(originally posted to the LT1 specific forum, with no suggestions).


A few months ago, I installed Kooks headers, Yank SS3200 T.Q., and
FLT (Level 3) auto tranny to a standard rubber tranny mount. Just prior
to that, the only other things I've done is add a Meziere electric pump,
ignition wires, plugs, MSD cap/rotor, new Yokohama tires all around.

A couple of months ago, I replaced the broken stock rubber tranny mount
with a poly mount and noticed an increase in vibration. But in the last
month, or say, 1,000 miles, I've noticed a fairly "violent" vibration. I've
rotated the driveshaft 180 degrees with no improvement.

The vibration occurs mostly between 1,800-2,000 RPM, and seems to be
most apparent in 3rd gear - if i manually shift through the gears, it's not
so noticeable in 1st or 2nd, but is most noticeable when i shift into D. In
fourth, (circle D), it's not as apparent.

I have also noticed one other thing. Let's say I'm running at 50+ MPH and
above 1500 RPM and I let off the gas. When the RPM's hit 1500, the tranny
downshifts to 3rd VERY noticeably (never did this before). Also, if I mash
on the gas (say, from a stop) conservatively, the tranny shifts into overdrive
prematurely, at around 1500 RPM at 45 MPH, which it never did before.

So here's my question - and I'm mostly interested in determining the
vibration problem now. What is the best method to determine where the
vibration is coming from? Could it be the flywheel, harmonic balancer,
torque converter, tranny, or driveshaft components?

How can I reliably eliminate where the problem could be coming from?

 

The best place to start for your vibration is tires and driveshaft. Check your tires for balance. Have the driveshaft checked for balance, and have the u-joints inspected. After so many the years the bearings sometimes get a little play in them and can vibrate.

Have you checked the fluid in the tranny? See if it smells burnt, that will be the first indicator there is something wrong with the tranny. If the level is good and it smells and looks ok, I would change the fluid and filter. That's where I would start.

 

+1 on tires and DS. I did mine and its tons better at 80 + mph.

 

I've checked the tranny fluid - looks and smells fine and is at the proper level.

I'm pretty certain it's not the tires - they are all brand new and have only
about 1,000 - 2,000 miles on them. Plus, I'm not getting the shake through
the steering wheel and / seats. (BTW - no codes are set).

This is something along the driveline. It's almost like the engine is shaking
badly - as if it's running on 4 or 5 cylinders and out of balance. it's a crazy
thought, but I want to say the flywheel or torque converter is out of balance.

If I drive along and put it in neutral, and rev the engine to the RPM's where
the shake occurs, I feel the shake a bit, but it's not near as bad and when
it's in gear.

When I rotated the driveshaft 180*, I checked the u-joints and they are fine.
The only thing I could not do is check the driveshaft for balance.

I should also mention that the FLT 4L60E tranny and SS3200 torque converter
have somewhere in the neighborhood of 5,000 miles.