1988 350 engine in Silverado. Engine hard to start. Unplug map sensor or disconnect vacuum line and it will start and run for approximately one minute then cut off. You can connect the vacuum line and it will cut off immediately. I installed a new map sensor. I really don't think its in the sensor. Could be a related problem.
Check the fuel pressure it should be about 8 to 13 pounds of pressure. Make sure you have the 2 second prime when you turn on the key with out starting the engine. The fuel pump should run for 2 seconds and go off when the key is turned on.
Get back to me with your results we can go on from there.