2008 6.7 Cummins Engine Knock

One word. Borescope. Pull the injectors and have a look around.
 
Using a stethoscope, the knock is definitely coming from cylinder #1 or #2.

When the motor is cold, the knock is almost undetectable. After running for about 2 min, the knock gets louder.

If you rev it to 1500 rpm and then dump the throttle, it will almost die and catch itself around 500 rpm where the knock ceases momentarily for 1/4 of a second, then it comes back.

I checked the flex plate bolts and they are tight and the flexplate looks just fine.


I pulled injector #1 and #2 and bore-scoped cylinder #1. The piston looks "too" clean and there are what looks like the circular pattern of a valve impression on the top of the piston. I can actually see the part number on the piston and the area next to the circular impression is shiney like polished aluminum. There was also a little bit of fuel in the bowl on piston #1. Can a hung-open injector cause a knock? Plausible?

It got late before I could really take a good look at cylinder #2, however, just looking in the bowl, it looks to have 10x as much carbon buildup.

The next task is to fully bore-scope cylinder #2, and then pull injector #3 to compare cleanliness/carbon build-up between 1, 2, & 3.


One more funny quark, when you put the truck in gear, it's an automatic, it will pull the motor down and almost die, and then recover. When you bump it into neutral, it will surge up to about 1500 rpm. Bad-fuel injector related????
 
To give a little more history, we have shut down injectors using the star scan tool and the knock does not go away. I then physically removed the electrical connectors from injector #1 and the knock doesn't go away.

However, in my mind, if the injector is slightly hung open, it will spray regardless of electrical input and therefore could still cause a fuel knock. Does anyone have experience with hung-open common rail injectors, is this possible?
 
Have you done a compression test, or better yet a leak down test? Is the indentation of the valve from the exhaust valve between #1 and #2 cylinder?

Paul
 
Big Blue24 said:
To give a little more history, we have shut down injectors using the star scan tool and the knock does not go away. I then physically removed the electrical connectors from injector #1 and the knock doesn't go away.

However, in my mind, if the injector is slightly hung open, it will spray regardless of electrical input and therefore could still cause a fuel knock. Does anyone have experience with hung-open common rail injectors, is this possible?
Click to expand...

Yes. The only way to truly eliminate an injector is to remove the line and cap the rail.
 
paulb said:
Have you done a compression test, or better yet a leak down test? Is the indentation of the valve from the exhaust valve between #1 and #2 cylinder?

Paul
Click to expand...

Tonight I plan to pull injector #3, then scope out 1, 2, & 3 to see which piston doesn't look like the others.

Then I'll probably pull the last 3 injectors and send all (6) out to be tested.

In the mean time, I suppose it would be prudent to do a leak down test, I have a hunch that at least one of the injectors is bad but I'd hate to get it fixed, put everything back together, just to find out that there are other problems like a valve seat or a rod bearing.


The valve impression is from the intake valve on cylinder #1, closest to cylinder #2. The impression is faint enough that it could be normal, and not a dropped valve seat, especially since it's the intake valve.
 
The knock is quiet while cold cause the oil is thick enough to cushion the blows of the rod on the bearing journal. THis doesn't not sound like a fuel knock. Have you cut the oil filter open?
 
I'd first pull valve cover for visual inspection and the turn engine over for further observation.
 
All (6) injectors were pulled last night. They are being sent out for testing this weekend.

The bore scope showed that #1 is really really clean with faint marks that look like a valve impression. #2 has moderate carbon build-up, no marks. #3 is a little cleaner than 2 but dirtier than 1, it has a few small 1/16" long nicks on the top of the piston, looks like foreign debris marks.

We'll do a leak-down test after we hear back on the injectors. If the injectors come back with a clean bill of health, the head is coming off next. If the injectors are bad, we'll leak-down test and then possibly reassemble with the rebuilt injectors.
 
Last edited:
An injector should have showed signs in the exhaust, that stink from advanced fuel or white smoke/haze.

That sound sure sounds mechanical to me. Tough through a tiny laptop speaker.
 
my vote is a bad injector caused to wash the cylinder wall/walls are destroy the rings. this happened to my 04
 
Looks like we'll have to buy an adapter to do a compression or leakdown test, dang odd-ball 6.7!!!
 
Why won't a regular 24v adapter work? I use a 24v adapter on 5.9 CR's. And 5.9 injectors fit in 6.7 and vise versa minus the hold down grinding.
 
Gibson said:
Why won't a regular 24v adapter work? I use a 24v adapter on 5.9 CR's. And 5.9 injectors fit in 6.7 and vise versa minus the hold down grinding.
Click to expand...

Yes. Snap-on EEPV313A 24v adapter works on a 6.7 head.
 
All (6) injectors tested good.

The owner has decided to quit messing around and the head is coming off this week.
 
You must log in or register to reply here.
Back
Top