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

billr

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I have been struggling with how to help here. I do have some suggestions in mind, but let me study your posts and my reasoning a bit more.

I agree the plugs look pretty good. Compression (150+) at your altitude (3900') is reasonable, but we should keep in mind that compression would be fine even if an exhaust valve was completely inop. That is one long-shot suggestion, pull the valve covers and check things visually, especially springs, and verify all exhausts have similar lift. Not to dial-indicator accuracy, just to the nearest 1/32" will be sufficient.

I don't think this is a CKP sensor problem, that should affect all cylinders equally. Yes, it could be an ECM issue, but also there are many wiring/connector possibilities, too.

Part of my "struggle" is simply because I am a 1-8-4-3-6-5-7-2 guy, I have to get used to thinking in terms of the new Chevy firing order!
 

rae61

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I doubt it is relevant but I noticed that in the firing order the misfires happen in two groups in the firing order 1-8-7-2-6-5-4-3.
 

billr

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Hmm... I just peeked at a picture of the coils. Looks like you have 8 individual coils, "CNP". Most (all?) of the below is wrong!

There is still a hint that the CMP (cam) sensor has failed and it has defaulted to "wasted-spark". Try swapping the 1-6 and 5-8 wires. If it has defaulted to wasted-spark the engine will run no different than it is now. If it is not in wasted-spark only the other four cylinders will fire, but you won't hurt anything by running it briefly.

bogus first reply:

Do I understand correctly, one coil fires 1and 6, the other fires 5 and 8? (Ignoring coils for other cylinders)

Try swapping the wires at the coils, 1 to 6 and 5 to 8. That should make no difference, but let's see if misses on 1 and 5 become more prevalent.

I assume this has no separate ICM, the coils are fired by the PCM/ECM directly; is that correct?

When you swapped coils, you swapped the 1-6 and 5-8 group for the 4-7 and 2-3 group, correct? Not the 1-6 for the 5-8.

Yes, I think the grouping of the misses indicates it is definitely an ignition problem, not injectors or anything else.

If you have not disconnected and inspected contacts in connectors to the coils and ECM, be sure to do so. Just disconnecting/reconnecting may solve the problem by "wiping" the contact surfaces and restoring good connection..
 
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rae61

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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.
 

rae61

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I will pull the valve cover an check valve springs and check the exhaust valves for lift.

There are 8 coils and I will check the electrical connections especially on the cylinders in question. I swapped coil #8cyl to #4cyl and swapped #6cyl to #2cyl. Then on the other bank #1cyl to #3 and #7cyl to #5cyl.

Try swapping the wires at the coils, 1 to 6 and 5 to 8 I am not sure I can do this as the connections are not long enough to swap.
 

billr

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I can understand that the stock CNP wires are way to short to swap. Consider getting cheap set of "universal" wires to do this.
 

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billr

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No, more like this. Leave the coils in place and just extend the high-voltage wire.


Just be sure to get a set with wires long enough to cross over the engine, that particular item doesn't seem to spec the length. You can get cheaper ones with copper-core, but I would pay the bit extra to get suppressor type wires, even for this temporary usage. You don't want to add one more unknown (EMI effects) to this diagnosis process.
 
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Hey...the barometric psi. I'm at +2500 ft. I used to live in Chicagoland area were it's sealevel and last place of residence in whiting, In. The key on engine no start here in Arizona in Tuscon I wasn't aware of the bpsi and threw me off course a little season considering I looked over and gave no thought. So, I would do an engine vacuum test first. Secondly, a good running vehicle would show your barometric psi at the MAP. Barometric psi should be same specs even though different vehicle or same in your altitude. Mine is at 15.7 in Tuscon,az and in Chicago was 17.4. being that many miles also, consider intake manifold maybe walls are Carboned up. I take them to machine shop if made of metal and have it cleaned, boiled process and they come out nice. A good indicator sometimes at pcv being plugged or I didn't check but the reading if you had the old spark plugs. So, here you go man.....

1) Check Map sensor and specs for your area.
2) Check intake and pcv. While at it use a camera scope to check inside intake manifold.
3) vacuum test. You could get one fairly inexpensive. Usually has a handy instructions/diagnosis packet.
4) Is the fuel line routed in such a way it's sort of over the intake manifold and the heat shield probably rusted out, fallen, it's missing,,,,and causing fuel to boil in line. (Happened to me- that was school of hard Knox! Repair done at 27 below). Your will need a infared thermometer.
5) did anyone mentioned the fuel psi regulator. Is it getting sufficient vacuum. Or holding vacuum?
6) Cylinder leakdown test. Test when cold and then engine at operating temp. (Best to do when noise around.)
7)'Also, I'm not Mr. Know it all so please.if any misunderstanding, I apologize. Sort texting this stuff and halfway asleep. So finally, how was the spark condition and did you check your ground points. I would have to
 
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Hey...the barometric psi. I'm at +2500 ft. I used to live in Chicagoland area were it's sealevel and last place of residence in whiting, In. The key on engine no start here in Arizona in Tuscon I wasn't aware of the bpsi and threw me off course a little season considering I looked over and gave no thought. So, I would do an engine vacuum test first. Secondly, a good running vehicle would show your barometric psi at the MAP. Barometric psi should be same specs even though different vehicle or same in your altitude. Mine is at 15.7 in Tuscon,az and in Chicago was 17.4. being that many miles also, consider intake manifold maybe walls are Carboned up. I take them to machine shop if made of metal and have it cleaned, boiled process and they come out nice. A good indicator sometimes at pcv being plugged or I didn't check but the reading if you had the old spark plugs. So, here you go man.....

1) Check Map sensor and specs for your area.
2) Check intake and pcv. While at it use a camera scope to check inside intake manifold.
3) vacuum test. You could get one fairly inexpensive. Usually has a handy instructions/diagnosis packet.
4) Is the fuel line routed in such a way it's sort of over the intake manifold and the heat shield probably rusted out, fallen, it's missing,,,,and causing fuel to boil in line. (Happened to me- that was school of hard Knox! Repair done at 27 below). Your will need a infared thermometer.
5) did anyone mentioned the fuel psi regulator. Is it getting sufficient vacuum. Or holding vacuum?
6) Cylinder leakdown test. Test when cold and then engine at operating temp. (Best to do when noise around.)
7)'Also, I'm not Mr. Know it all so please.if any misunderstanding, I apologize. Sort texting this stuff and halfway asleep. So finally, how was the spark condition and did you check your ground points. I would have to
 

rae61

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So I have been working on do testing as suggested. I went to the wreckers and found the longest HEI plug wires from a GM, but they were not long enough. I disconnected the ECM and checked the pins and nothing visible but cleaned with electric contact cleaner and also cleaned the coil electrical connections.

A follow-up test drive did not change the misfires on the cylinders, and it stayed constant.

Then the computer threw a P0332 code. So I then replaced both sensors and harness, cleared the codes and took for a test drive... same route as before, about 10 miles and speeds up to 55 mph.

Knock senor code never came back... problem fixed.

However, I noticed that the misfires were almost gone. Example... CYL 6 went from 872 to 34 using the same length of drive... CYL 5 went from 752 to 12!!

Not sure why or what happened, but it most definitely happened after the knocker sensor change.

Going to take it for a 200 mile road trip tomorrow and see if the misfire count stays down... I will updated when back.
 

billr

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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.
 

rae61

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Sorry, I was out of town and will try splicing the wires and test on Friday-Saturday.

On my road trip, 400 highway miles, I had misfires the whole time... eg 37,200 on cyl #6 over that trip with similar numbers on the three other cylinders.

The CEL light would flash at speeds over 55 mph. If I hit a hill and gave it more throttle up the hil the CEL would stop flashing. As soon as I back off the throttle on flat ground the flashing CEL started again
 
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