nitro496

I have a 1990 ls 400 and the front end starts shaking over 40 mi an hr someone told me this could be a bad streering rack issue is this tru and how do you check that ?

RA40

iTrader: (6)

Welcome to Club Lexus.
How many miles on car? Can be many possibilities. Does the shaking happen when braking? Is it just at this particular speed? Front or back?

Typical areas to inspect are the suspension bushings, steering components, wheel balance, tire issues, engine, tranny and carrier mounts.

dicer

Like RA40 says, front end looseness, alignment, the shimmy damper (its a little shock absorber).
If its speed related I would lean more towards one of these....... Tire balance, a blister on the tire, a bent or distorted rim.
Are the tire lug nuts tight?

Stroock639

jack the front end up and see if there's any play in the front wheels when they're off the ground. if you grab the wheel at 3 and 9 and it wobbles back and forth you likely have a bad inner or outer (or both) tie rod. i've been getting a bit of shaking recently and replacing my left tie rods fixed that. if you feel the steering wheel shaking its probably that. otherwise make sure you rule out wheel balance before going and checking everything else.

AkumaUCF10

The front end shaking at speed is 100% going to be a wheel balance problem
Ball joints and tie rod ends can cause this kind of vibration but I have only seen it when they are really bad and even then they are accompanied with a semi out of balance wheel. Furthermore its very easy to check for these, put the wheel in the air and do the 9 & 3 and 12 & 6 test to look for play (just like you would for a wheel bearing). The other thing is that if you did have one of these ball joints or tie rod ends going out than you would most likely hear a clunk as well and have really bad tire wear, which, now that we're on that subject, is something you should also check for

djamps

+1 on wheel balance and tire/wheel round-ness. That's where your shaking is coming from -- possible made worse by other stuff like mentioned.