My In-laws just bought an '08 Kia Optima, and got a very good price on it. Don't you just love when someone else absorbs Korean depreciation? Anyway, the only complaint I have about it is that it's almost impossible to get it rolling from stop smoothly, since just a little throttle creates a whole bunch of lurch. Is there any easy fixes for that? What about adding a second throttle return spring?
Is it drive-by-wire? If so, a return spring won't do any good. Also, the 'fuzzy logic' ECUs prevalent today will 'learn' driving habits and maybe the previous owner's technique made the ECU do this. If so, enough learn time and it may cease.
Can you re-boot the ECU and start over?
On older obd2 bmw's disconnecting the battery was enough to reset adaptations, I don't have a clue if that's possible with current cars.
I've long noticed with EFI that certain engines are remarkably touchy to the slightest opening of the throttle. Try a Toyota Tundra V8 for example. I don't know how you four wheel the rig because they explode forward under the slightest throttle touch.
So I really doubt doing anything like adding a throttle return spring will help. Perhaps re-engineering the throttle lever cam to slow the opening of the butterfly off idle, but otherwise, you just have to learn to drive it.
A one year old Kia Optima was likely a rental car in its former life. If such and if the ECU learns driver input, just think how confused it may be from a new driver every day. I think the reset idea sounds very good.
manufacturers do this on purpose to make people think they have more powerful cars than they actually do.
The LS400 is fly by wire. It's difficult to feather the throttle starting out. It's rather complicated, of course, but the pedal goes by cable to the throtle body, where a pedal sensor is. If you step all the way down, you get about 1/3 open throttle mechanically as a fail safe. Otherwise, the sensor reads the position, computer does its thing and opens the throttle. The cable between the pedal and the throttle had some slack. When I took the slack out, it made it a lot easir to control at low speed. Still, a 300 HP V8 that wants to run can be a handfull at slow speed, especially if you're used to high strung 4 cylinders.
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