I hope this helps you maybe save some bucks.05 chevy astro 215xxx
-only after driving at hwy speeds under a load for over and an hour
-then drive around town for 5 min and engine light goes back off and stays off unless i get back on the hwy for more than and hour
things ive tried
-good distributor, cap and rotor(no pancake syndrome)
-new plugs and wires GM genuine wires AC delco plugs with correct gap.
- good MAF
-good fuel pump/clean filter KOEO 62psi/55ish when driving around
(hold 60psi for 20 min before it starts to blead down which leads me to believe the fuel pressure regulator is not leaking and the injectors are not leaking)
I am thinking clogged injectors or a plugged up cat... But if i had a bad injector why would it be a p300 and not that specfic cylinder?
Fuel trims long term are negative 7 and negative 10 this makes me think there is extra fuel in the intake plenum(leaky spider or fuel pressure regulator) but then why does it hold 60 psi for 20 min?
I have read through all the p300 codes on here and im sorry to create another but i have not found somebody with my exact problem and after weeks cannot figure it out. thanks so much!!
Not sure how many miles on your engine but mine has 275000 miles. I kept getting a P300 code. Installed new complete distributor, Gm spider injectors, plug wires, plugs coil, and every other sensor on the engine.
It was a bad distributor cap. If you put an aftermarket brand on the 4.3 there is a good chance you will start getting random misfires after a short time of driving. Its also worse during damp environments. Fog, Rain etc.
I bit and had to purchase a OEM cap and my problem went away.
This particular cap has electrodes embedded in the cap and very close together. The after market caps are made with different material than OEM part.
One thing to keep in mind, those 4.3 engines run great if they get them OEM parts.