Duramax Diesels Forum Truck of the Week
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Old 03-25-2009, 12:43 PM
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Default bent rod!

well i took my piece to gm today because i thought i had a fuel knock and my injectors were bad so i took the twins off and got up there and nope, not injectors, return rates are fine, balance rates even better, but when you ramp up the rpm's and cut out cylinder 7 the noise pretty much stops. tech said it sounds mechanical to him. he seems to think cracked piston, but i think maybe the rods bent and maybe the wristpin is hitting. the crap sucks as i never even got to get on it from a standstill with the twins.
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Old 03-25-2009, 12:46 PM
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oh and to add to it, the BADASS lbz modded cp3 pump i just put in, wont do anymore then 23,200 psi.
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Old 03-25-2009, 12:52 PM
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crap
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Old 03-25-2009, 12:54 PM
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Quote:
Originally Posted by 02bigstrokin View Post
well i took my piece to gm today because i thought i had a fuel knock and my injectors were bad so i took the twins off and got up there and nope, not injectors, return rates are fine, balance rates even better, but when you ramp up the rpm's and cut out cylinder 7 the noise pretty much stops. tech said it sounds mechanical to him. he seems to think cracked piston, but i think maybe the rods bent and maybe the wristpin is hitting. the crap sucks as i never even got to get on it from a standstill with the twins.
just a thought but if it was a bent rod why would the noise stop when you cut fuel to #7 cylinder , the crank and rods are still spinning ? could very well be mechanical but compression check should be a first step in deciding if the rods bent , and maybe balance rates but you said they were fine . and 23,200 psi what is the pulse width of your tune ? that pretty decent for a lbz cp3 depending on how modded it is, with my stock lb7 cp3 i could not go past 3250 usec without going under 21k
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Old 03-25-2009, 12:54 PM
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Originally Posted by 02bigstrokin View Post
oh and to add to it, the BADASS lbz modded cp3 pump i just put in, wont do anymore then 23,200 psi.
it really should should it?

It should just hold that pressure, stock one would just fall off....
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Old 03-25-2009, 12:56 PM
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sorry to hear about this
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Old 03-25-2009, 01:02 PM
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Do compression test.
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Old 03-25-2009, 01:27 PM
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That sucks, but look at the bright side, with a set of rods you can take full advatange of your twins.
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Old 03-25-2009, 01:41 PM
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If it's a cracked piston you'll have excessive crankcase pressure. Fuel or not you would likely (but not necessarily) have the noise if it was mechanical.

Looking at a log from a full pull that Chris got with his LBZ it held about 22,000psi of rail pressure with a bone stock CP3 and no lift pump. With a lift pump it holds over 23,000 down the track during 12.4 passes but has areas over 25000psi.

The max pressure a stock LB7 tune demands is 160MPa which is 23,206psi - sounds like it might be a tuning issue. Even with dual CP3s you won't get more pressure unless you ask for it and allow it.
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Old 03-25-2009, 01:45 PM
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Quote:
Originally Posted by 02bigstrokin View Post
well i took my piece to gm today because i thought i had a fuel knock and my injectors were bad so i took the twins off and got up there and nope, not injectors, return rates are fine, balance rates even better, but when you ramp up the rpm's and cut out cylinder 7 the noise pretty much stops. Tech said it sounds mechanical to him. He seems to think cracked piston, but i think maybe the rods bent and maybe the wristpin is hitting. The crap sucks as i never even got to get on it from a standstill with the twins.
X2, if you have a bent rod, the piston skirt would be hitting the crank all the time, not when you cut a cylinder out will the noise go away. More than likely the injector is at fault. Is it blowing oil out the vent? Do you have smoke out the exhaust? I thought i had a bent rod and i did a compression test, and found i have 400-410 psi on all cylinders. your issue is after the lbz pump swap and the tuning it to idle right.
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Old 03-25-2009, 02:17 PM
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keep us posted with what you find , but i would do a few other test before you let the dealer tear your motor down .
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Old 03-25-2009, 02:38 PM
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Originally Posted by trentnell View Post
keep us posted with what you find , but i would do a few other test before you let the dealer tear your motor down .
The dealer wont be touching this one. I am on the hunt for a compression tester right now
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Old 03-25-2009, 02:39 PM
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when we checked this today the tech had the truck in gear with the parking brake on and would cut out number 7 and it stopped. he said it quits because with out the fuel in there, theres not any pressures on that cylinder, and thats why it quits. i dont know much about them, but i do know we checked eveything we could about the injectors and he said they were some of the closest tolerances hes seen. note: this tech is my foremans good friend not just some dumbass thats just trying to get me out of his hair. hell he was going to do the injectors under warranty knowing that it had twins on it and all. as far as the pump goes, im mistaken because i took it to him with a stock lb7 tune in it with just b1010 changed to make it run smooth, so 23,200 is right. and for smoke, its been smoking white when first started, and really this happened the first time i got down on it with the twins. i was going to my friends house and rolled on it hard in 3rd and afterwards it sounded like i had an exhaust leak. well its not an exhaust leak and its steady getting worse.thanks for all the advice. i will do a compression test when i get my hands on a tester.
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Old 03-25-2009, 02:40 PM
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I had a noise in mine too not too long ago. The tech found it by turning the injectors off one at a time, turned out # 6 injector was leaking past the seal, replaced that one injector and the noise went away. If your not experiencing the other symptoms of a cracked piston/bent rod I'd have them look at the injector they turned off when the noise quit. The noise I had sounded like a ticking or exhaust leak under acceleration, got worse of the course of several months.

On edit, mine started with the addition of the larger turbo.....................check your injector seal
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Last edited by LarryJewell; 03-25-2009 at 02:42 PM.
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Old 03-25-2009, 02:43 PM
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I had a noise in mine too not too long ago. The tech found it by turning the injectors off one at a time, turned out # 6 injector was leaking past the seal, replaced that one injector and the noise went away. If your not experiencing the other symptoms of a cracked piston/bent rod I'd have them look at the injector they turned off when the noise quit. The noise I had sounded like a ticking or exhaust leak under acceleration, got worse of the course of several months.

On edit, mine started with the addition of the larger turbo.....................check your injector seal
thats exactly like mine. cant really hear it bad in park, put it in drive and its real noticable with a load on it. sounds like a ticking noise and is steady getting worse.
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