I'm sure this is the millionth title like this but it is the only way to describe my problem, I have a 2000 Blazer LT with the 4.3. the blazer will just crank and when its cranking, its almost as it is backfiring out of the air filter. I have changed the mass air flow sensor, spark plugs(yes they have spark), fuel filter, crank position sensor, battery, and distributor cap. The rotor to the distributor cap spins when I crank the engine. I am not sure if it is a fuel related problem because I can hear the fuel pump building pressure when i turn the key to the run position. I really have no clue at this point and I would like some answers, I cant keep throwing money at this car with no results. Thank you anyone who takes time to read this and help me out.

 

Might be dumb, but did you double check your wires?

The backfire while cranking sounds like you have wires in the wrong location. I was gonna say distributor might be off, but I didn't see anything about that, just the cap...

 

Mostly this happen when you have wrong engine timing.

 

Checklist
Fuel Pressure and leakdown, Build to 60 and no more then 10 PSI leakdown over if i remember correctly 30 minutes
Spark Quality (Hot blue spark over a one inch gap is gold)
Distributor timing
Wire location - Cylinder Numbers front to back are 1,3,5 Drivers Side, 2,4,6 Passenger Side
Compression, should be >110 PSI and no cylinder should be less then that or less then 15% from the highest pressure obtained
You mentioned backfiring out the intake, which can be either Ignition or Mechanical timing, does it crank smoothly? next time i get my rig out of the garage i'll pull the fuel pump relay and record what a smooth crank should sound like.