I have a 2002 Blazer 4.3L. with just over 60,000. It had been sitting for 7 years. I have dropped and cleaned the gas tank. Replaced fuel pump and fuel filter. I have also replaced the plugs, wire, cap, rotor and air filter. I had to replace the spider injection system since it had sat for so long also replaced the IAC. It will start with little effort and idles around 600. If I give it gas it bogs down i can work the throttle and get it up to 2400-2600 rpm but will not go past that. I drove it up to the gas station and filled it up with mid grade fuel and a bottle of sea foam. Could not get it over 20 mph and any time I got on the throttle it would bog down and try to die. I thought that since it sat so long maybe the muffler had something living in it so I disconnected it and still no change. I am at a complete loss on what to do next. I ran codes and had the P0300 after driving it but since I cleared it; it hasn't come back maybe because it hasn't left the garage since I don't know. Any help would be greatly appreciated

 

My son's Pathfinder did the exact same thing and the same code. It was one of the downstream cats being fully clogged. Might or might not be the same with yours. Yours is more likely partially clogged because you only one of each cat.

 

yeah...good thing to inspect...if ya had disconn'd the exhaust system @ the header, that would have give u a starting point to rule in or rule out both Cat and muffler
...and, when you said "worked the throttle"...did you mean under the hood, w/your hands, on the throttle body?...also, remove the air duct system and verify the throttle body plate fully opens and closes