[Supras] Vibration

[berniek at technicaldevelop.com]

Bob:

    This may be a shot in the dark, but about the only thing left are 
the rear tires or the rubber mount for the driveshaft center bearing 
(very unlikely).  From all you have stated (car must be warm, engine can 
be off, etc.) you know it is not related to the fan clutch (not enough 
mass to cause vibration if all the blades are present) and engaging the 
clutch with the disc at a different point relative to the pressure plate 
and flywheel shows the clutch to not be the problem. 

    I've seen instances where plies separate, forming a hernia-like 
bulge on tires.  As stated, this is a long shot, but you have several 
options for diagnosis, some of which I've used to discover bent wheels 
and thrown wheel weights: 

1.  Jack the car up as soon as you can after it is warmed up and place 
jackstands as far out as possible under the rear control arms to keep 
the half shaft axle angles near correct.  Run the car in 5th gear at 
60-90 MPH and see if you feed the vibration.  Despite the rigidity of 
the jackstands, I've found thrown wheel weights that way, the vibration 
was so bad.  Have someone else step on the gas while you watch for 
eccentricity in the wheels and tires. 

2.  Interchange the tires between front and rear (hopefully you have the 
same size wheels on each).  See if the vibration then seems to be coming 
from the front, or see if you  notice a shimmy in the steering.

3.  Jack up as in (1) and remove the rear wheels.  Again run the car in 
5th gear and see what difference you notice. 

4.  After the car is hot, pull quickly to the shoulder and feel the 
temperature of each of the wheels to see if a partially stuck caliper is 
causing heating and disc warping, resulting in a weird humming 
vibration.  I have a '92 Nissan Maxima "mule" car which has this 
condition at the moment. 

    Hope this helps.  Let us know how you make out.  As far as the 
timing belt is concerned, you can remove the top two timing belt covers 
(the ones which also hold the wiring harness cover, and pry it open 
about 3/8", carefully.  Any oil on the belt will make its way to the 
outside, which will then be visible.  Chances are it will be dry.  That 
can be checked frequently if the oil leak gets worse.  Do you notice oil 
on the ground under the timing cover?

       BernieK