This is good. Divide and conquer is what it is all about.

If you can separate "RPM dependent" from "Vehicle speed dependent" from "Load dependent" then it creates a whole long list of things you don't have to worry about.

K

It's definitely rpm dependent, but load effects the severity. In park you can feel the vibration grow between 2800-3000 rpm and it's at its worst from 3000-3500rpm. It is still there, but subsides after 3500 rpm.

While driving the car, the vibration is worse during moderate acceleration. At full throttle the vibration is present but not as severe.

Motor mounts perhaps? Those have been in the car since I purchased it in 98.

I would say to check the converter to flexplate mounting.
If this is a new converter (to you?) it could be bolted up crooked.
Unbolt converter from flexplate and push converter away from flexplate.
Start engine and see if it still vibrates?

Check the flexplate while under there to see if there is any noticeable problems. (bent/crooked, holes wallowed out, etc)

When bolting back up, make sure the converter is pulled against flexplate evenly and tighten bolts without cocking the converter as it is rotated.

:confused:

This is definitely on my agenda to check. The flexplate was a new SFI unit that was installed about a year and a half ago. Doesn't necessarily mean anything however.

The converter is a new TSP unit. One of their billet faced triple clutch deals.

One thing I just realized is that the vibration range tends to be in the range where the converter starts to couple. It flashes to about 3100 rpm under load.

Update on this. I spent some time diagnosing over the past couple days and weekend. There's no visible wobbles or shakes coming from the flex plate or torque converter. All the bolts are tight etc.

On a hunch and some extended reading, I changed out the transmission mount from the poly mount that was installed with the trans, back to a rubber mount. To my surprise this almost entirely get rid of the vibration. I would say a reduction of over 95% in fact. I can still very faintly feel a vibration, but it's almost imperceptible now.

I still need to track down what exactly is creating the vibration to begin with.

how worn is the slip yoke and output shaft of trans?
Also check the bushing in tailshaft

Sounds like a NVH problem which can be hard to deal with.

It also might suggest a phase problem with the 2 u-joints on the shaft.
If they are a little out of phase can be a problem that is hard to trace.

Or the factory spends ton on NVH so maybe the rubber (durometer?) is the correct reduction for the vibration?

:confused:

The transmission mount that was on previously was terribly worn. It was probably 50% as soft as the stock rubber replacement I just put in. I would gather there may have always been some sort of latent vibration, that was soaked up by the fact that the trans was sitting on a sponge.

I've got the engine out of the car for unrelated issues now. It has factory type motor mounts, but they appear to be in decent condition. I may decide to change those out while I have the engine out. Whether or not that will help, I'm not sure.

There's a lot of solid mounting in this car, between the delrin suspension bushings and delrin/aluminum subframe mounts. Some amount of NVH transfer has to be considered acceptable.

Final update on this.

After getting the engine back in the car for it's unrelated issue, I did decide to replace the motor mounts with a set of mighty mounts. Upon further inspection you could see the rubber starting to tear and delaminate from the steel casing.

Nothing changed with the vibration moving to new motor mounts.

After doing further testing to isolate the vibration, I did narrow it down to engine side with the torque converter dismounted from the flex plate. This week I replaced the unknown age, but seemingly in decent condition factory type balancer with a BHJ from Nitemare performance. I'm happy to say that the vibration is now gone with exception of a very slight buzz near the very top of the rpm band. Based on the drill marks present in the factory type balancer, I believe whoever did the rotating assembly on this engine back around 97-98 likely tried to neutral balance the stock balancer and there may be a slight imbalance in the rotating assembly itself. All of this likely masked by the poor and soft mounting the car had.

As I've replaced bushings, engine and trans mounts and gone to more rigid mounting of the subframe and suspension components, that slight imbalance is there. I've been beating on this thing for many years and I assume I'll be able to continue to do that. But, I'm socking away money for a rebuild at this time.