Duramax Diesels Forum Truck of the Week
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Old 01-21-2017, 10:41 AM
Duvel Duvel is offline
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Default L5P tapping/knocking noise when coasting???

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.
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Old 01-21-2017, 11:13 AM
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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..
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Old 01-21-2017, 12:57 PM
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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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Old 01-21-2017, 03:39 PM
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Crazy loud on mine, it would be cause for great concern to me if I hadn't seen what Kory posted awhile ago...
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Old 01-21-2017, 03:52 PM
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Quote:
Originally Posted by M.A.M. View Post
Crazy loud on mine, it would be cause for great concern to me if I hadn't seen what Kory posted awhile ago...
Well don't be shy, what did he post?
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Old 01-21-2017, 04:41 PM
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Old 01-21-2017, 05:07 PM
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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...
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Old 01-21-2017, 06:45 PM
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My LMM does that when you let out of it, sounds like marbles
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Old 01-21-2017, 09:06 PM
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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.
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Old 01-21-2017, 11:36 PM
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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.
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Old 01-22-2017, 05:42 AM
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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.
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Old 01-22-2017, 06:47 AM
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Quote:
Originally Posted by PureHybrid View Post
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.
Fuel has to be injected at 0% throttle or the engne would shut off.
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Old 01-22-2017, 06:54 AM
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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???
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Old 01-22-2017, 08:10 AM
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Quote:
Originally Posted by DMAXchris View Post
Fuel has to be injected at 0% throttle or the engne would shut off.
Not at above idle speed
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Old 01-22-2017, 08:42 AM
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Quote:
Originally Posted by PureHybrid View Post
Not at above idle speed
I think they're taking about that couple seconds when you let out of the throttle, mines always been pretty bad, if I run the stock tune it's horrible, I'd say it's just more pronounced with the new injectors and higher rail pressure
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