Fluctuating RPMs

This does not sound normal but it may be a little difficult to find depending on how far it revs back up. When it reaches 1,500 does it just raise for a split second or does it rev to a certain RPM?

Also, can you get a video of what this does so that we can see it and I am sure that will help get us on the same page.

I am thinking this is either a vacuum leak or a leaking injector. So let's start with checking for a vacuum leak with this guide:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

The higher RPM will decrease the amount of vacuum in the intake but when you close the throttle the vacuum spikes and if it is pulling air in it could cause the RPM to raise. however, more fuel is normally what causes this so a leaking injector is more likely.

Let's start with a vacuum leak and go from there. Thanks

Got it. That is 100% normal. That is the PCM slowing the RPM down so that it doesn't free fall to a lower RPM and potentially stall the engine.

Think of it like someone who is skydiving and they release their parachute. They are dropping rapidly and the parachute comes out and it dramatically slows them down. Well the PCM doesn't want to have the engine decelerating to a low RPM at such a fast rate so it interrupts the deceleration so it can then slowly drops the last 600 or so RPM to proper idle.

In other words, it is the PCM's way of gaining control of the RPM again. When you let the RPM drop like that, the PCM is actually cutting the pulse width to the injectors so there is no fuel and the RPM fall. If it doesn't slow the deceleration then the engine will stall because it has no fuel. So the PCM is bringing the injectors back in and it slows the deceleration and then levels out.

Hope that makes sense. Thanks for the video!

You are welcome. Please come back to 2CarPros next time. Thanks