Drivetrain Vibration Questions

Discussion in 'Troubleshooting & Diagnosis' started by Glava, Nov 15, 2021.

  1. Glava

    Glava Veteran Member Lifetime Gold Member

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    As I've mentioned, I bought my Z28 in September 1979. As I also may have mentioned, I have had a slight, seat of the pants, vibration gremlin since I got it.
    Since the vibration is at about 70 mph, it didn't bother me much, because with 55mph speed limits I didn't go much over 60 most of the time.
    Plus, I now only have 31k miles on it, so there were never any really long, fast highway trips.
    But, this past summer, I started to drive the Z more and at higher speed -basically 70mph which seems to be the "slow" speed on the interstates. So, after 42 years I have decided to finally go all in after the gremlin.

    Here is the starting point.
    My Z has a 350 engine, a close ratio 4 speed BW T-10 tranny and a 373 posi-traction rear end and 31k miles.
    I bought the Z in Sept 79 and it was off the road shortly after for the winter.
    Mine came with Uniroyal steel belted radials and in the winter I always put it on floor jacks.
    I don't remember when I first noticed the vibration, but again it was at higher speeds, so it was not a major concern.
    I'd say after maybe 3 years of ownership, the Z started pinging bad one day like the timing was off. I took it to the dealer and they found that i had a bad "soft" camshaft and that a lobe was whipped. It was out of warranty, but it only had 19k miles on it, so GM replaced the cam, the timing gears & chain and I think some lifters - that is the only time that anything on the Z was touched -it is still100% stock.
    I had the Uniroyal tires rebalanced several times over the years and one time even replaced one rim, that they said was slightly bent.
    The vibration still always showed up at about 70mph.
    My tires never wore funny and I changed them at about 29k miles because of age.
    I actually sold them on the internet instead of keeping them, because I could get at least half of what replacements were going to cost.
    I bought a set of retro Goodyear radials and had them mounted and balanced -I went for the first ride on them and had the same vibration at about 70mph...
    The tires have about 2k miles on them and earlier this summer, I took it to a great shop and they road force balanced them -the vibration is still there and the tires are wearing evenly.
    So my questions at this point are what are the best steps to take to track down the gremlin;
    1) Do I assume that the tires/ rims are not the issue, since replacing the the tires and 1 rim never changed anything?
    2)If I my memory is wrong and the vibration came later, could the cam replacement come into play?
    3)On here, I have read that a bent driveshaft or axle can cause vibrations -would that be just at a specific rpm?
     
  2. COPO

    COPO Veteran Member Lifetime Gold Member

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    I ordered new Z28 4 speeds in 1977, 78 and 80 and never had a problem with vibrations at any speeds.

    Here is what I would check out if it was my car.

    1) Front wheel bearing preload - Backoff nut, spin rotor and tighten spindle nut to 12 lbs. ft. Backoff nut to 1 flat and insert cotter pin. If hole doesn’t line up then back nut off an additional 1/2 flat. Rotor should spin freely. Insert cotter pin.
    Bearings should have a .001” to .008” end movement when properly adjusted.

    2) Transmission- If you leaking trans fluid at the tailshaft driveshaft seal and a new seal didn’t seal it then the aluminum bushing needs to be replaced and of course a new oil seal which would get damaged from the removal.

    3) Driveshaft - Look for obvious signs of missing balance weights. Also if you have a dial indicator, see if there is lots of runout at each end and the middle.

    4) Rear universal joint - Check for loose play by grabbing it in your hand and turning it when in gear. Change universal if you have excessive end play.

    5) Tires/rims - Move the wheels that have the most balance weights to the rear and have the correct air pressure when cold.

    6) Unibody rubber mounts - if these are shot then the car will want to bend at the front in a downward direction towards the driver.
     
    Last edited: Nov 15, 2021
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  3. Glava

    Glava Veteran Member Lifetime Gold Member

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    Thanks for looking.
    With you on moving the tires around!
    I was thinking of doing it with just the rear on jack stands -and maybe take the front out of the mix. That way I could try it without tires & rims -which may rule them out if the shake is present.
    Then I could maybe see if the rear end is visibly moving, which would lead to checking the universals and drive shaft for damage or missing weights. Maybe I should just bring it to a drive shaft repairer and have it checked for balance and true to totally rule that out.
    Even though I only have 32k miles -I have done long term preventive maintenance and repacked the wheel bearings at least a couple of times.
    Maybe it could have been built with bad body mounts.
    Thanks again!
     
  4. Chevyforever

    Chevyforever Veteran Member

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    My two cents is on the driveshaft, being the culprit.
     
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  5. MChamp

    MChamp Veteran Member

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    Why replace just one rim?? And which one...
     
  6. cadillac_al

    cadillac_al Veteran Member

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    It doesn't vibrate at 60 mph but it vibrates at 70 mph, what does it do at 80? A tire vibration will usually lessen at a higher speed. A Ujoint will keep on grinding at higher speeds. A bent wheel or axle flange will get worse at higher speed. Some of these clues might narrow it down.
     
  7. biker

    biker Veteran Member

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    When my 81 with a T10 4 speed was stock, I had a similar vibration in mine. Long story short, a 1/8 spacer under the transmission cured it. The factory springs had sagged far enough that it changed the pinion angle and caused the high speed vibe.
    At least, that's the theory I'm going with.
     
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  8. COPO

    COPO Veteran Member Lifetime Gold Member

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    Maybe and maybe not. I owned my 80 Z28 4 speed for 10 yrs and racked up over 100,000 Km without changing the front or rear springs and had no issues. Maybe your body mounts were toast.
     
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  9. Glava

    Glava Veteran Member Lifetime Gold Member

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    The garage told me that it was bent -it was about 30 years ago, so it has been rotated etc. and is most likely not in the origimal spot
     
  10. Glava

    Glava Veteran Member Lifetime Gold Member

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    That helps!
    I will take it out for a run and verify what happens at 60mph and 80mph
     

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