o4 chev truck misfire

Hey guys I have a 04 chev truck 5.3 engine vin T with p0300 random misfire code. Most of misfires are on cyl 1-5-6 fuel trims are +or- 5%.
Misfires both cold and hot..
idle and under load
Maf G/s 5.5 @ idle
Map i think it was around 18"
fuel pressure 49 psi running
plugs & wires 4 years old not alot of miles since changed all AC Delco parts
Did injector balance test. they all passed

I dont remember all of the freeze frame data but it was under a load at 195 temp any other data that you may want I can get
Give me some direction please. I am at a fork in the road and not sure which way to turn. :ohmy:
Any help is much appreciated

There's a TSB about p0300-p0308 covering this truck concerning a PCM ground issue. Might get lucky.
These are known for intake gasket failure, but the info you've supplied doesn't support that theory very well. There's always the mechanical health of the engine to consider. A compression check and leak down test may be in order.

Is there a noticeable miss, or just the p0300 code?

I second the notion for the need of a compression test next.

Thanks for the help guys. I really don't think its a gasket leak. because FT are great And if it wasn't for bad luck I wouldn't have any luck so probably not the tsb that was mentioned. But I am definitely checking for it, and a compression test.
A mechanic will notice the misfires, the average person probably wouldn't, the customer never noticed it. He only noticed the flashing check engine light.
Thanks again guys I will let yinz know what I find

Hey John!

Any chance you recall which cylinder had the highest miss count out of the three? Just trying to see if I can tie the flashing CEL to a single cylinder, may or may not work out.

I was about to suggest a CKP Relearn, but then you said it has a noticeable misfire Couldn't be that easy, lol.

Sorry, but I gotta go with the others, possible mechanical issue. Not uncommon for these engines to have lifter or flattened cam lobe issues. I'd suggest doing some running compression testing along with static, to get a better idea of how the engine is breathing.

Any ideas on how to post pictures on here I tried 4 times and nothing happens. After spending all day trying to figure this out with no luck. as a last ditch effort I swapped plugs and wires from my Tahoe and it didn't change anything, I didn't think it would but I am at the point of saying F it I cant fix it, But I cant do that I have never given up on anything.
The most misfire was on #6 then 5 and 1. The other cyl had a few of them but not that many.
I checked the crank and cam they have no drop outs
I checked amps on bank 2 with a 1 min frame rate and I could see drop outs but not sure if that was scope aliasing because when I went 50ms I didn't have any drop outs.
Not sure where to go from here I have secondary waves that I have questions about I dont think they look good

is this a ground that goes from a harness to the engine or from the engine to the body? I have a ground from engine to body but couldnt find a ground from a harness to the engine

It's a PCM ground, so it must come from the harness. Here's the TSB word for word:

Bulletin No.: 06-06-04-046

Date: September 12, 2006

INFORMATION

Subject:
Information on Engine Misfire MIL/SES Light Illuminated or Flashing DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0420 or P0430
Models:
1999-2007 Cadillac, Chevrolet, GMC Full-Size Pickup and/or Utility Trucks
with 4.8L, 5.3L, 5.7L, 6.0L or 6.2L VORTEC GEN III, GEN IV, V-8 Engine (VINs V, C, T, Z, B, 3, M, 0, J, R, U, N, Y, K, 8 - RPOs LR4, LY2, LM7, L59, L33, LC9, LH6, LMG, LY5, L31, LQ4, LQ9, L76, LY6, L92)

with Active Fuel Management(TM) and E85 Flex Fuel

If you encounter vehicles that exhibit the above conditions, refer to SI for the appropriate DTC(s) set. If no trouble is found, the cause may be due to an ECM ground terminal that has corroded with rust over time. Inspect the main engine wiring harness ground terminal (G103) for this condition.


Zoom Sized for Print
The wire terminal (G103) attaches either to the front or to the rear of the right side cylinder head, depending on the model year of the Full Size Pickup and/or Utility Trucks. If the ECM ground terminal has been found to be corroded, then follow the service procedure outlined in this bulletin to correct the corrosion issue.


Zoom Sized for Print
Remove either the nut or bolt securing the main engine wiring harness ground terminal (G103) to the right cylinder head. Refer to the above illustration to determine where the ground is located on the vehicle (1).

Remove all rust from the ground terminal, the cylinder head and the retaining nut or bolt.

Position the main engine wiring harness ground terminal and install the nut or bolt.

Tighten:

Tighten the retaining nut or bolt to 16 N.m (12 lb ft).

Apply some type of electrical moisture sealant to protect the harness terminal from further corrosion.

johnphufnagel wrote: Any ideas on how to post pictures on here I tried 4 times and nothing happens.

You may be having trouble due to the 250 kb size limit the forum uses. You can use an image resizer ( resizeimage.net ) to get them below the max, or send them to me and I'll post them here. wrenchturnsyou@gmail.com

Any luck with compression testing? Since you noted #6 had the highest counts, I'd suggest starting there.

yes in one of my replies that didnt go through I checked static compression all 180psi

I sent you the pictures of the waves
Thanks

Got 'em, thanks sir!

















It looks like the firing line is missing in some of the shots, but I think that has more to do with the scope's sample rate. Turning peak detect on will improve the level of detail in your ignition captures.

Aside from that, all these seem to be fairly normal secondary waveforms. I'd totally welcome a second opinion if someone sees something I missed. Was the misfire taking place when you took these?

I sent Paul a picture last night on Facebook and he said that it was inverted, but its not, I thought the same thing when I took these and I inverted them and they look really weird.
Yes it was misfiring when this capture was taken if you look at 6 5 and 2 the cyl I am having problems with the most, they have a downward spike, not sure why, it should go up. And if you look at cyl 6 scope settings I don't have it inverted.
If it will help I will retake them.
I tried taking primary side but can only do 1 bank at a time if I connect to the fuse its a mess with the fuel injectors in there to
I knew it wouldn't make a difference but I swapped the 3 plugs and wires with my Tahoe and didn't make a difference
Thanks

Update I checked the ground it is not on the head like the tsb says it was on the back of the intake or block/ transmission I couldnt see what it was bolted to but the connection was good I looked all over the engine and that was the only ground that I found on the right side

Come on guys help a brother out! There has to be more than a few smart people on here that can help me. I am willing to try anything :whistle:

Hi John. I read your case and I wish I could help. I'm new here so I would be interested in what you find as the problem. Good luck.

I was able to isolate just the coils and look at the currant ramps all together and they look good. So now I am going to do the same for the injectors, since it looks like I am flying solo I will let yinz know what and if I find anything before the customer picks it up

Sorry I haven't gotten back to you sooner!

You did an injector balance test, right? If they balanced properly, then I doubt you'll see anything in the injector current ramps.

I see that you did static compression tests on all cylinders, but did you try running as well? I bring this up again because you can have a worn cam lobe or broken valve spring, but get perfect cranking compression. Ask me how I know

yes I did a balance test and were all equal. No I forgot to do a running compression test The customer needed the truck back and will let me know if it gets worse

johnphufnagel wrote: yes I did a balance test and were all equal. No I forgot to do a running compression test The customer needed the truck back and will let me know if it gets worse

Understood, thanks sir!

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