Hello! First post. I have a 2004, with a newer engine rebuild from Mazda. I am not a car person, so please bear with me.
When I turn the ignition, the starter spins up, except it the sounds is like there is no engine compression, or, "...like an engine with no spark plugs... Spins freely." Almost like a brushless motor. I don't hear any piston ignition cadence/interruption.
- I have taken the starter out, and had it bench tested: The starter was still good.
- Battery terminals have been cleaned thoroughly, and battery has been tested and charged. That all checks out, and my electronics are all functioning.
- I do not have any noticeable leaks, foul smells, smoke, or residues. The car has been in the drive way for a month, no puddles. Nothing startling underneath either.
- Fluid levels are all in check as well.

From what I'm reading on other threads and posts/responses, I see "engine compression" coming up frequently. My help, who is more car savvy than I, mentioned the "lack of compression" sound as something I should detail in my post.
Please let me know if there are more details I can elaborate on. Thank you.

 

Since you have the starter pulled. It’s good opportunity to inspect the ring gear on the flywheel.
if your starter is back on, you can let another person to crank and see if the engine spins, if it does not, it’s likely the flywheel ring gear has separated from the flywheel itself. Assume the starter operates normally like you said.
This is something I have encountered and sounds similar based on your description.

 

Check to see if you have spark and injector pulse. If not, your eccentric shaft position sensor has likely failed. You can test it with a resistance test to start. You will need an oscilloscope to confirm that its bad however. I went down this path, my engine sounded the same as yours when cranking after the car was stored for the winter. I confirmed I had no spark or injector pulse. I went through everything as the eccentric shaft position sensor passed a resistance test. I replaced it as I went through everything in the car and it fired up.

 

Maybe flooded the engine? A flooded engine sounds a lot like what you described as well. Definitely pull the spark plugs and have a look.

 

Can you record a video?
Can you confirm if the engine is actually turning with the starter? You should see the belts at the front move.

Possible the starter is not engaging the flywheel, but need more info. If not that, the above are all good lines of investigation

 

that’s what it “sounds” like to me too, either that or somebody forgot to install the rotors