My car wont crank and the fuel pressure is low on regulator

make sure there is a good spark, then check fuel pressure and flow, if pressure is low check fuel pump and fuel regulator . fuel pressure must hold with engine stopped . if flow is low replace fuel filter.

do a fuel pressure test. Sounds like a weak pump or faulty fuel pressure regulator.

wouldnt start before all that or just after, (hummmmmmm)
wont crank or cranks robustly but wont start.
check fuses yet?

run the fault codes and check for cam/crank position sensor . check that the cam shaft is turning (broken cam belt)

Could be a fuel pressure regulator or lost signal from crank sensor that gives inaccurate data.
The crank sensor is responsible for sending data to increase fuel pressure as a function of increased RPM. If the RPM reads low, then fuel pressure will be low.
Check fuel pressure.
Thinking that air pressure is affecting pressure regulator and compensating for a weak regulator diaphragm.
Would be interested in final fix of your problem.

Sounds like a lack of fuel. I would start with the fuel filter being that it is the cheapest option. Check that the fuel pump is working and if that checks out then you should have the fuel pressure checked, the fuel pressure regulator may be bad. Most of the newer GM vehicles I have dealt with have a pressure regulator that I found out is only available if you purchase the entire fuel injection rail system.

what is the fuel pump pressure for a 2001 chevrolet blazer 4.3

the fuel pressure if leaking could cause this the regulator is located under the intake plenum but you should smell gas .if you dont then most likely the pump is bad ,the pump will come on for 2 seconds when the key is on then go off it will not stay on with just the key on only when you crank it will it .so have your fuel pressure checked with a gauge.

When you hook the pressure test guage to shrader valve on fuel rail what it reads is what the pressure is from pump, around 60 p.s.i. all you need to do is hook up the guage and turn on key, you do not have to turn engine over, and if pump is good it will shoot up to 58 or 60 right now.

Sounds like either the fuel pressure regulator or a clogged fuel filter.