L5P tapping/knocking noise when coasting???

Duvel

New member
Aug 7, 2016
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Do any of you L5P owners notice a tapping or knocking noise that is synched with engine rpm when you let off the accelerator and coast? It only seems to happen between 1,000 and 1500 rpm. My 16 LML made a similar noise sometimes, but this L5P seems to do it more. It's not the typewriter tick, that's for sure. I'm probably just paranoid about the new platform, it's probably nothing. I'd still like to know what causes the noise.
 

kiwilml

Member
Nov 4, 2015
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16
Well I thought I'd noticed the same thing a time or two on mine already but wasn't real for sure.. I've only got about 500 miles on it so far..
 

S Phinney

Active member
Aug 15, 2008
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Quncy, Fl
I noticed a little more engine noise on the one I took home over a weekend. It isn't anything to worry about. It is just timing causing a little more noises than other years.


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M.A.M.

Member
Jan 9, 2016
426
7
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Ohio
Crazy loud on mine, it would be cause for great concern to me if I hadn't seen what Kory posted awhile ago...
 

M.A.M.

Member
Jan 9, 2016
426
7
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Ohio
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Duvel

New member
Aug 7, 2016
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Awesome guys! I'm super psyched there is an explanation for this! Glad it's not trying to come apart already, but why in the heck would GM do such a thing? Is there some benefit to it? Inquiring minds want to know...
 

THEFERMANATOR

LEGALLY INSANE
Feb 16, 2009
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They all do it to some degree. Look at an LB7 timing table, and how high the timing goes at 0 fuel. I always go in and bring it down to sane levels for coasting so you don't get that knocking sound during coasting. Another thing you may also notice is GM staggers when pilot turns off and on in the later trucks. It doesn't just turn it off for all cylinders at the same time, but instead does them in sequences to lessen engine noise when pilot drops out. With the new controllers, GM may now be running multiple timing tables for each cylinder so that when pilot shuts off, those cylinders will run a different timing table than those with pilot active. It would create an engine knock/rattle sound, but my bet is GM mapped it so cylinder pressures are still normal for all cylinders when it occurs.
 

2004LB7

Well-known member
Dec 15, 2010
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How much fuel is being injected at coast down? I'm surprised there is enough to cause a knock. I was under the impression that when coasting down the ecm cut fuel.
 

PureHybrid

Isuzu Shakes IT
Feb 15, 2012
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Shouldn't be any fuel injected so timing would be irrelevant. Unless it's making noise when the fuel comes back on to keep the engine from stalling.
 

NC-smokinlmm

<<<Future tuna killer
May 29, 2011
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I tought this engine was more quiet? Could that be the reason that you can hear it now? I test drove one, I couldn't put my finger on it but I knew I heard something different. I will say it has good throttle response, it was quite peppy. I still think GM did something with the tuning on the 16 LML's bc mine has zero dead pedal, my 15's was so bad it was scary. I really didn't realize it fully until I drove that 17, it didn't drive any different than my 16. Anybody else with a 16 LML notice this???
 

2004LB7

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Dec 15, 2010
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I know on my LB7 and my LMM when I let off the throttle, as long as the wheels are turning the engine, the fuel goes to 0mm until just before idle rpm. Both show timing values but 0mm fuel injected

The LB7 actually has a DFCO (deacceleration fuel cut off) in the tuning. Haven't seen it in the later years but they seem to do the same thing when coasting.
 

Awenta

Active member
Sep 28, 2014
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CT
Yes the main injection goes to 0us when you let off until you're going slow enough that it needs help to keep running. Hm now I'm curious to see if there is any pilot injection going on at that time though.


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THEFERMANATOR

LEGALLY INSANE
Feb 16, 2009
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When you're in a situation where the ECM is coasting, it will cut fuel. Even though your scanner may show 0 fuel, sometimes it will be enough for it to activate the pilot tables. It's supposed to take .016 of main injection iirc for the ecm to activate pilot for a stock lb7 i believe. So just because you show 0 on the main, it doesn't mean the pilots not active as I have 1st hand seen pilot come on while it showed main as 0. Also I know at least on mine, unless the converter is locked, it will normally inject some fuel even on decel. GM may have also made it try and keep fueling active for longer to try and keep it smoother on decel. Going to have to datalog one to see what is going on.