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Chevy 350 light knock

Discussion in 'Engine & Performance' started by TrucksNameIsEarl, May 21, 2020.

  1. TrucksNameIsEarl

    TrucksNameIsEarl Junior Member

    Posts:
    4
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    Joined:
    May 4, 2020
    Location:
    St Louis, MO
    First Name:
    Brian
    Truck Year:
    1985
    Truck Model:
    High Sierra K1500
    Engine Size:
    Rebuilt 350
    Got an 85 GMC K1500, 305 from the factory, swapped rebuilt 350 in it currently, it's developed a light knock! On start-up it's not there, after about 10 minutes of driving it's really noticeable (audible and feel it under your feet in the driver's seat) when braking. Now it could be that it's always there and the noise of the engine and exhaust drown it out, but you can only feel it when braking. Any ideas? Hoping it's some kind of detonation issue. Going to be changing out the spark plugs this weekend, throw some oil thickening agent in the block, and try a slightly higher octane gas.

    Any other suggestions would be much appreciated!
     
  2. TrucksNameIsEarl

    TrucksNameIsEarl Junior Member

    Posts:
    4
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    1
    Joined:
    May 4, 2020
    Location:
    St Louis, MO
    First Name:
    Brian
    Truck Year:
    1985
    Truck Model:
    High Sierra K1500
    Engine Size:
    Rebuilt 350
    Forgot to mention.. I've taken it to a mechanic buddy of mine to make sure drive components and suspension are good to rule out any possible drive-shaft or random suspension tolerance issue, but he seems to think that all checks out
     
  3. Bextreme04

    Bextreme04 Full Access Member

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    Location:
    Salem, OR
    First Name:
    Eric
    Truck Year:
    1980
    Truck Model:
    K25
    Engine Size:
    350-4bbl
    There is nothing I can think of off the top of my head that would make an engine "knock" so hard that you can feel it in your feet only on deceleration. The engine is under the highest load at acceleration and should be basically free wheeling during deceleration. You might want to check the torque converter bolts, transmission bellhousing bolts, and engine/trans mounts
     
  4. Ricko1966

    Ricko1966 Full Access Member

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    Joined:
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    Location:
    kansas
    First Name:
    Richard
    Truck Year:
    1975
    Truck Model:
    c20
    Engine Size:
    350
    Look for a cracked flex plate.
     
    Bextreme04 and 84 M1008 like this.
  5. TrucksNameIsEarl

    TrucksNameIsEarl Junior Member

    Posts:
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    1
    Joined:
    May 4, 2020
    Location:
    St Louis, MO
    First Name:
    Brian
    Truck Year:
    1985
    Truck Model:
    High Sierra K1500
    Engine Size:
    Rebuilt 350
    Thanks for the replies, guys!

    After driving around a bit and seeing if I could recreate it, it occurs when braking hard. Braking softly doesn't seem to upset the vehicle. I will check out mounting bolts and flex plate!
     
  6. TrucksNameIsEarl

    TrucksNameIsEarl Junior Member

    Posts:
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    1
    Joined:
    May 4, 2020
    Location:
    St Louis, MO
    First Name:
    Brian
    Truck Year:
    1985
    Truck Model:
    High Sierra K1500
    Engine Size:
    Rebuilt 350
    Ha! Ok, found a bolt missing on the transmission mount! 12mm bolt I presume? I'll pick a bolt up tomorrow and give updates
     
    Bextreme04 likes this.
  7. 1987 GMC Jimmy

    1987 GMC Jimmy Automobile Hoarder

    Age:
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    Joined:
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    Location:
    Mississippi
    First Name:
    Jesse
    Truck Year:
    1987
    Truck Model:
    V1500 Jimmy
    Engine Size:
    350
    74C9E7CE-0DBF-4E10-BB44-A242FBA37A79.gif
     

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