97 chevy blazer4+4 4.3 cranks over but won't run it started to die now and then when stopped

failed after the job,
so FPR good, fuel pressure regulator tests are first.
per the service manual all tests FPR and related
seen here,
the FPR (most) have fuel return line and if blocked , or pinched or connected wrong the FPR SLAMS to max pressure, and bingo flood city. .use a pressure gauge and learn how to do
all rail , FPR and injector balance tests.
the FPR my have vacuum line dropped off, and if so
the FPR goes to max PRESSURE,. (fuel)
the EFI can not hide a bad FPR, see? can NOT

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Check your positive battery connections. You may have lost power to the fuse box. Crossing the solenoid to make it crank will not engage the fuel system so it will crank but not run.

You need a new battery-
It takes a great deal more power to crank a motor than it does to light up the lights. This is why your lights still come on even though the battery does not have enough power crank the engine over. If you install a new battery keep an eye on it...if it dies quickly then you have a bad alternator.

could be fuel pump or even crank or cam , timing sensor. Did it start running strange before this?

Have you checked fuel pressure with a gauge ? Vortec engines require 65+ psi to run.

you must RUN the scanner/computer for easy diagnostic and fix it .
Remember the electronic from vehicle NOW is very complicated.
You can have PCM or BCM faulty, or ignition COIL, or spark plug or sensor, faulty. etc
see the diagram attached.
God bless you

Check your TPS Sensor

Back firing is usually an indication of a timing issue

The timing chain is most likely worn & jumped a couple
of teeth or the distributor was installed wrong,if you recently
had work done

check and make sure the powrr wite has a solid cconnection the check the grounds to the starter.

does the motor crank when key is turned?If so check for a spark at the plugs.Get an auto electrician bypass your alarm