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engine knock at start up

This is a discussion on engine knock at start up within the Internal Engine forums, part of the LSx Technical Help Section category; Hey everyone whats up. I need some imput. I got a 99 Z28 with 73k miles. I just bought a ...

  1. #1
    pewterz28
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    engine knock at start up

    Hey everyone whats up. I need some imput. I got a 99 Z28 with 73k miles. I just bought a new 2500HD to pull my boat so now i can park the Z and start to beef it up. My problem is when it sits overnight then you start it up it knocks for about 15 seconds then it wont do it the rest of the day. Full throttle, idleing whatever it makes no noise. So what the deal. Are some lifters collapsing overnight, or do i have some piston slap. If a bearing was bad it would knock all the time. I have no experience with these small blocks. Ive build a few big block marine motors so any imput will be greatly appreciated

  2. #2
    Junior Member
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    Pewter
    1999 Camaro Z28

    Sounds like it could be piston slap to me.
    99 pewter Z28 M6, SS hood, 10 spokes, Brembo rotors, lid, K&N, Ripper, CAGS, Pacesetter LT's, Borla, TSP ORY-pipe, O2 simulators, STB, SFC's, free mods. Cant forget about my hot fiance in the passenger seat!

  3. #3
    Senior Member NeeD4SpeeD's Avatar
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    Collectors Edition Yellow
    2002 Pontiac Trans AM WS6

    How do you fix piston slap cuz i get the same thing when the car is really cold and a start it up... but my noise is only when i put the car in 1st and start movin from low RPM..

  4. #4
    pewterz28
    Guest
    If thats what it is and im willing to bet it is, you gotta get the cylinders honed or bored .010 over. Piston slap isnt that big a deal. Mine is still under warrany so maybe they will deal with it. if not i guess its time to make it a stroker. I want to get about 450-475 hp i just need to figure out what i want to do. Thanks for the imput

  5. #5
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    white
    1987 mc ss aerocoupe

    piston slap at start-up is pretty normal for the genIII small blocks. GM's stance is 2 minutes past cold start. these engines have a very short skirt on the pistons in order to eliminate drag friction. i have worked on hundreds of these engines with piston slap and long miles that show no abnormal wear at teardown.

  6. #6
    Senior Member NeeD4SpeeD's Avatar
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    Collectors Edition Yellow
    2002 Pontiac Trans AM WS6

    Thats good news then!

  7. #7
    pewterz28
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    sweet that makes me feel better.

  8. #8
    maroonZ
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    I have the same thing with my car.

    Living in Ottawa it has gotten pretty cold (-35 to -40C). I just let the engine warm up to about 60-70 C before I drive it anywhere. Usually takes about 3-5 mins.

    Don't see many Z's this time of year in this part of the world.

  9. #9
    Maybe the oil filter is draining back.What oil filter are you using?Try a Ac

  10. #10
    ruff ruff rotwiler's Avatar
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    LT's, 3400 stall, FIPK,CB
    2002 Z28

    I only have a little clatter in really cold weather, but a anti syphon valve should help on the oil filter. Mobile 1 10/30 seemed to quiet mine down to almost non existant, but 5/30 didn't help at all, kinda odd since they are so close. If it is piston slap, then neither would help much though.

  11. #11
    pewterz28
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    I use an AC filter i heard the frams were really crappy. Mine only does it in the cold anyway. Its gonna be a summer car now so im not gonna worry about.

  12. #12
    Space Cadet RainMan's Avatar
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    Red & Dk Gray
    2002 WS6, '08 G8 GT

    WIX Filters, excellent, been around for years. Excellent filter media, coil spring, burst resistant canister. Quality construction throughout.

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