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P300 05 Chevy Astro

588 Views 13 Replies 4 Participants Last post by  AstroAnt
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 coil
- 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!!
1 - 2 of 14 Posts
It sounds like an ignition misfire to me… the LTFT numbers say there is unburned fuel.

So you say distributor is good, has it been replaced? Has the drive gear been replaced?

Have you looked at the coil for viable sparking? You say “coil good”… what is this based on? Replaced? If so, how about the coil wire?
Well a cat backppressure test is done with a guage… but you could pull one of the upstream o2 sensors and drive it… of course you will get the code for the missing sensor but if a blocked cat is causing the 300 code it should not come back.
1 - 2 of 14 Posts