RPM drop by 200 to 300 while decelerating

[Tarek80]

I know mine is P80

[jimmy57]

Normal. When you lift off throttle with rpm above 1450 then the injectors are not operated and throttle is set to a greater angle to avoid rich mixture and high hydrocarbons. When the engine speed and road speed fails to target values then the throttle returns to lower opening and injection resumes and the idle is slowed back to normal rpm. Majority of people will not notice as you change your focus to the road as you slow as the feedback you need to choose brake pedal force for the slowing rate as closing distance is how your mind computes how fast you should be stopping. As you slow the transmission goes down through the gears and some gears have engine braking and some freewheel. The extra throttle opening also serves to make the linked gears not so noticeable as open throttle decreases engine braking effects. If you manually shift auto trans the gear that freewheel have a brake that overrides the freewheel. manual shifts are noticeable because of the braking. The coast linking for those gears allows them to give you engine braking.

[Tarek80]

Thank you Jimmy57, I will pay attention next time to see what gears have engine braking, I will take the car to a country road and have some time to experience. I have 3 more cars (non turbo) and I have never noticed such a drop, maybe it is Volvo specific?
I have been reading and learning about turbos. I have a theory here but would like to get your opinion
What about wastegate adjustment, what if the adjustment is wrong and the wastegate opens at lower boost, wouldn't that cause the drop? or maybe the turbo bypass opens too early
Thank you!!