1988 LX 302 with high speed vibration


New Member
Jul 27, 2005
the mustang vibrates bad at speeds above 80mph on accel and decceleration it seems to do it more so on deccel. the car has an aluminum driveshaft and a stock rear end (open diff) yesterday i put mobil 1 75w - 90 synthetic today was the first time i noticed the vibration but it may have done it before. also the tires are balanced so i know it isnt that. the only thing that is weird about the car is the harmonic balancer seems to wable at idle but i put the car in neutral above 80 so the rpms are down but it still vibrates any ideas?
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if your balancer is stock , look at the rubber insert between the 2 peices. if its severely cracked or rotted that might be your problem. best way to see it , is rev it up under the hood and watch it to see if it wobbles worse. if your car is a 5 speed, might want to check on the output shaft bearing in the tailshaft if it hasnt been replaced. both of those will cause vibration on accel and deccel. also a bad pinion bearing will vibrate on deccel.
i've had a problem only since my 3:73 install, but only at highway speeds also. i'm going to get an aluminum driveshaft, and hope that fixes it, but i was also told to check my control arms and bushings... i guess they can throw the pinion angle off and cause similar symptoms....
now that you say that it feels like rumble strips

i'd say your pinion bearing in the rearend is going tops up. buddy of mines 88 coupe did the same, get on it and feel it a little bit but let off and man would it vibrate like he!!. best to jack it up and see how much freeplay you got in the rearend, just twist your driveshaft back and forth and you'lll be able to tell if its the rearend or not lol

could also be bad u-joints, loose driveshaft bolts to the rearend, but i'd check the pinion bearing first. if you feel/hear like a whomp whomp whomp once the load has been lifted off when deccel, it might be a pinion bearing.
I had the same vibration around 80 and above. I have new aluminum driveshaft, and 3.73 gears. New bearing kit installed along with gears. The tires were balanced the day of my road trip. I can't figure it out either. I guess I never will.

I totaled my car last weekend anyway. Hopefully you figure it out.
you think this could be a road force problem above 80mph?

Road force can do all sorts of ****, I just like to start with the easy **** first, such as tires, then work your way to the driveshaft, then possible pinion bearing free play. Most shops will do a "balance/road force" check for little to nothing if you are having vibration problems. Also tire condition, bent wheels, ect. I would NOT hesitate to fix this problem as if it is something ugly such as the pinion bearing big trouble could be ahead and you could possibly ruin your whole rear, seen it. When was the last time the tires were rotated? when was the last time (if ever) the U joints were serviced/replaced? Rear end inspection? Ect. Chase it down and eliminate everything slowly and surely.

Hope this helps,