I was going to make the same thread, I have same issues, I have 1jz, w58, with open diff,megan coil, I installed poly mounts on the front, no the front is tight and feels hard on bumps.
Before I used to go to 170mph and was ok few months back, I had act clutch, now I have a new action clutch, the problems is when I step on the car ~15 psi the car start to fish tale and feels like out of control.
I need some input on how to fix this I have some theories....

Is the new clutch applying to much torque?
Is the open diff going bad? do I need to get the supra auto tt diff to fix this.
The rubber that connects the drive shaft to the diff is a little worn out, do this could be the cause?
The drive shaft have some align marks that need to be match to the diff? I am asking this because I never so any when installing the new clutch.

 

You should always align the drive shaft before and after install, it's balanced!

I would look at that, then get all four wheels up in the air, and pull on each wheel to listen for pops or excessive play.

 

i had a similar issue and its from the rear, i was going to order new traction rods and toe links from battle version but ive asked my fabricator teacher to build me a pair custom. i can prob get them made for half the price battleversion asks if more people are interested.

 

I had this issue when I first got my car. It was a bit frightening.

I cured it by changing the steering rack bushings and installing a new suspension (and getting an alignment). My OEM shocks were blown at the time.

In conjunction, those things fixed it.

Later on a TT rear swaybar took out any remaining semblance of the rear end "wiggle" for me but the primary issue seemed to be the bad rack bushings needing replacement.

 

suprised no one asked yet - what TIRES do you have? Mine had a very slight wiggle at high speed (and i do mean HIGH SPEED). I upgraded tires and it's like i'm GLUED to the road now

 

Mine ended up being a bad wheel bearing/hub issue in the front right it seems.

I also noticed and adjusted all the preload off the coil overs since the previous owner didn't know what he was doing in that area.

 

Ahh, tram-lining in an SC... I can help! This took me quite a while to chase down. Like me, you replaced all the bushings, struts, and swaybar links already. Hopefully you did the rear springs as well. So what next? Here are some suggestions:

A) The shop installed "discount auto" $10 sway bar links and they failed already. Don't do that. Get the $40 premium grade links, it's a sports car with 120HP/ton, not a 90's Camry.

B) The multi-link suspension out back needs work, and/or you have failing ball joints on the A-frames. The service manual explains the tolerances. But a good, reputable shop can perform this check.

C) Did you change all the vacuum hoses? This worked for me: replace all vacuum hoses, especially the two vacuum lines running to the power steering solenoid. To access from the top, remove the intake box and ducting (and mark them so you don't get them reversed!). Access from the bottom is possible but it is a terrible angle. I used "bent" needle-nose pliers to remove the hoses. Be careful not to break the plastic nipples on the solenoid.

D) Replace the lower control arms out front.... The Supra fits, for half the cost. You can find info and confirm Supra model & year using search on the forums.

I did option C as part of changing plugs/wires/etc for 300k service. It worked! Here's why:
The SC400 has a variable-rate steering rack that is speed-dependent (vacuum dependent). At highway speeds with a leaky vacuum hose or defective solenoid, the rack is in "parking lot / low speed mode" and the car can "tram-line."

Took me all summer to figure it out. Like you, this was after completing some "time-to-do-it" work: struts/strut tower mounts, sway bar links, bushings, etc. The car changed from "1 inch of play and I get blown around" on the steering to a "live" wheel at highway speeds that "goes where I point it." Mine's a 94 SC400 300k miles.

If that doesn't work, you can to the PITA check "Option B" yourself (with a $50 brake disc runout / ball joint check kit) or have a shop measure the ball joint play (IIRC, 3mm = fail). You can find the service manual online.

HINT: once any hoses hoses are bad, they show micro-cracks and are hard/not easy to bend. At this point replace every hose there is under the hood- especially the brake booster vacuum hose.

Good luck!

 

do you have a pic of this brace. Ive been under there so many times but its not ringing any bells? Im a little concerned now im missing both of them.

Also for anyone that has taken the long bolt out for the upper control arms, Don't forget the manual says to do the final torquing with the wheels on the ground so the suspension is somewhat settled, i think its something like 121 ft/lbs.

edit** found the diagram.

 

That brace is apparently only for 95+ which is weird as I don't see any structural point of it. Just to tighten her up a bit more.

I found what seemed to of been the last two problems.

1) I finally got around to replacing the stopper bushing in the LCA between the arm and bracket. The wiggle went away for the most part.

2) Then my power steering went into highway mode sitting in the drive through lane... *face palm* someone suggested that could be an issue and it seems to have been as well

 

Correct. It is 121 ft-lbs for the control arm bolts.