I have a 2009 Chevrolet express 1500 van with a 4.3L and was wondering if you could give me your opinion on something. Van has just over 200,000 miles on it and i have been experiencing a random misfire code P0300. Replaced plugs, wires, coil pack block a while ago, and recently replaced all 3 cat's and installed a Magnaflow muffler. Runs alot better since replacing exhaust, but miss fire still happens only at extremely high RPM's. I notice a slight hesitation after the C light starts flashing while climbing a steep hill for a while. I also have to be in the slow lane because i am going so slow. Around 50 mph or so. After i replaced the exhaust parts i also got a Running Rich code so i am not sure if its not getting enough air due to a possible vaccum leak or if something else is going on. I don't hear any rattling in the engine bay so don't think i have a bad lifter (yet). Any help or advice would be greatly appreciated. Thanks for your time.
A few questions ....
Do you know if it has distributor ignition or electronics? I'm not sure exactly what year they switched. Did you own it for the entire 200k miles? Your symptoms sound a lot like a dizzy cap that needs adjustment or replacing, but it could be a weak fuel pump too. Check the fuel pressure.
When you have misfires, you can expect rich codes. Misfire means incomplete combustion, which means more unburned fuel going downstream, right. Unburned fuel will also quickly ruin catalytic converters as you found out.
A vacuum leak causes TOO MUCH air in the mixture. So that would cause you LEAN codes, not rich.
By the way ... not sure if you have run into it... but the 4.3s suffered from failing intake gaskets. Especially near the rear of the engine. Mine started leaking badly around 150,000mi. The mating surfaces were pitted. If you haven't already, change those out before too much damage is done. Not difficult.