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2005 chevy 2500hd with 6.0L. I have code p0300 for engine…

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2005 chevy 2500hd with 6.0L...
2005 chevy 2500hd with 6.0L. I have code p0300 for engine misfire detected. It seems that cylinders 1,2,3,and 4 are misfiring. 6 and 8 have a small history. 5 and 7 never skipping. I replaced the spark plugs, wires, and intake manifold gaskets. truck has 167k miles on it. I t doesn't seem that theres is much of a pattern on the misfires. I did an injector balance and all 8 injectors were the same. starting at 55psi fuel pressure each injector dropped it to 45psi. I don't suspect faulty converters or a fuel pressure or vacuum issue because its not randomly misfiring. and also 5 and 7 have no history of missing. I have good spark and good fuel pressure. I sprayed everything with starting fluid and didn't find any vacuum leaks. also I don't hear any metallic noises as if it had a broken valve spring or something internal. 02 sensors seem to be operating within their values per alldata.
Submitted: 3 months ago.Category: Chevy
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Answered in 11 minutes by:
1/5/2018
Chevy Mechanic: david craig, Auto Mechanic replied 3 months ago
david craig
david craig, Auto Mechanic
Category: Chevy
Satisfied Customers: 5,145
Experience: owner/mechanic at 3D Automotive
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Hi my name is ***** ***** I'll be more than happy to assist you. Please allow me a few minutes to review your information. Thank you

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Chevy Mechanic: david craig, Auto Mechanic replied 3 months ago

do a compression test on all cylinders. also, dont be so sure the cats aren't bad. misfires will eat cats for lunch. do a back pressure test also

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Customer reply replied 3 months ago
ok, I also forgot to say that if I hold it at 2500rpm with it in park it runs smooth. soon as I bring it back down it starts skipping
Chevy Mechanic: david craig, Auto Mechanic replied 3 months ago

are you watching the fuel trims while doing that?

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago
Todd Audette
Todd Audette, Chevy Technician
Category: Chevy
Satisfied Customers: 416
Experience: Chevrolet Technician
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Hi my name is ***** ***** will try to assist you with your truck. Please allow me a little time to review and i will be back with you soon

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Sorry for the delay but i lost internet connection....are the misfire counts incrementing steadily or random

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Customer reply replied 3 months ago
its worst at idle. when you give it throttle cel starts to flash. I have 135psi compression across the board.
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

ok it's not a compression issue and i didn't think it was....does it seem to be mostly 1 bank or random on both banks

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Customer reply replied 3 months ago
both banks but mostly the front 2 cylinders on each bank. pass bank rear 2 cylinders (5 and 7) don't skip. they don't show any history of skipping. driver side bank rear 2 cylinders (6 and 8) skip once and awhile randomly but not as much as the 2 and 4 or 1 and 3.
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Hang tight let me look at something...brb

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Silverado or Sierra

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Customer reply replied 3 months ago
Silverado 2500hd
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

any other codes other than the p0300

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Customer reply replied 3 months ago
had a code for evap vent control p0449 and a maf sensor code p0101. I tried a new maf sensor but that didn't change anything.
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

2 things i would try.....a fuel injector balance test and unbolting the exhaust from the manifolds on both sides and run it and see if the misfires go away. I have seen this before where the cats get blocked and will cause a misfire on 2-4 cylinders.....not 2 and 4....but 2-4 cylinders on a bank. Reference the attached Document

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Customer reply replied 3 months ago
I did the injector balance earlier all were the same. start at 55psi then when injector activates, it drops to 45psi. I will try and get the exhaust off and fire it up
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Don't just take the o2 sensors out....that never works. It's not enough of an opening for all of the exhaust to escape

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

And don't waste time trying to do a back pressure check with a gauge....unbolting is the best and most accurate way

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Customer reply replied 3 months ago
I cut the pipes bc the bolts are way too rusty. still skips.
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

You cut the pipes that quick?....Do you have spark on the misfiring cylinders?

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Did you cut before the cats?

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Customer reply replied 3 months ago
before the cats. just saw zawed them. and yes I do have spark on all 8
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

did you run it for a few mins.....revving it up to clear the cylinders out?

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Can you actually feel the misfire or is it just incrementing on you scan tool

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Customer reply replied 3 months ago
I ran it at idle then reved it to 2500 then let it come back down and still skipping. long enough to burn out the cylinders. only took a min to cut the pipes. and I can feel it. at idle the whole truck shakes.
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

does the balancer wobble at all

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Customer reply replied 3 months ago
no
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

On your scan tool can you watch engine rpm and see if it's drastically jumping all over the place....like crazy numbers....1000........12000 at idle

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Customer reply replied 3 months ago
no seems accurate. might need to pick back up on this monday
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

ok....whatever you want to do...i will continue to try to help you

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

I had another thought about your misfires....not sure if you have a vacuum gauge but if you don't try to get one and with the engine running hook the gauge to a vacuum port and let me know if it's steady vacuum at idle or bouncing all over the place

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Customer reply replied 3 months ago
im back, I will try the vacuum gauge sometime today.
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Ok

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Customer reply replied 3 months ago
at idle I have about 10 inHG of vacuum. it bouces a little bit but im assuming its because its skipping. if I hold steady at 2k rpm it runs pretty smooths and vacuum at 15inHG.
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

You should have like 20in of vacuum. When you say it bounces a little like how much. It shouldn't bounce at all even if it's misfiring Unless you have broken or weak valve springs. That was the reason for me asking you to do a vacuum test

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Customer reply replied 3 months ago
the gauge just flutters a little bit between like 10 to 12 or 13in. the customer said that its been acting like this for about a week. it was driving fine then I guess on e day he went to take the truck and it was acting like this.
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

I'm running out of ideas.....How about a fuel sample. Maybe ask the customer if he got fuel before this started

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Customer reply replied 3 months ago
I don't know how to test it. I would think that something like fuel or vacuum would be causing a random multiple misfire. I did a crank variation relearn just to do it and nothing on that either
Customer reply replied 3 months ago
what should my fuel trims be at idle, I don't really know how to read those, maybe those will tell something. the 02s seemed to be working fine
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Try to hook up a fuel pressure gauge that has a relief on it and put some in a clear bottle and let it sit and see if anything separates to the bottom or if it has a funky odor or color

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Fuel trim should be close to or around zero...+ or - 5-10

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Customer reply replied 3 months ago
ok I just noticed that the fuel cap says e85. if someone put e85 in this and it had been running 87 its whole life, could that cause a misfire? or is there a way I can test if it has e85 in it or 87
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Possibly

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

There is a tester for that

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

On your scanner can you look at fuel alcohol content

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Customer reply replied 3 months ago
just talked to customer and it has 87 in it. and it doesn't show alcohol content
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

I'm at a loss my friend....we've covered everything i thought it may be from here but without being at the vehicle myself i'm sorry but i don't know what else to suggest

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Maybe ECM?

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

I wish i could have helped you get this figured out and i hope you would kindly rate this chat session 3-5 stars for my assistance in trying to help you

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

I would still get a fuel sample

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Just checking in to see if you have made any progress or come up with anything

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Is this setting a P0300 or not

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Customer reply replied 3 months ago
I just replaced the intake plenum with a brand new updated version. the truck seemed to be running a tiny bit better. I watched the misfires on the scanner. now it seems to be cyl 2 and 6 mostly now. but mostly 2. I switched coils with 2 and 4 to see if it moved but it stayed at cyl 2. the passanger bank is definitely better then it was before.
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Maybe throw a new plug in 2 and see what happens...might just be fouled

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Customer reply replied 3 months ago
I switched the plug, coil, and injector with a good cylinder and nothing is changing. seems to be 2 and 6 now. mostly 2. could I have a broken valve spring or something and not hear and noises from it
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Absolutely.....if a valve spring is broken you won't hear any noise. You'll have to remove the valve cover and physically look at it

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

You could also have a burnt valve....best way to check that is to remove the rocker arms from the misfiring cyl....remove the shrader valve from your compression tester,,,,,screw the hose in to the head, attach an air hose and put air into the cylinder and see if you have air escaping out of the intake manifold or the exhaust

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Customer reply replied 3 months ago
just pulled the valve cover and everything looks good in there. surprisingly clean also
Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

try the other thing i suggested about air testing the cylinder...there's only 4 things that can cause a cylinder to misfire.... spark, fuel, compression and a leak

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Chevy Mechanic: Todd Audette, Chevy Technician replied 3 months ago

Any progress

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