Odd fuel knock

[C.bowles2]

Heres my problem. When i start my truck and its cold im experiencing a fuel knock at idle, but when i go from park to drive or reverse. So when theres a slight load at idle, when i increase rpm's it goes away. And when i put it in park or neutral it goes away also. Also another odd thing is if i put it in reverse and move backwards at an idle and hit the brake real hard i can hear the knock worse. And sometimes when in drive and right iff the get go while accelerating i can hear it too. Any ideas? 230k miles on cp3 and injectors. Balance rates are in spec. More recent things ive done is a turbo and running 1/2inch fuel line from lift pump straight to injection pump at 9psi.




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[Ridin'GMC]

I have a similar thing going on right now, I can hear it more whenever I'm in 6th gear but don't hear it in the other gears. I'm curious what yours comes out as. I haven't checked my rail pressure yet, hoping it's not the cp3. Have you checked your rail pressure?

 

[C.bowles2]

Yea my rail pressure is fine and truck runs great. Its odd




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[henery97]

I have the same thing going on as well. Injectors, cp3, and pistons have 8k on them. thought I had a bad injector (return rate was wayyyyy high) so i swapped it out and it made no difference. Kind of a knock/tick at low load.

 

[WVRigrat05]

Colder weather and winter blend fuel. Mine does the same thing. Put the cover on the front last night on my way home from Ohio, after a few miles it quit

 

[C.bowles2]

I know my merchant motor mounts probably pronounce it even more. Well if its normal then i guess i wont worry about it.




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[WVRigrat05]

Speaking of which, I need to order a set, I broke mine the day before yesterday. Time for the nickel and diming to begin.

 

[C.bowles2]

Bump, anyone have any idea what this could be? Also when im in drive at and idle and hit the brake my lift pump pressure spikes a couple psi from 9 to 11




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[02greysixer]

Try running a lubricity additive in the fuel see if it changes. My money is on an injector slightly overfueling at pilot injection. I've chased ghost knocks just like y'all are describing on several lly/lbz trucks. Every time I've ended up pulling the injectors and testing them. Every time at least one has failed at pilot injection on the stand but had perfect balance rates, return rates and pressures on vehicle.

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[C.bowles2]

Hmm odd ok. Well im due for slightly oversized injectors anyhow. So this is a good excuse to upgrade. Would running a half inch line straight to the injection pump from the lift pump cause this?




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[C.bowles2]

Also this is kind of off topic but is there anyway to tell with efi live data of an lly has over sized injectors?




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[02greysixer]

Hmm odd ok. Well im due for slightly oversized injectors anyhow. So this is a good excuse to upgrade. Would running a half inch line straight to the injection pump from the lift pump cause this?




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Could have if the line was contaminated or picked up some trash while you were routing it under the truck.

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[Burn Down]

This was my fuel knock experience, damn near pulled all my hair out Very similar to yours...
http://www.duramaxdiesels.com/forum/showthread.php?t=65367

 

[C.bowles2]

This was my fuel knock experience, damn near pulled all my hair out Very similar to yours...

http://www.duramaxdiesels.com/forum/showthread.php?t=65367

All that and it never three a code? Glad you figured it out. Im gonna check mine although i dont think thats it at all




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[henery97]

I pulled my injectors out and sent them to exergy and they found scoring on the armature pins & 2 junk nozzles. I haven't ran it since they came back but I hoping this fixes my knock/tick.

 

[WVRigrat05]

Mine knocks too under light throttle and wfo, but I 95% know I have an injector or 3 going.

 

[C.bowles2]

I can only hear it sometimes. It comes and goes. I also hear it when i hit bumps? Does this make any sense? I wiggled injector 2 and 7 connectors and couldn't get it to act up.




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[Burn Down]

Did you check the ground wires?

 

[C.bowles2]

Yea they were all tight




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[C.bowles2]

Well update..... Turns out it wasnt even anything fuel related and it was my steering shaft vibrating at an idle, who would have known lmao sounded just like an injector




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