Hey guys I have a 99 LS and when im on the freeway going about 70-80mph the car shakes, its really annoying. It happens when im accelerating but as i let off the gas it stops? anyone got an idea of what the problem might be?

Also when im on the freeway, sometimes the car will feel like a have a flat tire, then it goes away. It usual comes from the rear

 

One common cause of vibration at highway speeds is bad front strut bar bushings -- you can see the strut bars in front of each front wheel suspension assembly if you get down on your knees and look. I had the ones on my 00 LS400 replaced at 86,600 miles due to the vibration. On my car, the vibration was almost always apparent at about 65-70 mph but was sometimes worse depending on how much the road was crowned and even whether or not there was a strong wind.

Apparently it is less expensive to replace only the bushings if you are doing the work yourself -- the bushings have to be pressed into the bars if I remember what someone on this forum said. Due to the labor rate I was paying at the Lexus dealer, it was less expensive to buy the complete strut bar assemblies.

 

Welcome - some may say engine and/or tranny mounts since it is only experienced with accelerating.

 

While you may well have bushings/strut bar the FIRST thing I would do is make sure your tires are not the problem.

Most of the vibrating shimmying and comming/going steering wheel problems I have had were due to out of balance or out of round tires or a separated carcas. Check those tires.

 

True - should have mentioned that first. If your tire shop can do a high speed balance it would be best. Some shops have road force machines where tires are balanced against a hard surface simulating the road versus a free standing balancer. Recently one shop balanced mine but they were still out. Another shop down the road did the same and they were perfect - differences due to calibration.

 

ohhh ok got it! thanks guys ill have this stuff check out this weekend

 

Interesting that you brought up balance. I had my wheels/tires rebalanced three or four times within a 2 - 3 month period, including on road force balance machines, trying to get rid of the vibration at highway speeds. I didn't even use the same shop twice.

I finally gave up and took the car to the Lexus dealer. They replaced the strut rods and the vibration was totally gone.

But, for sure, other issues can cause a similar vibration problem.

 

I had a 98 ls400 do the exact same thing ,ended up being the transmission mount ,now it's smoothe as silk !!!! Part was $65.00 from Lexus ,and very simple to install .