Hi
My 97 chevy blazer 4.3 vortec runs rough. It starts fine in the morning but begins to run rough after about 15 minutes of driving. I sometimes get p300 codes. I changed the plugs, cap, rotor, airfilter, fuel filter, wires. My fuel pressure goes right to 60 psi for a second and drops right away with key on. With it running to stays at around 55 psi and then drops down quick when I shut it off. My suspicion is the fuel pressure regulator or spider assembly. Since there are expensive parts does anyone know how to test the regulator.

 

Sure. Start by following the instructions in this thread.