2008 6.7 Cummins Engine Knock

[kengarr12]

For what ever its worth mine turned out to be number 6 rod bearing,.,never was low on oil and never ever showed low oil pressure but number 6 is shot,.,,,it sounded exactly the same as yours so my guess is same answer
*bdh*

 

[mr_hahn73]

I know there is a ton of opinions, I wished I would of made a video of mine. Idk if you even have a rail pressure gauge on your truck but my truck sounded just like that. It sounded like metal on metal actually. I looked at my rail pressure and my truck was idling at 26k RP. I replaced the FCA on the top Duramx cp3 new I mind you(less than 2000 miles). They go bad all the time and seems miles play no part. Hope you find the problem.

 

[JWhite]

$100 says it isn't the fuel system. If a cooling nozzle went bad, you would most likely have a scuffed cylinder and would notice in the blow-by aspect of things. The "nicks" in the top of #3 piston leads me to believe that a valve / seat has messed up, generally when a valve or seat goes bad they will throw trash through the intake and you will see that in the other cylinders.

$.02

 

[Big Blue24]

****UPDATE****

I think we found the problem, it looks like the #1 piston is hitting the head. The faint valve marks I could see in the borescope turned out to be the edge of the valve recessed pockets.

We're going to raise the motor, pull the oil pan, and remove #1 piston/rod assembly to look at the bearings. Hopefully the crank is still in good shape and we can just buy a used rod/piston, or have this rod rebuilt.

There is enough wear that #1 cylinder at TDC is about .020" below the deck surface, whereas all the other pistons protrude a few thousandths like their supposed to. What this means to me is that when the motor is running, #1 is loose enough to protrude past the deck far enough to smack the head. When the motor is cold, the oil is super thick and the piston is cold and "not swelled". After running a few minutes, it gets warm enough to make contact.

I'll bet we could get the knock to go away for a while with an oversized head gasket, but it would only be a matter of time before the connection rod assembly gets loose enough to make contact again.

This is #2 piston for comparison sake as it looked without cleaning:

The next two pictures are the head @ cylinder #1. All of the valves and seats visually look in-tact and correct.

Cylinders #2-6 look great, perfect cross hatch, no signs of wear. Cylinder #1, however, pictured below has signs of wear up near the top. I assume this wear is from the piston hitting the head and wobbling a little till the slack in the bearings or piston pin or whatever is taken up and piston starts travel down. None of the marking can be caught with your fingernail or even felt by hand or with a tool, but you can see the discoloration.

I assume this will hone out when we do an "in-frame" rebuild of cylinder #1.

For these pictures, the piston was cleaned with a shop rag and a little brake cleaner. This is at #1 TDC and you can see that the piston does not protrude. #6, the companion cylinder protrudes roughly .015" if I had to guess, far enough to feel with your bare hand or finger nail. This along with the wear on the top of the piston between the valve relief pockets lead me to the diagnosis that #1 has vertical slop and is hitting the head. The other 5 pistons have legible factory markings, #1 is only legible in the valve relief pockets where it can't make contact with the head.

 

[Big Blue24]

$100 says it is the fuel system....

I'll take that bet!!! Please gift the money through paypal so I don't get screwed over on the 3% transaction fee..... LOL




Back to the rebuild:

Tips?

Advice?

Suggestions?


Would anyone dare slap it back together with an oversized head gasket? The owner wants to keep the truck for a while so he decided to fix it correctly. Just curious if anyone has had this problem and "patched-it" together with a thick head gasket.

 

[Joesixpack]

GAWD NO.....thicker gasket.....LOL Either rod bearings or wrist pin % on the bearings, more than likely the crank will need to be cleaned up. Horseshoe planted up ass and its in the wrist pin a relatively painless fix. (except for the horseshoe)

 

[JWhite]

I'll take that bet!!! Please gift the money through paypal so I don't get screwed over on the 3% transaction fee..... LOL

I'm sure you would.

How long was the engine knocking before he took it somewhere? That isnt the sound of a spun rod bearing, it is too consistent. BUT I suppose that if the bearing is completely GONE....

Did you ever cut open the filter?

 

[Big Blue24]

I'm sure you would.

How long was the engine knocking before he took it somewhere? That isnt the sound of a spun rod bearing, it is too consistent. BUT I suppose that if the bearing is completely GONE....

Did you ever cut open the filter?

The knock is because the piston is so loose, it flies up past the top of the block deck when running and hits the head. In other words THE KNOCK IS THE METALLIC SOUND OF ALUMINUM HITTING CAST IRON The only way for this to happen is sloppy worn bearings, or possibly a damaged piston that has vertical play on the piston pin, or loose rod caps from maybe a half-failed rod bolt.




Next week we'll get the bottom end torn apart and I'll take pictures of the rod and bearing surfaces on #1. Then I'll come back and ask advice on how to fix a mangled crank "in-frame"

 

[coorsman2005]

wow atleast you caught it before it made a window

 

[JWhite]

I get what you are saying, just was curious if the bearing had spun to the point where it spit out all the material. If it is the crank journal it is probably flat spotted and cant be fixed in-frame.

 

[DRT086.7]

If it were me Id just pull the motor completely at this point, its alot easier to work on like that.

 

[Big Blue24]

I've never pulled or read up on pulling the entire 6.7 motor. I assume the tranny, turbo, downpipe, CAC, Radiator, AC, etc., etc. have to come out to pull the motor, sounds like a lot of work.....
Unless of course the crank is toast, then you'll be pulling the whole motor anyway (fingers crossed this isn't so...)

 

[Big Blue24]

Looks like all of the damage is consolidated to cylinder #1 rod, bearing, and journal. Also, the oil ring was broken, don't know how or why, but the compression rings are just fine.

 

[duck_04]

is the engine still in the truck?

 

[Big Blue24]

Should we polish the crank in-frame with some emery paper, measure, and get an oversized bearing?

What about new rings? Maybe just install a used rod, piston, and rings on #1 with a new oversized bearing?

 

[Big Blue24]

is the engine still in the truck?

Yes.

And it didn't take long at all to drop the pan and raise the motor a few inches.

 

[duck_04]

From the looks of the pics, the crank is probably gonna have to be turned. I would do it right while you are doing it.

 

[chance cobb]

ouch

 

[DRT086.7]

Crank has to come out or you will just be putting a short term bandaid on it.

 

[LReiff]

I have a complete 6.7 short block for sale.