P300 05 Chevy Astro

My van was throwing a P300 and eventually stayed with a P303 code. After changing everything like coil, injectors, complete distributor, wires, plugs, and anything else that could cause this issue here's what I found.
After pulling plugs for the second time I noticed cylinder 3 plug was a little oily looking. Fouled plug was the culprit. Changed the plug and code cleared until I drove so many miles. The issue I found and fixed to clear up this issue was bad valve guide seals in cylinder 3. Pulled the valve cover put engine on TDC for #3 cylinder pressured it up with air compressor to keep valves closed, removed springs on both intake and exhaust valves, ( one at a time of course), and changed valve guide seals, put everything back together and no more P303 code since. The old seals were hard and petrified from excessive heat. These seals are umbrella type. I only did the one cylinder because I have plans to drop in a 5.3 LS in the future.
If anyone pulls plugs, observe closely for oily residue, it could be bad valve guide seals specially if the engine has a lot of miles, has overheated or ran hot at some point.
Hope this helps anyone pulling their hair out over these misfires. I'm not upset I changed so many parts but that's what I do when I get a vehicle I plan on keeping. By changing everything ended up being a learning curve to finally find the issue.

Ant

redconverse21 said:

Thanks for all the help so far guys!
broke down and took it to a shop for a 200 dollar diagnostics... they didn't come up with anything except doing a crank relearn with there fancy scanner and clearing the codes... bummer
also with the fancy shop tool they were able to tell me most of the misses were on 1 with some on 5...
fuel trims are still way negative aboout -10 but can go as high as -17
this makes me want to go check out the spider and the fuel pressure regulator and its the only thing i have not done.
Only problem is KOEO it holds 60psi for 20- 30 mins so i don't think there is a leaky injector or fuel pressure regulator...
My thought is that maybe under vacuum something is leaking/going wrong in there that doesn't happen KOEO...
Im a time crunch and need it tip top in about 3 weeks any and all suggustions welcome! THANKS!
For those just joining
P300 at highway speeds
p1072 -10 to -15 LTF on both banks now

New filter and pump KOEO 60psi for 30min before it slow starts to blead down
New cap, rotor, distributor, plugs, wires, coil, ignition control module
New mass air flow
New crank position sensor W/sim and relearn
o2 sensor's 30k ago all read good on torque app

unplugged upstream cat no difference.

Click to expand...

I had a dreaded misfire the same as you. If you getting a high negative trim means to much fuel is being dumped. If your fuel system is holding pressure like that there is no pressure regulator leaking.
A P300 random misfire my guess could be a few things.
Plug wires, fouled plugs, bad cap, or could be a bad crank sensor. I changed all these with OEM parts.
As for as fuel trims, that could be a faulty temp sensor. ( I ran into that problem recently). My van would stall while driving then miss real bad. Computer thought engine was cold when it was actually hot and kept dumping fuel as if the engine was cold. I watched these trims during transitioning from open to closed loop.
Another issue can be your intake air flow and air temp sensors. They make a cleaner you can use to lean this sensor.
Like I mentioned in my earlier post I had bad valve guide seals on my #3 cylinder. My P300 code was an aftermarket distributor cap made from cheap materials. Had to put a GM cap that cost close to $30 but it stopped my misfires.
Right now my fuel pressure is between 45-55psi and drains down instantly when key is off. I put in new GM spider injectors when I got my van. I just bought a new GM fuel pump to install to get my fuel pressure to specs and hope to get a little more power. My van has 277,000 miles on it. No smoking yet.
One more thing, if your engine has been dumping excess fuel it will foul your plugs and then send unburned fuel down your exhaust system and compromise your O2 sensor function. I cleaned my O2 sensors. They were really black from unburned fuel. I'd start with the cap.
Worth a try.

Ant