Posting for a friend have you ever known a IS250 to shake under acceleration? He bought 19" rims staggered and had them balanced twice, factory lexus wheels off a 500 mile loaner installed & tested and still shakes. he had it looked at by lexus and couldn't figure it out.

 

What's his setup?

 

could be the engine having carbon build up.

 

Yea how many miles? I'll guess the carbon build up to

 

where would the carbon build up be?

 

inside the engine....

 

duh...the injectors?

 

no, more like the valves. since it is a direct injection.

 

Any Lexus tech worth their pay would spot engine imbalance quickly, especially if viewed on a tester or scope. I doubt I would put a lot of weight to that idea.
Shaking under acceleration could be attributed to shot motor mounts, or maybe a u-joint going bad?

V

 

Does it shake when you rev it in neutral or only in gear?

I was driving with a friend in his '71 Road runner back in HS once. As we turned out of his sub the car starts shaking like mad. We look at each other like our fillings were going to fall out. As he slowed to the stop light it subsides. Then as it turns green and he accelerates the car starts shaking again. This time we felt a huge clunk from the back. The rear of the car dropped like 8" and we hear scraping. As I looked out the window I see this huge tire rolling past me! My friend shouts at me and says "don't just sit there go get it!" I chase down the wheel and as I return laughing all the way I see him standing at the rear of the car shaking his head. It turns out he sheared off the lug nuts from doing too many burn outs and lost the wheel.

Moral of the story, if your car shakes when accelerating don't keep driving it.