I just bought a 09 is250 with about 98k miles, i been noticing the car dosent really have much a response on the throttle atleast not compared to my stock is300, and once in a while it kind of "jumps" or hesitates, i figured maybe its just the spark plugs needed changing since its reaching 100k and i have not seen that job done on the lexus maintenance timeline (that car was maintained there its whole life before i bought it).

But today i noticed that when im in the highway and i kind of press more on the accelerator the rpms jumps drastically while the car does nothing, as in ill be going 70mph its at 2500 (just a example not exact numbers) and it will jump to 3200 quickly with a light press of the gas pedal. It also does this when im driving around town too, but if i firmly press the gas pedal further instead of lightly it will moves accordingly.

To my knowledge a jumpy tach and no MPH means a slipping clutch, but that it usually on a manual transmission, not a automatic well maintained lexus.

Could this bad something else other than the transmission?

 

Doing a drain and fill and verifying correct level at correct temp. should be your first step.
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The car weighs in at 4000 lbs and is a bit under powered. That said, it sounds like its doing what it's supposed to do which is to drop gears and hold it's own, increasing slightly. Now if pinned to the floor, it should accelerate.

Does it accelerate from a stop and pull through each gear? Not knowing how it was used, the intake may be carboned up which will hurt performance pretty drastically. This is best viewed when you have the intake off.

 

that i understand, and its what NORMALLY happens on a automatic car when you go WOT, the car drops gears to "hold its on" and get up and go, but this is not whats happening, its the opposite, im no where near WOT when it does it, and im LIGHTLY tapping on the accelerator. And it jumps foward in rpms not back.

 

It could be a vacuum leak or engine coolant temperature sensor.