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Thread: Highway speed vibration, worse on deceleration

  1. #1
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    Default Highway speed vibration, worse on deceleration

    Hey everyone,

    Starting to get depressed with my Kat. It has a highway speed (65 mph and up) vibration from the rear of the car. I don't really feel it in the seat but I can hear it pretty good. If I take foot off of the gas it gets worse. It's been into the dealership where I work 3 times on this problem with someone who's been with Ford for over 20 years. First time was bent axle shaft so those have been changed, but did not fix. Second time we changed the upper control arms to CHE adjustable to correct the pinion angle (car is lowered), did not fix. Third time we changed the tranny mount as it was collapsed but this did not fix either. Car has 3.73's and problem seems like it got worse after gear install. U-joints and all rear axle bearings, shafts, gear set are new (Ford Racing, Moser). Wheels are balanced. Driveshaft hasn't been balanced but looks clean and original weight is still there.

    Any ideas? Help with this is appreciated!
    88 XR7 5.0L low mileage Florida Cat. AOD, 8.8, headers with x-pipe and flowmaster cat-back, 3.73's, lowered 1". 1 of 421 according to Marti Report.


  2. #2
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    I had three weights on my '86 when I got it, and there was a noticeable, repeatable vibration between 25-45 mph. Had a new driveshaft made and that solved the problem. A good driveshaft should either balance out with no weights, or have one weight. Don't know how mine got 3 but it sure looked factory.

    You could try removing the counterweight ("dogbone") from the front of the axle housing, that may help a bit. Sounds like everything else has been fixed or checks out alright though.

  3. #3
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    Tried removing the "dogbone" too. Didn't help, but made it slightly worse at about 60 mph, so back on it went.

    Thought I had all the bases covered but still no answer.
    88 XR7 5.0L low mileage Florida Cat. AOD, 8.8, headers with x-pipe and flowmaster cat-back, 3.73's, lowered 1". 1 of 421 according to Marti Report.


  4. #4
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    3.73 gears and the stock driveshaft? Yeah it's the stock driveshaft. I had the stock one in my car balanced 3 times trying to fix the vibration issue after 3.73 gears. I gave up and had an aluminum driveshaft made. Fixed the issue instantly.
    '88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, Edelbrock 70mm TB, 76mm C&L MAF, 30lb injectors, 2.5" exhaust, AOD with 2800 PI converter, 8.8 with 3.73 gears, 31 spline Traction-Lok, 31 spline Moser axles, 04 Cobra front arms, Maximum Motorsports extreme duty rear arms, subframes.
    '11 Focus, '12 Mustang 3.7, '17 Accord EX-L V6

  5. #5
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    Try unbolting the driveshaft and turn it 180*. I reused my driveshaft from my 2.79s, then on my 3.73s, and finally on my 4,10s without any issues.
    What about the trany bushing?
    Last edited by QUICKSHIFT; 10-05-2017 at 09:16 AM. Reason: additional info
    84 COUGAR/88HO, 1.7RRs, performer RPM,700DP, equal length shorties, stainless EXH ,T-5,Hurst pro-billet, KC clutch, 8.8/ 4.10s, line-lok, bla ,bla, bla.
    71 COMET/289,351w heads, 12.5 TRWs, 750DP, Liberty TL, 9"/6.00s, 11.9x @112 , bla,bla,bla.

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  6. #6
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    Hmmm, don't think we've checked the tranny bushing. Problem seems to be coming from the rear end but I guess the sound might be traveling down the driveshaft. Worth a look.

    Still using the stock DS. DS has been rotated/turned 180* - didn't fix either.

    I wouldn't mind spending money to get an aluminum shaft made up if I knew this would fix the problem but I've already spent over $1500 or more getting rear end done, upper adjustable control arms and tranny mount etc. It's getting expensive with no results. I'll have to see if that's the path to try next.

    Does anyone recommend a good place to get an aluminum shaft made up in Southern Ontario (KW area preferably)?

    Thanks!
    88 XR7 5.0L low mileage Florida Cat. AOD, 8.8, headers with x-pipe and flowmaster cat-back, 3.73's, lowered 1". 1 of 421 according to Marti Report.


  7. #7
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    Holler at Vinnie. I believe he got his out of a Crown Vic and had it shortened.

  8. #8
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    Quote Originally Posted by AT8 Cat View Post
    Hmmm, don't think we've checked the tranny bushing. Problem seems to be coming from the rear end but I guess the sound might be traveling down the driveshaft. Worth a look.

    Still using the stock DS. DS has been rotated/turned 180* - didn't fix either.

    I wouldn't mind spending money to get an aluminum shaft made up if I knew this would fix the problem but I've already spent over $1500 or more getting rear end done, upper adjustable control arms and tranny mount etc. It's getting expensive with no results. I'll have to see if that's the path to try next.

    Does anyone recommend a good place to get an aluminum shaft made up in Southern Ontario (KW area preferably)?

    Thanks!
    http://www.dennysdriveshaft.com/ or go to ebay and search custom aluminum driveshaft

  9. #9
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    If it's a buzz and you see it in the rearview mirror it's more than likely the driveshaft. Unless the gear install was fubar but, in that case, you would feel it in the seats.
    '88 Thunderbird LX: 306, Edelbrock Performer heads, Comp 266HR cam, Edelbrock Performer RPM intake, Edelbrock 70mm TB, 76mm C&L MAF, 30lb injectors, 2.5" exhaust, AOD with 2800 PI converter, 8.8 with 3.73 gears, 31 spline Traction-Lok, 31 spline Moser axles, 04 Cobra front arms, Maximum Motorsports extreme duty rear arms, subframes.
    '11 Focus, '12 Mustang 3.7, '17 Accord EX-L V6

  10. #10
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    If you put the trans in neutral with a manual gearbox and you still have the vibration then it isolates it to the rotating assemblies from the output shaft of the trans to the tires. This includes bushings, bearings, shafts, etc. With a slushbox I do not know because I have never educated myself on how they work so I don't know if this holds true with putting one of them in neutral.

    So the other issue is also something that has been talked about for years over in the Mustang community with regards to 3.73 and steeper gears which is called critical speed.

    http://www.foxtbirdcougarforums.com/...shaft-cures-it!

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