Vibrations upon acceleration

Cougar Community Classic Cougar Maintenance Restoration and Repair
1

I have a 68 cougar GT with the 390 and the C6 trans.
My problem is that when I get to about 60 mph there’s a strange, wavy vibration coming from the rear (or my at least i can feel it on the seat from underneath).
Now the weird thing is : I vibrates only when I’m stepping on the gas, and it goes faster the more i press. When letting off the gas it moslty disappears regardless of the speed.
I thought it could be the rear tires as they are 20+ yrs old but i wonder why it’s gas pedal dependant and not speed… any ideas ?

2

bad rear u joint?

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3

I would have the driveshaft rebuilt and balanced. They replace the U-joints and balance the drive shaft. It costs $105 at the shop near here. Otherwise it might be time to replace your tires.

4

I live in France so wherever you are might be a bit far. I’ll look for a shop near me.
And yeah i just got the car so tires are on my to do list, i can’t believe they never got changed for the last couple of decades

5

If the tires are that old they would be the top of my list. Over there you have a lot of good options.

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6

Could be the isolaters that the springs are mounted in…allowing too much change in pinion angle

7

Oh meaning the load that the acceleration is pulling could cause the rear springs to vibrate if they’re bad ?

8

Check the transmission fluid level. Low level vibrates my transmission.

9

Springs are probably ok but upon acceleration the spring tries to wrap up causing the pinion angle to become excessive…ie vibrations. The u joint could be bad also
Old rule of thumb is pinion should be 1 degree down at the u joint in relation to trans angle

10

Okay thanks ! I needed to remove the diff anyway

11

Fluid is okay so im thinking about the drive train

12

I agree with Royce. 20 year old tires must be changed immediately. Once you have that, you can baseline the issues / performance of the car.

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13

Yeah next month i’ll get some bfgoodrich T/A i think.
Also : My leaf springs are sagging bad, and I figured it might be the cause of the vibration after reading the comments above regarding the differential pinion.

14

Mine did a similar thing , was axle bearing rear wheel

15

I had a very similar issue with my 67. From 60-65mph, the car would shimy and shake. It was the tires. Replaced them and haven’t felt anything similar since.

16

Check u joints first …but also as Royce said when they balance they install new u joints 2 birds one stone