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Discussion Starter #1
Started noticing a slight clacking, but not quite what I would consider a “knocking” sound at low rpm. It only really seems to happen from about 1000-1300ish rpm (it does seem to lessen the longer the truck is running or in use). New fuel filter, and air filter, fresh tank of fuel. Truck is bone stock, doesn’t smoke or surge, heads were done about 15k ago,

just this clacking sound that’s driving me crazy.

anyone have any ideas?
 

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Sometimes low-octane fuel is to blame; sometimes it's deposited on the piston crown. Fuel with too low an octane rating for your engine can sporadically ignite prior to the piston reaching TDC. The resulting shock wave rattles the piston inside the cylinder, creating the knock, ping, or can-of-marbles sound you hear.
 

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Since this is in the LLY section, we don't talk about octane :). Sometimes we talk about cetane.

Diesels inherently make ticking, tapping, clattering, and clacking noises. So most of the time, it's normal. The first time I drove up to 9k+ feet mountain pass, and stopped and let the truck idle, it clacked so loud it concerned me. Since that was about 13 years ago and it does it every time I go over a high mountain pass, I am no longer concerned. In other situations there's ticking, tapping, or clattering. Yeah, I ignore those, too. Sometimes on the freeway (especially with bio-diesel) it's so quiet you'd never guess it was a diesel. So there you go!
 

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Discussion Starter #4
Thanks for the replies, this usually happens pretty regularly when pulling out of a driveway, parking lot, or away from a light, usually if I get on it very lightly, between 1-1.5k rpm... I did notice today it definitely happens less once it’s been running and is fully warmed up.

full disclosure we’re at 328k and this is my first ultra high mile vehicle, so if it’s normal, I wouldn’t know lol.
 

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Possibly sounds like your injectors are getting a little gunked up and sticking a little when cold. They can't do the split shot sequence correctly so it runs a little noisier, like an old diesel.
 

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full disclosure we’re at 328k and this is my first ultra high mile vehicle...
Lol, "ultra"

Anyway, how about when cruising at almost 0% throttle? Just before engine braking, do you hear it then? Why were the heads done? Aside from the regular diesel noises which are pretty loud, a certain rpm and low load could be piston slap. Then again mine is over 642k and the pistons are fine...
 
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Discussion Starter #7
“Ultra” high mile in my world, I usually off a vehicle before 150k.

I don’t hear the sound at idle or close to idle before engine braking. It’s honestly usually just low load acceleration, like pulling out onto the road from a parking lot etc, or very minimally from a standstill at a traffic light.

heads were done after a gasket failure at around 300k. Was done by a reputable shop in FL, however it was before I purchased the truck. I did pick up an AutoCal and checked balance rates on injectors and they’re between -2 and 1, so I think I’m okay there. If this is piston slap at that low rpm low load condition, is it something to worry about or just nature of the beast for a higher mile duramax?
 

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Do you have anyidea of the previous owner's use of the truck? As in was it tuned and abused?

These engines are living wayyy past the 200-300k range, thats why I giggled a little, yes "ultra" compared to the typical daily driver...

If it is piston slap, thats not good, it could go a long time or not. This usually ends up breaking the piston skirt and scoring the cylinder wall and if run long enough can cause extensive damage with metal bits everywhere...
 
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Discussion Starter #9
Previous owner used it primarily as a commuter vehicle, with occasional towing of a small trailer and some payload in the bed, never a fifth wheel or goose neck. It led a pretty easy life of 90-150 miles a day of highway. I’ve listened to some audio of piston slap and it really doesn’t seem to fit here, I’ll try and get some decent audio of the symptom and post, hoping maybe just an injector cleaning cycle will do the trick, or maybe I just have bitchy sensitive ears and I’m worrying about nothing...
 

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Piston slap is kinda a long shot... but unique... can sound as mild as valve lash (ticking) hopefully its not a big deal in your case, and maybe it is an injector issue. The other end of the spectrum from abuse is if maybe, just maybe the truck sat for a while while there was a slight bit of moisture in the fuel and that tiny ball check corroded just a little...

Tell you what, my LLY never went more than 3 days without running, I ran injector cleaner a lot, not every tank, too many tanks to think about... BUT when I changed the injectors and high pressure fuel lines at 613,000, they were unbelievably dirty inside... and they may not have needed a change... I just started getting nervous about an injector maybe sticking open and melting a piston...
 
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Read up on the split shot injection. The ability of these injectors to fire multiple times per single combustion event is what makes the motors so quiet. What you are hearing is likely just noisier combustion since it can't time the sequence properly or shuts it down entirely. Common rail injectors are known for building internal deposits, once the injectors get hotter they are likely not getting as much resistance from the deposits and the fuel timing gets closer to normal.
 

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Discussion Starter #12
This is all amazing info guys, I truly appreciate the knowledge sharing! I will do some more homework and read up. Does anyone happen to know if there are any PIDS I can read with efilive through my autocal that may give a better idea than just the balance rates?
 

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You can monitor pilot injection quantity and timing with a cts, so you just have to look up the pids, should be listed in efi scan tool if you have it. I have a v2 so im not sure how functional the autocal is. Pilot is disabled about 2500rpm anyway and you know it. Thankfully the LLY only uses one pilot event... my friends 2012 Cummins uses 5 or 6 events per stroke... I think 3 or 4 pilot and 1 post at certain times. I like to keep it simple.

The pids will tell you what is commanded, it doesnt have a clue if its actually doing it. I had an injector connector start to lose connection on a pin and you would hear an occasional knock for a little while before it would throw a code because the ficm realised the impedance was off, but it doesnt know if any fuel is passing through the injector except for idle where it monitors rpm several times per revolution and even then it's adjusting main injection not pilot as far as I can tell.
 
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