P0300 on my 2005 GMC 2500HD with a 6.0L and 350k KM.

[grcauto]

Yes it starts up right away, and idles well. The idle speed once warm is about 580 and fluctuates about 10 RPM either side of this.

I'd be looking at temp sensors since it runs good cold.

 

[billr]

Have you tried it with the EGR disabled, both ports blocked-off?

 

[rae61]

Have you tried it with the EGR disabled, both ports blocked-off?

I could not find the EGR... looked on Rockauto for a EGR listing and they had nothing listed. I am wonder if because it is a 2500 HD the emissions are different?

 

[rae61]

Take two of those "not long enough" HEI wires and splice them together end-to-end. Put a couple of layers of tape over the splice and route/tie the long wire so that the splice is several inches away from any ground.

Do you see any difference in the knock-retard events in live-data now? I would be looking at both the frequency of occurrence and amount of retard.

I tried the longer wires on 1-6 & 5-8 and there was no noticeable change in the number of misfires. At idle the advance was 15.5 deg and would go to ~ 31 deg at 2500 rpm. My reader has shown 0 deg knock retard from the start, so I just assumed it was not able to detect this PID... or maybe there was no timing retard happening... just not sure.

 

[rae61]

I'd be looking at temp sensors since it runs good cold.

The obd2 is showing 195.2 cruising at 65 mph.

Ok, not sure what I should check on the temp sensor or what to look for?

 

[billr]

After swapping the plug wires, is it still 6 and 8 that have the (most) misfires?

 

[rae61]

After swapping the plug wires, is it still 6 and 8 that have the (most) misfires?

Yes, 6 & 8 still have the most... on average they have about double the counts of 1 & 5

 

[billr]

I doubt, then, that this is an ignition problem. Not the plugs, coils, PCM, or ICM (if it has one).

As I recall, you have swapped injectors too, with same results, problem stays primarily with 6 and 8; correct?

So, I am suspicious of valve operation. I would take off valve covers and check valve-train visually, and try to measure lift on valves for 6 & 8. Precision not required, just to the nearest 1/32" will be good enough.

Also, a simple vacuum gauge may give some clues..

Oh, I forgot about the EGR. RA does show an EGR gasket, did you see that? You can't rely on RA listing all parts. If this engine doesn't have VVT, then I would expect it to have EGR somewhere.

 

[rae61]

I doubt, then, that this is an ignition problem. Not the plugs, coils, PCM, or ICM (if it has one).

As I recall, you have swapped injectors too, with same results, problem stays primarily with 6 and 8; correct?

So, I am suspicious of valve operation. I would take off valve covers and check valve-train visually, and try to measure lift on valves for 6 & 8. Precision not required, just to the nearest 1/32" will be good enough.

Also, a simple vacuum gauge may give some clues..

Oh, I forgot about the EGR. RA does show an EGR gasket, did you see that? You can't rely on RA listing all parts. If this engine doesn't have VVT, then I would expect it to have EGR somewhere.

Correct I swapped injectors and the problem did not follow the injectors originally from 6&8 and 1&5.

I will make time in the next few days to pull the valve covers and measure the lift on 6 & 8.

No VVT on this engine, so I will look again for an EGR, I did see the gasket listed. I have searched the internet and see some later model Chevy 6.0 engines are not equipped with an EGR system. I will see what I can find.

I do have a vacuum gauge, how should I test with the vacuum gauge?

Thanks!

 

[billr]

Vacuum should be very steady, like varying only +/- 1/2"Hg at idle, with reading being in the 16-22" range. If the reading flutters, like +/- 5 " or so, then I think that indicates a valve problem. I am not an expert in interpreting vacuum readings, but there should be many guides online. Tell us what you get and we can all make guesses what it means...

When you have the VC off, be sure to look for anything that looks not-quite-right in the valve stuff. Loose rocker, broken spring, excess lash, collapsed lifter, etc. Maybe even pull pushrods and check for a bend in them.

Since this seems to be only 6 & 8, adjacent cylinders, I'm wondering if a head-gasket issue or something in the fuel-rail is involved?

 

[rae61]

So I did confirm that these 6.0 truck engines did not use an EGR after 2002.

I had a local guy that has built some performance LS engines check the engine with Snap-on scanner. The injectors tested fine, one thing he found was the o2 sensor Bank 2 Sensor 2 was sticking at a high voltage and not fluctuating as it should. I checked the temp at the front and back of that cat and found the the outlet temp was about 50f higher that the inlet. So seems like the cat is operating ok? I changed out the Bank 2 Sensor 2.

I checked vacuum, warm engine, and got a rock solid reading of 16, no fluctuation. Rev to 2500 rpm and let off the gas and vacuum quickly jumps to 21 before dropping back to about 16.

I found something that says at 3000-4000 ft elevation that 15-19 is the range, so a bit on the lower side.

Used carb cleaner all over the intake no noticeable increase in rpms.

I have not had a chance to pull the valve covers but hope to have time this weekend, and it is my last hope at finding an answer/cause.

 

[grcauto]

Yes, 6 & 8 still have the most... on average they have about double the counts of 1 & 5

If those are side by side you may have a head gasket issue. Did we do a running compression test?

 

[rae61]

I did do a running compression teat on 1 & 5 and the results were good. Cyl 6 & 8 are side by side I have not done a running test on them, but will give it a try.

Also, forgot to mention we did a crank sensor relearn with the snap-on tool, no real change or improvement.