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Fuel knock sound - Lonestar10 - 05-11-2025
I have a fuel knock sound starting or atleast audible around 1300 rpm and throttle of 50%+ started a little bit after overhaul.
Also when you first start truck engine will run smooth while building air as soon as it finishes building and air dryer blows off it will start acting like a gasser with an audible miss sound in exhaust note.
Also after going down road when parked hot will run smooth but for a while then after heatsoak wears off will start to stumble again.
Engines pulls smoothly no signs of miss but on takes offs from stop will throw more black smoke then it used to. Used to just be a little poof now it's a second or 2 haze (black in color).
Fuel mileage down used to actually have to purposely try to get below 6-6.5 now 5.3-6.3mpg is normal used to be 6.5-7.5 and really nice tailwind 8+
Here's what's been done with no changes.
Injectors, air compressor, overhead few times, turbo, ex back pressure sensor and cleaned tubing, pressure tested charge air system,
I bought a timing wedge kit so going to check for cam slip today or tomorrow.
Just wondering where else to look if cam hasn't slipped. Also head was new on overhaul also new rocker arms, crossheads
Isx 2350, 13sp, proper approved tune, 450/1650 ratings.
RE: Fuel knock sound - Rawze - 05-11-2025
do an injector leakdown test and make sure it passes.
ref: https://rawze.com/forums/showthread.php?tid=8728&pid=75483#pid75483
Otherwise, Its somewhat normal for fuel mileage to drop a bit on a fresh inframe until it wears in good. My own truck took almost 80k miles before I started seeing it come back and the extra smoke to start to dissipate.
RE: Fuel knock sound - Lonestar10 - 05-11-2025
Leak down passes, sorry forgot to add around 250k on overhaul been chasing my tail a while, also rebuilt fuel pump about 30k ago, no visible air bubbles in davco... davco to hp pump I have no idea how to check. Banjos are tight no visible damage to oring washers.
Also my oil used to stay pretty clean almost like new with 15k oil changes now it isn't black but it's very dark.
My dad just had his 2350 overhauled start his truck with hood open you can still talk normally to someone where mine is a lot louder and idk how to explain but mechanicly louder, clangy idk how to say it his is 550/1850 proper tune.
RE: Fuel knock sound - hhow55 - 05-11-2025
Under sympton search in quickserve, you will find info to troubleshoot your problems for the black smoke, and engine knock.
RE: Fuel knock sound - mikkhh - 05-12-2025
Are the temperature sensors reading ok? Fuel temperature, air temperature?
RE: Fuel knock sound - Lonestar10 - 05-12-2025
(05-12-2025 )mikkhh Wrote: Are the temperature sensors reading ok? Fuel temperature, air temperature?Never thought of that will check tonight
RE: Fuel knock sound - Fleetman13 - 05-12-2025
There is a sh*t ton of tsbs on the 2350 for excessive engine noise in quick serve. I don’t remember the details on them but was thumbing through them looking for a one off problem the other day before I dug my heels in….might start there
RE: Fuel knock sound - Andre_The_Giant - Yesterday
I have a cummins cm870 that is just loud! My father bought it brand new & sold it to me used that way and it’s still that way. 19 years later she still runs. Loudness isn’t always an issue, but a skip could be. If it doesn’t skip all the time & is very hard to diagnose I would let it get worse. Eventually a leaking injector or bad whatever will show its self. Or be easier to spot out. I chased a small rattle for the first few years of owning the truck. 10yrs of owning it later it still does it from time to time. I gave up.& it thought me this lesson
if your pulling hills loaded strong, just drive the truck
RE: Fuel knock sound - Lonestar10 - Yesterday
I've been putting up with noise for almost 2 years slowly changing parts that I think could cause it and the skip. So far crank sensor, imap, exhaust, air compressor,rebuild fuel pump, injectors, turbo, cam sensor will be in today, doing wedge timing check today.
After that only things I can think of is fuel pump wasn't timed when front case was rebuilt, excessive gear lash, or clutch issue.
After it builds air when cold it starts the skip and fueling jumps erratically from 0 mg/stroke to 20-28 mg/stroke. Commanded stays around 3 grams.
As far as I know fuel actuator, fuel gear pump, fuel lift pump, hp fuel pump (cam lobes look good) are all original 1.2 mil miles