only problem with the truck so I need it fixed before I burn something up with hard starts and excessive cranking.
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Please be more specific. 1. You turn the key to run (not start). Do you hear the fuel pump run for a few seconds? 2. You turn the key to start. Does the motor turn over? Does the motor fire? 3. You release the key. Is the motor running or not running?
Here is some background on how I understand the system works. 1. When you turn the key to start, the fuel pump runs for ~2 seconds to pressurize the system. Key to start, ignition fires, injectors fire. motor starts. Release key. Oil pressure indicates to the ECM that the motor is running. ECM then provides power to the fuel pump. No oil pressure means no followon power to the fuel pump, and the motor stops as the fuel pressure drops to zero. The purpose of the convoluted power path is to ensure that the fuel pump does not continue to run when the motor is not running (no oil psi), and the ignition is on (as in a crash). Some common problems here are: 1. The Relay fails. Symptom: Motor cranks but is slow to start, but does start. Immediate restarts are good, but the problem repeats if some time between restart. 2. Oil pressure switch fails. Motor cranks and starts fine, but runs only a few seconds. Restarts perfectly, but stops after a few seconds again. ECM can cause the same symptoms.
Since you are in a very cold climate, one thing I would look closely at, is the oil pressure switch, and/or actual oil pressure. Very thick oil can cause a delay in oil pressure buildup on cold starts. If this delay is more than a few seconds, the symptoms might be, crank, start, but shuts down quickly, with subsequent restarts working fine.
KenG
Chevrolet_Camaro wrote:

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1.The fuel pump pressurizes the system just fine and shuts off soon after the engine fails to start. 2.The motor turns over with the starter and tries to fire but dies right after done cranking over. 3.The motor is not running
As for the oil im running 5w-30 which i changed due to this problem with no luck. I always run 5 weight in the winter and it has never been a problem. As for the oil pressure registering...it does if I can get it to start by pressing on the gas after the motor runs, otherwise it doesn't register by just turning the key.
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