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Discussion Starter · #1 ·
05 chevy astro 215xxx
-p300
-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!!
 

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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?
 

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Discussion Starter · #3 ·
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?
Thanks for the reply Sailing_Faith
The distributor has been replaced along w/ the cap and rotor. It has a lot less play then it did with the old gear so I was hopeful that was the problem but no luck.
The coil is new as well as the coil wire... hmmm
 

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Discussion Starter · #4 ·
I am thinking i have a plugged cat or bad/leaky spider. I would like to confirm one or the other before just troughing parts at it. It holds 60psi for about 20 mins so i would say spider/injectors/fuel pressure regulator are good so bad cat but that's all i would be going off of. If someone could explain how to confirm diagnose Spider/injectors/fuel pressure regulator over cat that would be awesome!
 

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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.
 

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'95 Astro AWD Hi-Top Conversion
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if you can't feel a distinct series of "puffs" from your tailpipe, but only a steady "whooshing" then you may have a clue about your catalytic converter.

be mindful that a partially clogged catalyst will result in super hot exhaust gases, so don't just shove your bare palm into the stream. a welding glove is a good idea.

as an exhaust mechanic i found this to be the quickest first step in troubleshooting and detecting a clogged converter. it's pretty accurate too in helping determine what's next. in severe cases, listening alone is good enough to diagnose a clogged exhaust.
 

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Discussion Starter · #8 ·
Okay, drove the van without the upstream o2 out with no change. Also it does have a decent puff puff its not strong but definitely not whooshing out.
should have mentioned in original post new o2 sensors and a properly shimmed Crank Position Sensor
 

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2004 Chevy Astro Cargo 5 window. 276000 miles.
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05 chevy astro 215xxx
-p300
-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!!
I hope this helps you maybe save some bucks.
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.

AstroAnt
 

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Discussion Starter · #10 ·
Okay I will stick to OEM parts.
The one thing that stands out to me that is that its only at hwy speeds for a deration. drive around town for 10 min and the light goes out and it clears from pending...
I changed nothing and the P300 went away and was replaced by a p0172 system to Rich.
IKD why the p300 went away but the p0172 makes sence as my long term fuel trends are -16 and -17

fuel pressure
new pump, new filter
primes to 63/64 for the 3 seconds then drops down to 60 and holds 60 for at least 10 min this has led me to rule out the injectors or the fuel pressure regulator.
where is the extra fuel coming from?!?!
 

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Discussion Starter · #11 ·
Also now it has been about 30 mins and it fuel pressure has dropped down to 54... how long should it hold the 60psi?? it holds it perfect for about 15 min and then slowly starts to blead down.. yeah after a half hour its at 54
 

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2004 Chevy Astro Cargo 5 window. 276000 miles.
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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
 

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Discussion Starter · #13 ·
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 sensers 30k ago all read good on torque app

unplugged upstream cat no difference.
 

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2004 Chevy Astro Cargo 5 window. 276000 miles.
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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.
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
 
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