Engine revs dip briefly when foot off from gas pedal

[UpSideDown]

I'm having this annoying problem with my IS250 from 2012. When I'm cruising on highway gentely accelerate and lift the foot off the gas pedal, the engine rev dips briefly and comes back. It feels like a very brief engine brake. This seems happens only before the engine is fully warmed up and in any gear. Anyone know why it does this? :uh:

Here is a recording of this strange behaviour
https://youtu.be/aJCsd3QMMuY

[dazed]

The car looks like it might have been in 6th gear for the duration of the Youtube clip so the behaviour may have been due to the torque converter lockup being released and reapplied.

When you accelerate in 6th gear (possibly 5th as well?) the lockup in the converter is usually released to allow the revs to rise more easily and the torque to increase. When you release the throttle the revs will momentarily fall back until the converter lock is reapplied. At the moment this occurs the revs will rise again as direct drive is re-established from the wheels back to the gearbox. (Some people mistake this for a gear change as it feels very similar)

It may be that these transitions are exaggerated in your car some reason - perhaps bad timing.

[UpSideDown]

That seems to be a reasonable explanation of this behavior. And yes this happens in 5th and even 4th gear as well. However, this seems only happen when it is cold out side and the engine is cold. When the engine temp reaches the optimal temperature all goes back to normal, the transmission/engine stops doing this. The car is from 2012, I think it starts doing this for a year ago or so. Is this a sign that something needs to be replaced down the road? Or should I take easy on this? I'm planning to keep the car few more years.

[chr15gb]

I'm new to lexus but having come from a 5 series, I'm familiar with torque converter issues.  

If it were a 5 series,  I could guarantee you need a gearbox fluid change,  this is bourne out by the fact it only does it when cold and gets better as it warms up.  If the T/C itself was faulty,  it would do it all the time. 

Have a good check for leaks or evidence of weeping around the pan. 

If these are anything like the ZF gear box in a bmw,  there is a very specific filling routine so best given to a garage. 

[Linas.P]

8 hours ago, chr15gb said:

If these are anything like the ZF gear box in a bmw,  there is a very specific filling routine so best given to a garage. 

It is A960E Aisin gearbox, but indeed refilling procedure is very specific (as for all modern auto) e.g. gearbox should be at specific temp etc. Looking back to my service history I seen dealership only doing top-ups e.g. 1L-out, 1L-in, but not complete refiling. Actually, I believe Lexus dealership was cheating on previous owner, because gearbox refill is "lifetime" or at very least 120k miles and then 60k afterwards (dealership did first top-up at 31k).

Now that said A960E Aisin can survive nuclear attack and never goes wrong, you might need slight top-up if your car has 120k miles + (which doesn't seems to be the case). Secondly, engine temp has little to do with gearbox oil temp, so engine being cold should not impact T/C or gearbox operation. 

the explanation by @dazed seem to explain the behavior, the reason why it is more noticeable on cold engine might be that on IS250 engines revs noticeably higher rpm while cold which would make T/C locking/unlocking more noticeable and less smooth. Finally, is hard to tell how hard you press accelerator and how quick you release it, but it seem  quite normal that when you press it starts building up revs and torque and when you release it before changing gear it just drops the revs as no longer needed, I am pretty sure mine does the same.