i am having this same exact problem with my IS250 AWD since i installed my eibachs. Glad im not the only one!

 

Guess im not the only one with this prob, maybe its a AWD prob??...Anyways, if im not the only one (maybe more lowered AWD's pips should chim in) and this is expected, i can totally live with it, just want to know if theres something wrong....

 

i am going to try a different set of springs that give an equal front and rear drop. the spring i have now lower the front 1.4" and the rear 1". Maybe if the car is lowered all the same it won't be noticed because everything is in the same place just lower.

 

I just talked to my dad about my cars' vibration problem. He's a Nuclear/Mechanical engineer. I gave him the specs of the springs and let hims drive my car around a little bit. He came back after 20 mins and it seems to be fixed. What he did was inflate the front tires more than the rears. He put 47.5 psi in the front and 42psi in the rear. He had some science and logic behind it, talking about the front being lower which the extra psi in the front with being the front and the rear closer to normal etc etc.

I want you guys to try it and tell me what you think. Im thrilled so far. I just took it on the highway and nailed it past 100 mph on the on ramp. No vibrations. Went 90mph and let it coast down to 60mph, still no vibrations. I just hope that this is not a placebo thing were i think its fixed because my dad said it was. Now with the car in D or R and stopped the vibration/shaking is to a minimum.

So try it out and let me know if it works for you.

Pete

 

Overinflating your tires to compensate isn't safe to do. I would highly recoomend against this. It doesn't take a mechanical/nuclear engineer to be an automotive mechanic. Overinflating your tires on one side will drastically cause potential problems with vehicle handling and possible tire failure.

All lowering springs do not give an equal drop in the front. This is just a charateristic of any lowering spring including OEM. The wheel gap from the Left front and Right front are different. On the 3GS, it tends to sit lower on the driver's side.

 

Ur dad doesnt seem that smart i guess...inflating ur tires to near 50psi is kinda stupid, specialy for this issue, that is not gona fix anything...True, spring allways lower the front slightly more than the rear, but this is deff. not the reason im havin the idle issue...Deflate those tires as soon as possible, bc u prob exceeded or are extremely close to the tires maximum pressure, dont want to total ur car with a blowout!!!

 

I have a '06 GS300 AWD with no modications performed yet. Around 1 month into owership (Feb '06) I took my GS to Lexus for a TSB (brake adjustment for pulling at breaking) work & when I got it back from them the next day, I noticed rough idle (vibration felt in steering wheel, in drivers seat, & door when arm is resting on the door sill) while sitting at a light...almost as if the car is idled too low & it actually shudders now & then, as if about to cut off, when stopping quickly at traffic light. I've complained about it to Lexus Svc everytime I take it for anything. Naturally the Lexus Tech said it was normal & occurs after the "break-in" period. Of course, I questioned their initial response & over the course of nearly 2 years now. Recently I was told that factory set of idle in drive gear is 500 RPM & the load of being in drive vs neutral or park (idle smooths out like butter then) is normal. If this is normal, it sucks! Not sure if the new springs is compounding your vibration issue, but I feel your pain. Other than the wheel gap, this vibration while sitting still in drive is my 2nd peeve with the GS3.