Vibration Help

So I'm trying to track down a vibration that's very confusing. I'm not sure when it started but I think its been a pretty long time. Before I switched wheels/tires. 99 GTS

I've replaced the following:
Motor Mounts (within 100 miles)
Clutch/Trans Mounts (within 2000 miles)
Suspension (BC coils within 500 miles)
Tie Rod Ends (within 2000 Miles)
Steering Rack Bushings (solid. within 500 miles)
Wheels Tires (within 500 miles)
Fluids in trans/diff are all within 2000 miles.

Car needs an alignment after the suspension change, but tracks straight. There's no wheel rubbing.

The vibration seems to only occur above 75-80mph and only in 6th gear. It's not constant it's cyclical. like 1 second on, 2 seconds off, 1 second on. It seems to go away with the clutch down or in neutral. It seems to go away if I drop to 5th gear (or maybe it becomes less cyclical and I can't notice it as much?). It's noticeable through the steering wheel and the pedals, although to me it feels like its coming from the rear...its very hard to tell. It's not a crazy vibration, as its cyclical. 75-100mph didnt seem to change it much one way or another. Revving the engine in neutral while coasting at 75-80 in various RPMS couldnt recreate it.

I'm set to get an alignment but i dont think that will fix it. I'll try throwing the stock wheels and tires back on as well just to eliminate that.

There doesnt seem to be any excessive play in the driveline/driveshaft. Car has 15k miles most everything is still original.

Any ideas on where to start looking after the alignment/wheels tires? It seems like it has to be driveline related somehow.

Couldn't hurt to start here. GTS Dean was chasing down some vibrations as well. Some good drive train vibration info that thread.

BoondocSaint said:

Slight driveline vibration after trans. swap.

I had a bad synchro or blocker ring in 3rd gear, so I swapped in a known good transmission, put some new u-joint straps on and went for a spin. There's now a minor shifter vibration under accel/decel above 3k engine rpm. No vibe when declutched. Prop shaft was indexed to the diff yoke pre/post...

www.viperclub.org

Couldn't hurt to start here. GTS Dean was chasing down some vibrations as well. Some good drive train vibration info that thread.

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Thanks...seems like he didnt get to the full bottom of it tho yet. I'll check some of those items!

You need to figure out the order of the harmonic vs engine or wheel RPM, as that will tell you what area to look in.

Also of note, is that wheel issues can masquerade as other things depending on loads, wheel bearing play, etc.

In any case, it sounds like a tough one- switching gears causing a change indicates engine/clutch/flywheel/trans input vibration, but the fact that you cant recreate it in neutral is also suspect. I suppose its also worth mentioning that its possible you can have small vibrations in certain systems that "stack up" with other vibrations. This can cause the "oscillation" effect you are referring to. Wheels are most likely to cause this, as you have 2-4 sources of very closely matched vibrations, and when the vibrations "align", you get a rhythmic oscillation.

Viper Specialty said:

You need to figure out the order of the harmonic vs engine or wheel RPM, as that will tell you what area to look in.

Also of note, is that wheel issues can masquerade as other things depending on loads, wheel bearing play, etc.

In any case, it sounds like a tough one- switching gears causing a change indicates engine/clutch/flywheel/trans input vibration, but the fact that you cant recreate it in neutral is also suspect. I suppose its also worth mentioning that its possible you can have small vibrations in certain systems that "stack up" with other vibrations. This can cause the "oscillation" effect you are referring to. Wheels are most likely to cause this, as you have 2-4 sources of very closely matched vibrations, and when the vibrations "align", you get a rhythmic oscillation.

Click to expand...

thank you! great advice/explanation

You mentioned steering rack bushing are solid. What kind/type of mounts are the motor and tranny made of?

Without me looking up/calculating rpm in 6th @ 80mph. See what happens in the other gears at that exact same rpm.

Then you also mentioned that it doesn't change much from 75-100mph. So does that mean the frequency (the cyclical reference you made) is the same in that mph range (plus what is the rpm sweep from 75-100mph)?

Happens on the throttle (load) or off it (coasting) or both?

What happens when you change lanes both left and right when it is doing it? Does the lateral load change anything? Too keep you safe and in control you can try more aggressive left right changes BUT at LOWER speeds but at the rpm you report back with when it is happening at the 80mph.

The other thing that can be done is with a multi channel chassis mics placed around the areas.

Goggles Pizano said:

You mentioned steering rack bushing are solid. What kind/type of mounts are the motor and tranny made of?

Without me looking up/calculating rpm in 6th @ 80mph. See what happens in the other gears at that exact same rpm.

Then you also mentioned that it doesn't change much from 75-100mph. So does that mean the frequency (the cyclical reference you made) is the same in that mph range (plus what is the rpm sweep from 75-100mph)?

Happens on the throttle (load) or off it (coasting) or both?

What happens when you change lanes both left and right when it is doing it? Does the lateral load change anything? Too keep you safe and in control you can try more aggressive left right changes BUT at LOWER speeds but at the rpm you report back with when it is happening at the 80mph.

The other thing that can be done is with a multi channel chassis mics placed around the areas.

Click to expand...

Motor and trans mounts are stock.

Tried the other gear and it's either not there or oscillating differently such that its not as noticeable.

The oscillation does change with speed slightly from 75-100 but not significantly. I.e. it doesnt seem to get worse or get better. Slightly different frequency.

Only in gear, but on or off throttle.

I'll have to try the last one, but turning the wheel slightly didnt change anything. I'll try bigger inputs.

I recently had a similar vibration. It would occur for a few seconds after on-throttle 4th shift to 6th light throttle. Went over the entire driveline and found the ball joints were bad. I swapped out the upper an lower (complete car) to Howe and so far the vibration is gone.

TEALLIFE said:

...
Tried the other gear and it's either not there or oscillating differently such that its not as noticeable.
...

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Didn't answer my question fully as I read your post carefully. At the same rpm as when it does it at 80mph?

Goggles Pizano said:

Didn't answer my question fully as I read your post carefully. At the same rpm as when it does it at 80mph?

Click to expand...

Yes I'm pretty sure it's only in 6th gear for some reason at those speeds. Or at least that's only when its noticeable.