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Codes P0300 and P0306?

Tiny
TROUBLESOMETAHOE
  • MEMBER
  • 2004 CHEVROLET TAHOE
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 277,000 MILES
I have the vehicle listed above throwing a random miss/ cylinder 6 miss code you can feel and hear it at idle. Checking counts, it's really only cylinder 6 missing. I've changed plugs (Delco Iridium), swapped wires, coils, and injectors. The miss stays consistent on cylinder 6. Fuel trim is also about -17% to -20% on bank 2, bank 1 is staying in single digits, around -3%.

I pulled the MAF to clean it (correct cleaner, no contact) and tried starting the truck with the MAF unplugged. The miss counts dropped to 0 and idle smoothed out, but the fuel trims stayed the same. Cleaning the MAF changed nothing, with it plugged in the misfire starts again and the fuel trims stay the same.

I'm really just looking for a direction to go before I start throwing parts at it. I don't have a scanner but do have Torque Pro with GM PIDs in case you need more information.

Thank you
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Sunday, April 2nd, 2023 AT 6:57 PM

10 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 43,033 POSTS
It sounds like you have low compression on cylinder #6 which can be caused by a burnt intake valve which would cause the P0300 as well. Here is a guide to help us confirm the failure:

https://www.2carpros.com/articles/how-to-test-engine-compression

Check out the images (below). Please let us know what happens.
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Monday, April 3rd, 2023 AT 11:23 AM
Tiny
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  • MEMBER
  • 6 POSTS
I appreciate your reply. Looking like I've got 155 PSI on cylinder 6. 2, 4, 6, and 8 are all 155-165 dry and cold. The weather is getting bad so I can't check bank 1 right now.
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Monday, April 3rd, 2023 AT 1:38 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 43,033 POSTS
Nice work if you have 155 PSI on #6 it is okay, I would try a new spark plug because the one in there now may be defective, also when the engine running is there a ticking sound from the bank above the #6 cylinder? If so, you may have a flat camshaft or broken valve spring. Also, when the engine is cold let's do a vacuum leak check by using this guide:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Please go over this guide and report back.

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Tuesday, April 4th, 2023 AT 9:37 AM
Tiny
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  • MEMBER
  • 6 POSTS
It was doing it with both the old and new plugs. Old plugs in 4 and 6 both had some carbon on them, black but not wet. 8 and 2 looked good. Not really any valve train noise to speak of, very quiet and smooth aside from the miss.

I should have included in the original post that I sprayed around the vacuum fittings, lines, and intake with no change in idle.
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Tuesday, April 4th, 2023 AT 10:24 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 43,033 POSTS
I am wondering if you have an EVAP purge valve stuck open, let's remove it and see if you can blow through it. If so, the valve is bad, here is the location. Check out the images (below). Please let us know what you find
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Wednesday, April 5th, 2023 AT 10:10 AM
Tiny
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  • MEMBER
  • 6 POSTS
Okay, I can't blow through it. Installed and plugged in on cold start it seems to be shut then opens around the time it goes into closed loop, judged by putting my finger over the nipple.
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Thursday, April 6th, 2023 AT 11:09 AM
Tiny
TROUBLESOMETAHOE
  • MEMBER
  • 6 POSTS
I can't edit my earlier reply, but I put a gauge on the fuel test port. 45 psi KOEO, holds the same with the engine running. It's a Z code flex fuel engine, I can't find the spec. Some say 48-54 others say same as T code.
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Thursday, April 6th, 2023 AT 1:31 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 43,033 POSTS
That shouldn't affect it, I would remove the valve cover and then crank the engine over to see if you have a flat camshaft. You can remove the fuel pump relay, so the engine does not start. Let us know what happens and please upload pictures or videos of the problem so we can see what's going on.
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Saturday, April 8th, 2023 AT 1:15 PM
Tiny
TROUBLESOMETAHOE
  • MEMBER
  • 6 POSTS
Hey man, I wanted to thank you for your time and help. 4 and 6 had leaky injectors. Those are the injectors I swapped and both plugs were carboned up when I pulled them. New Densos in both holes now and fuel trims look great, codes are gone!
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Monday, April 10th, 2023 AT 3:06 PM
Tiny
KEN L
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Thanks for letting us know, we are here to help, please use 2CarPros anytime. Can you please leave us a rating? (Copy entire link)

https://www.google.com/search?q=2carpros&gs_ivs=1#lrd=0x80dcd47364be5d0d:0xba091aa4209f4497,1,,,&tts=0

and

https://www.sitejabber.com/reviews/2carpros.com

Thanks again.
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Tuesday, April 11th, 2023 AT 10:52 AM

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