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

rae61

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Make
GMC
Model
2500 HD
Year
2005
Miles
350,000 KM
Engine
6.0
I am getting a P0300 on my 2005 GMC 2500HD with a 6.0L and 350k KM.
My scanner shows the misfires are on cylinders 1 & 5 and 6 & 8, with the misfire, count higher on 6 & 8. Zero misfires on the other cylinders

This is what I have done so far.


-New plugs and wires on all cylinders/problem the same.
-Moved coils from 1,5,6,8 to cylinders 3,7,2,4 / problem did not follow the coils, no miss fires on 3,7,2,4.
- Changed out fuel injectors on 6,8 / no changes to these cylinders, still miss fires
- moved fuel injectors 1 & 5 to 3 & 7 / miss fires still on 1 & 5

I did a compression test and the lowest cylinder was 150 psi and the highest was 156 psi.

I am hoping to get some suggestions on what to check/test next... thoughts?

Thanks!
Rick.
 

grcauto

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Can you do a running compression test? What are the fuel trims doing?
 

rae61

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I can't do a running compression test. I have attached a screen shot showing fuel trims from today.

Fuel Trim.png
 

grcauto

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The vehicle is obviously running so I don't understand why you say you can't do a running compression test.
 

rae61

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What I should have said is I need to get someone to help me with the running test so I can watch the gauge when when starting the engine. I will try to get done this weekend.
 

grcauto

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What I should have said is I need to get someone to help me with the running test so I can watch the gauge when when starting the engine. I will try to get done this weekend.
It'll show issues with valves and or cam. Do the test at idle and 2500 rpm.
 

rae61

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I did a fuel pressure test 60lbs at idle, and revved to 3500 RPM... the pressure held steady at 60lbs.

I did the running test on Bank 1 (cyl 1,3,5,7) and took some video. I am not sure what I am looking for... let me how a good cylinder should perform, and what will indicate issues with cam or valves.

Thanks!
 

grcauto

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I did a fuel pressure test 60lbs at idle, and revved to 3500 RPM... the pressure held steady at 60lbs.

I did the running test on Bank 1 (cyl 1,3,5,7) and took some video. I am not sure what I am looking for... let me how a good cylinder should perform, and what will indicate issues with cam or valves.

Thanks!
What were the numbers?
 

rae61

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I am not sure if it is normal for the running number to be less than the static test numbers?

Cyl #1 running 115 - Static 150
Cyl #3 running 105 - Static 151
Cyl #5 running 111 - Static 155
Cyl #7 running 113 - Static 155
 

billr

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Yes, running compression at idle is significantly lower than regular compression readings. You have to momentarily open the throttle to wide-open on a running engine to get the same readings as you are used to.

When you took the regular readings, did you hold the throttle wide-open? What altitude are you at? 150-160 psi seems kind of low for a decent engine at sea-level.
 

grcauto

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Yeah. The problem with a running compression test is it's useless if you don't know how to run the test and interpret the results.
 

rae61

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We are at about 3900 ft here. I did not do wide open, but revved it to about 2500 RPM. Did not see more than about 5-10 psi increase at the higher RPM.
 

rae61

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These pics are of the plugs I removed, they had approximately 80k KM on them when pulled. I am not sure if they may help with trouble shooting the misfiring on 1,5,6,8?IMG_8263.JPGIMG_8264.JPG
 

rae61

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I am running out of things to check, but still not ready to give in and take it to a shop.

In poking around the internet I see where some people have resolved a P0300 with having a crankshaft sensor relearn done. Any thoughts on this?

Also any chance it could be an issue with the ECM?
 
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