2008 6.7 Cummins Engine Knock

Big Blue24

Big Blue24

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Jan 5, 2008
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Help me identify what's causing this engine to knock. It's a bone stock 2008 6.7 3500 dually 4x4 with 109,000 miles. It was used to plow snow by the previous owner.

Believe it or not, the knock will come and go away at times. No visible smoke from the tail pipe. No excessive blowby out the oil filler cap when running. The coolant is brown, I assume that's the new non-green coolant they fill these things with. The oil does not appear milky like it has coolant mixing, however, it does appear to have abnormal sludge build-up. Does the new coolant not leave a milkshake when it mixes with engine oil?

Check engine light is on, key dance will not pull up codes. The owner took it to a diesel specific repair shop and they said the code came up as a fault in the EGR system / O2 sensor. They wanted to dismantle the motor on the spot and luckily he walked away.

So what is it?

Dropped valve? Blown Gasket? Bent Rod? Spun Bearing? Piston Slap? Clogged piston jet and resultant melted piston?

[ame="http://www.youtube.com/watch?v=iBKEKuk5xgo"]http://www.youtube.com/watch?v=iBKEKuk5xgo[/ame]
 
Id start by pulling the valve cover off and checking valve lash and push rods. Then I would start looking for a small chinese man hiding inside your engine with a hammer.
 
MAKE IT STOP!!!!!!!!!!!!!

Geez I was actually thinking a rod was gonna come through the computer screen and smack me in the face....

Why isn't that thing deleted 1st...

I'd check your top half first as mentioned.
 
Oh god. Before I listened to it I was thinking maybe injector knock but damn.......
 
Check flywheel, flexplate, torque convertor bolts.
 
Update, the owner told me that the diesel repair shop that looked at it last week said they manually disconnected the wiring harness to each injector and it didn't change the sound. They used this to determine that it wasn't a bum injector or a melted piston making noise from weird combustion.

I think I'll help them look over the valvetrain and if nothing turns up "it shouldn't since a trained diesel tech was under there disconnecting injectors" we'll look at the flex plate bolts.

The knock is much louder on the exhaust side of the motor.
 
Have an oil analysis done. Or in the meantime cut the oil filter apart and check that out.

That does not sound good at all.


Sent from my iPhone during class
 
Sounds like the big end of a rod double tapping when you rev it up. Frequency seems to quick to be in the valve train.

Did you throw your head under it for a listen?
 
Im putting money on a valve seat. Just did an 03 with the EXACT same sound. Pull the head before you do what my buddy did and let it run so the chunks of the seat crack the cylinder wall!!!!
 
Find a doctors old stethoscope and try to listen to the bottom of the pan. We have one that has a metal tube on it that is a little more geared towards mechanics but a regular one will work.


Sent from my iPhone during class
 
It's a video, but it doesn't sound deep like typical bottom end issues. The knocks I've heard that were rod related had a distinct deeper tone/hit.
 
Gwoody said:
Check flywheel, flexplate, torque convertor bolts.
Click to expand...


This has my vote. Sounds almost hollow compared to a rod knock. Every bad rod I have been around was a knocking sound and didn't echo. Never dropped a valve seat though.
 
Tc bolts is my vote. Does it do it in reverse but at a stop? How about drive at a stop? 1st? 2end? I think something with tc or flex plate imo
 
If it does it more/less in other gears I would lean towards tc/flexplate. Something of that nature!!!
 
Ouch, that sounds horrible!


Sent from my iPhone using Tapatalk
 
It sounds too slow to be in the valve train, the knocks would be a lot faster, it sounds a little tingy or echoey though, I would lean more towards bottom end though, compression test would be next on my list,
 
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