Noticed something strange with my 6.2..

Discussion in '1982-Present GM Diesel' started by diesel4me, Nov 18, 2019.

  1. diesel4me

    diesel4me 1 ton status Premium Member

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    I put about 50 miles on my 6.2 every weekend lately,mostly on back roads ..

    The subject is the noise the 6.2's all seem to make..

    I've noticed the usual diesel "clatter" the 6.2's make (like hailstones on a tin roof) is most pronounced at a fast idle,when you are cruising along between 30 and 45 mph..
    When I let off the throttle and coast,it gets almost silent in comparison..that isn't unusual really,its always been that way..

    This engine in my pickup is a lot noisier than the 6.2 in my '85 K-10 Suburban for whatever reason...I looked at the injector pump timing marks ,and its dead on the scribed lines,so I doubt it's been fooled with..sounds like it is advanced more than the Suburban's engine though,that one is much less "clattery"..it also doesn't seem as peppy or responsive as the pickup either..

    What puzzles me is on the ride I take ,there is a long downhill grade about 1/8th of a mile long,and I have to make a 90 degree turn at the bottom of it,so I must slow almost to a complete stop to take the corner..
    I often pull the TH400 down into second gear so engine braking keeps the truck from gaining a lot of speed down the hill..then I take the corner,put it back in drive,and go on my way..

    Then comes the puzzling part--the engine always quiets down after that,to the point its almost silent!--no "clatter" at any rpm/ speed,for at least 2 miles,then the clatter gradually returns,and remains "noisy" as it always sounds the rest of the ride..

    Makes me wonder if the engine gets a lot of oil puddling in the timing chain area while it is pointed downhill and I had downshifted it,or maybe oil gets sucked past the valve guides and lubes them up better temporarily--there is no oil smoke while coasting ,and no difference in how it runs,I would expect any excess oil getting sucked in the combustion chamber would make it rev up for at least a few seconds as it got burned if that was the case..

    It does this at the same hill every time,so it isn't just a fluke that happened once or twice..I'm taking dozens of times..

    Just seems weird to me,like maybe the engine isn't getting oiled somewhere enough..but it has at least 25 psi at idle and over 40 psi above idle,just as it always has..engine has no other knocks like a loose bearing or rockers,lifters,etc either..so it's not like its going to pop or anything,its just puzzling to me why it can get so quiet temporarily..
    Any one have a theory ?..:dunno:...
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    The elevation change is causing temporary hearing loss? :D
     
  3. imiceman44

    imiceman44 Hoarder extraordinaire Premium Member

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    I am not sure what the clatter is from on diesel engines.
    If I knew I would probably have an idea of what is happening.
    I will do some research and get back here
     
  4. imiceman44

    imiceman44 Hoarder extraordinaire Premium Member

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  5. diesel4me

    diesel4me 1 ton status Premium Member

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    On a 6.2 the clatter never really goes away!..
    (Unless the driver goes deaf,as Rene suggested !)..
    Guess I'll never know the answer--the quora explanation doesn't say why it might quiet down temporarily..but does list the reasons why a diesel clatters..it's a mystery..
     
  6. imiceman44

    imiceman44 Hoarder extraordinaire Premium Member

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    Well if you read all of it you'll see it clatters more when it is pushing a lot of fuel so after you run it hard and you are coasting it will quiet down until it loads up again.
     
  7. Barrman

    Barrman Registered Member Premium Member

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    I have had 4 6.2 trucks and a mechanical 6.5 truck on the road for the past 9 years or so. I agree the clatter changes sometimes. Especially with a manual and pulling heavy loads. After a long grade, up shift and it seems to not make noise at all.

    Of course, that is in my M715. Which has no insulation so maybe I just loose my hearing at that point. But, a few rev matching down shifts when driving in town do the same thing.

    However, my GEP 6500 in the Cowdog V20 Suburban with a Banks turbo and diamond precup turbo heads is annoyingly loud with severe clatter. I thought it was the turbo heads with the military 6.5 IP when I first built it. Now I have a 4911 spec IP on there and still super clatter. Listening to a 2002 center mount 6.5 turbo in a van that makes the same noise I am convinced it is the head/pre-cup combination.
     
  8. longbedder

    longbedder 1/2 ton status Premium Member

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    The clatter is all from the combustion events.

    The nature of combustion changes according to the combination of RPM, fuel input, and crankshaft load.

    You're just putting it in a unique condition for a bit on that stretch of road, and it's repeatable.

    New diesels actually actively control noise with multi-squirt injection schedules.
     

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