Acceleration Vibration

Blown02svt · Dec 31, 2025

I have a 1993 GT with a 347 and it runs great! However, I’ve noticed that under “hard” acceleration (I don’t really drive it that hard), I’m getting a lot of vibration. It feels like drive train vibration but if I’m just driving, the car is smooth…it’s only under acceleration. It’s not at any particular RPM, just when in the gas. In addition, I don’t have any vibration while revving while stopped…again only under acceleration.

Not really sure what’s causing it, but I’m leaning towards a failed or failing harmonic balancer.

And just FYI, I bought the car already built and don’t really know all of the history behind the car and/or build but do know that it has an aftermarket balancer already on it.

Thanks in advance for any input…

Noobz347 · Dec 31, 2025

U-Joint(s)

Get underneath, grab hold of each end of the driveshaft, and shake, rattle, and roll. Look for any slop at all.

Get a good look at your pinion angle while you're down there.

Ethereal_Zer0 · Dec 31, 2025

Noobz347 said:
U-Joint(s)

Get underneath, grab hold of each end of the driveshaft, and shake, rattle, and roll. Look for any slop at all.

Get a good look at your pinion angle while you're down there.

I'd second this. My car had a weird vibration we couldn't figure out. It ended up being one of the little C-clips in the U-joint had somehow worked itself out of the groove (or more likely just didn't get installed properly) and everything had a little play in it.

Blown02svt · Jan 3, 2026

I crawled under the car today and it appears that the driveshaft is solid. I didn’t see any movement up/down or side/side. It does have a little movement/play when twisting it (maybe 1/32”…it barely moves at all), but it feels like it’s more movement within the transmission versus the u joints.

AeroCoupe · Jan 3, 2026

Pull it out and take it to a driveline shop. They can spin it up and see if there are any issues.

2000xp8 · Jan 4, 2026

Taking the driveshaft out isn't much work as long as you have the correct wrench and swivel socket. Changing the u joints with a press from harbor freight is not all that hard either.
When the history is unknown, I think you just change them.
With that said, I'm also a fan of taking the shaft to a driveshaft shop, but I think before I did that, I'd just buy an aluminum driveshaft if it fits in the budget and I didn't already have one. You could land up with a $200+ bill if they change the u joints, when you could just buy the aluminum for $450.

Blown02svt · Jan 4, 2026

giantpune said:
FIFY. All I've ever needed to do my 8.8 driveshaft bolts is a 12mm wrench with 12 point ratcheting closed end. Pretty much every brand that makes wrenches has a line of those.

Mine actually has a 9” rear end and an aluminum driveshaft already. I reached out to a driveline shop to see what they’d charge to inspect the full assembly, but I’ll probably pull it regardless and at least chance the joints. They look fairly new, but again, will probably change them out, it doesn’t look too difficult.

AeroCoupe · Jan 4, 2026

Just me but I would pull the DS and have them check that first. No sense in pulling the 9" unless they don't find a problem with the DS or if you just want to.

Blown02svt · Jan 4, 2026

Blown02svt · Jan 4, 2026

AeroCoupe said:
Just me but I would pull the DS and have them check that first. No sense in pulling the 9" unless they don't find a problem with the DS or if you just want to.

Sorry, I didn’t mean I was going to pull the 9”, just the DS to have it checked or at least change the ujoints.

2000xp8 · Jan 4, 2026

giantpune said:
FIFY. All I've ever needed to do my 8.8 driveshaft bolts is a 12mm wrench with 12 point ratcheting closed end. Pretty much every brand that makes wrenches has a line of those.

Yes, it can be done with a wrench only, but if i remember correctly, if you have an impact, extension and swivel 12mm and you rotate the shaft, you can take it out quicker.

AeroCoupe · Jan 4, 2026

With the torque arm I just use a 12mm wrench and a 13mm wrench to make the 12mm longer. I should just buy a long 12mm wrench but I just don’t have the need that often.

Qtrhrse23 · Jan 5, 2026

I've seen the mainshaft in trans bent before causing a vibration during acceleration. With the driveshaft removed, run it thru the gears.

General karthief · Jan 5, 2026

I just take a torch to remove the DS and let the drive line shop fix it.
What difference does it make what tool is used, Get-R-Done!!
Vicegrips FTW

Acceleration Vibration

Blown02svt

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Noobz347

Stangnet Facilities Maint Tech... Er... Janitor

Ethereal_Zer0

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Blown02svt

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AeroCoupe

lube between the nut and the face. I know my lubes

2000xp8

Blown02svt

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AeroCoupe

lube between the nut and the face. I know my lubes

Blown02svt

Member

Blown02svt

Member

2000xp8

AeroCoupe

lube between the nut and the face. I know my lubes

Qtrhrse23

Member

General karthief

wonder how much it would cost to ship you a pair

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Acceleration Vibration

Member

Stangnet Facilities Maint Tech... Er... Janitor

Member

Member

lube between the nut and the face. I know my lubes

Member

lube between the nut and the face. I know my lubes

Member

Member

lube between the nut and the face. I know my lubes

Member

wonder how much it would cost to ship you a pair

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