I currently own a 2008 Lexus IS250 AWD with 110k miles and recently I've almost experienced an accident/death due to the car not accelerating properly while merging onto a freeway. I'm a typical driver that does not have a foot made of lead but this recent event has lead me to be very observant on how I am driving. Currently the issue is completely random whereas I do not know when it will occur or why it occurs. The problem occurs when accelerating off of a complete stop whereas putting my foot down maybe 25-30% throttle (basically normal acceleration), the car barely moves and the rpms are stuck around 1.5k for a good 4-5 seconds (might be a little dramatic but it is long enough for the car behind me to honk) before it shoots up to 3-4k rpms quickly in 1st gear (while still putting down 25-30% throttle) and launches me forward. No matter how heavy throttle I put, when this problem occurs it occurs. Also on the (NYC) highways its very typical for traffic jams as well as slowdowns where traffic is flowing around 30-40mph but then it will quickly speed back up to normal highway speeds. Rolling at 30mph on the highway, I have the same problem whereas no matter how much throttle I input, there is the delay and then it launches me forward. Another note is I've noticed my MPG has dropped significantly to literally 8-9mpg when I used to average 20+ mpg. There is no check engine lights or any codes when it was hooked up to my mechanics scanner and he is also unsure what the problem is since this is also his first time seeing this problem Any suggestions or help would be nice whereas this is my personal car that I need for work. I will be looking at this thread everyday for anyone's response because this is literally driving me mad not knowing if the next ramp I take onto a highway will be my last.

 

I’d start with the easiest potential issue.

Are you using OEM floor mats? Are they secured in the lock holes in the floorboard and are they well clear of the accelerator? Do you have just one set of mats in the car or maybe rubber covers laid over carpet mats?

 

Did your mechanic see any live data, or did he just scan the codes?

Need more information. If your milage is suffering that bad something is definitely wrong.

 

I completely agree with Mike on this. You need to stream and log some live data.

I've seen the knock sensors go bad and pull back the timing which will kill acceleration and mpg's. That said, I doubt that's it.
I for one would start with a MAF cleaning and then go immediately to data logging.

Log: Calculated load, Ignition advance, Absolute Throttle Position, MAF flow rate and maybe one bank of STFT and LTFT.

Here I dumped some data from Torque Pro into MS excel to see ignition advance under load. Start there. It's pretty simple and will tell us a lot.

<<PASTE >> from -> FSport's IS-350 running list of service & maintenance DIY
Chart from Torque Pro, stock 2013 IS-350 @ 78,xxx mi.
Logging set to 0.20 seconds.
Rolling start at 4mph hits R-limiter at 86.x in 3rd.
Max MAF = 224g/s
Timing gets pulled back a good bit on the shifts...


Notice the ignition advance takes a pretty good hit at max load when it bangs the next gear.
Anyone care to share their data and compare?