Help identify blue plug-like cap off Engine or Tranny?

Hi. I'm new to this forum. In California and I have an 2001 E39 525i wagon. Nice to meet you all. Looking for some advice/help here.

Something broke off or fell off my 2001 525i Wagon under the engine/transmission area. I found it on the floor under my car and it looks very brittle. It's blue and looks like a plug of some sort. I'm attaching an image.

Can anyone identify this for me and tell me where this broke off from? Is it important and will it be fine without it?

Thanks so much for your help.

It might be the top of the drain plug in the bottom of the radiator.

I think it's probably in here ...
- Getting to know your M54 engine, a photo & identification guide to every visible part in the engine bay (1)

Appears to me to look like the plug where u place a flywheel locking pin to lock your bmws timing down when doing engine work. My e36 had one very similar looking. The picture blue bee posted makes me almost positive that is what it is. Nothing of concern. I'm surprise it fell out though, I couldnt get mine out...

Thanks for helping edjack, bluebee and Blitz355.

I did check the radiator lower plug as you mentioned and it was not it. The original blue plug was there. Bluebee hit the mark with the picture. It looks just like that and according to Blitz355 description, there was a small hole at that location where it could fit. As I was looking at that location, I just noticed a big hole about 3 or 4 inches above that little hole. I tried looking thought the forums per Bluebee's link but I can't identify this hole into the flywheel. Is this normal or should something be mounted here like a plug?

I think I sprayed water into that hole while degreasing my engine. Does anyone know if there is a drainage or might I have a puddle inside?

Attaching pictures. Thanks guys, knowledge is power and you guys have the knowledge.

I appreciate your help.

Blitz335 is likely correct that this is the plug for the flywheel locking hole. Any water that got in there would drain down & run out of the slots between the engine & trans. I would have to assume that the remainder of this plug is either still in the hole or has been chewed up by the starter ring.

Originally Posted by edjack

it might be the top of the drain plug in the bottom of the radiator.

+1

Originally Posted by samdayo

I tried looking thought the forums per Bluebee's link but I can't identify this hole into the flywheel.

I don't know the answer as I've never touched a flywheel ... but ... typing /flywheel F3 in the bestlinks nets me this which may have more information:
- How to make your own BMW flywheel locking tool (1)

Originally Posted by Blitz355

Appears to me to look like the plug where u place a flywheel locking pin to lock your bmws timing down when doing engine work. My e36 had one very similar looking. The picture blue bee posted makes me almost positive that is what it is. Nothing of concern. I'm surprise it fell out though, I couldnt get mine out...

Yes, it's somewhat strange that that plug top would break off like that unless something struck it on the outside, but it's protected pretty well by some aluminum "fins" from the engine casing.

The "hole" higher up ( in the second pic you posted) might be a threaded ferrous bit that receives one of the starter mounting bolts from the bell-housing side. I am not sure though; I can just make it out on my car but it doesn't look rusty.

Yea, don't ask me how it came off. On my e36 i was trying for an hour to get it out, but it was in there so good that I ended up busting the top part of it off... Surprised yours just fell out

I was googling for the very same question today; after several attempts the link to this page came up after searching "e39 plug under starter". The plug is disintegrated from dry rot after years of neglect. I found the end of it lying around under the intake manifold (along with many other interesting bits) which I have off to replace the wiring harness of my ultimate driveway machine.

Originally Posted by weh3

....my ultimate driveway machine.

LOL that's funny!

It's become my tagline