I have a 2000 blazer "W" eng. Have only a PO 300 code per Autozone. Started slight miss a couple weeks ago & now gotten much worse. Stopped by Autozone & they checked codes . Showed only a PO 300. Now it's popping in the cat. converter. Swapped dist cap & rotor -no difference. 3-4 mos ago, did intake gaskets
new cap & rotor
AC delco plat plugs
wires
thermostat
cleaned MAF
fuel filter
also now it's running VERY rich
checked most sensors with vom per Haynes OK
maybe injector o-rings??
thanks for any help or other ideas

i would say 1 (or more) of your o2 sensors may be going bad. another posibility would be a leaking injection spider.

Have you check the fuel pressure regulator? I't no fun on the CPFI system, but it could be leaking. One indiciation is to test running fuel pressure, then see if it drops off quickly after engine shut down.

FWIW, there are no 'O' rings on the injectors in that system - It sues the central injector pod and "poppet" stlye injectors buried in the intake runners.

http://72.19.213.157/files/CPFIInjectorAssembly.jpg

Checking standing fuel pressure would be a good idea anyway.

fuel pressure is: turn key on- 62 psi. key off- drops to 50 in about 2 seconds. while running- guage swings from approx 45 to approx 58 VERY fast- in almost a blur...
prob regulator?

Sounds like the regulator or the injectorrr