Hello, new to this forum. I have a 99 suburban with misfire problems. I have been trying to figure this out for five years off and on. I replaced fuel pump, new tank because it needed one, spark plugs, wires, ignition coil, cap and rotor, fuel filter, spider injectors and everything involved with fuel, spark and air. I cannot get rid of the po300 mise fire code. Before the spider injectors it had a po305 and that cleared it up. But it had a misfire. When I pull weight it will every so often bog down. It’s bogs sometimes pulling off. I also have new fuel rails. Please help
me. I drove the truck with 8,000lbs from New Jersey to Arizona. Now I’m driving it back but to Florida and don’t want to break down this time. Anyone have a clue? Thanks.

 

Welcome from Florida. There is a chance that you have a clogged catalytic converter. I have seen a P0300 caused by back pressure created by a clog in the exhaust. Something to look at.

 

I forgot to mention that I have new headers, Down pipe all the way back. new o2 sensors as well