What is this part attached to the throttle body?
 

I was taking off the throttle body to replace the driver side coolant line and the cap (see "b" in pic) of this part partially broke off. It looks like heat and time made it very brittle. Inside the unit, there's a screen filter. The cap seems salvageable as a little silicone sealant around the edges might be able to seal it back up. So what is it and is that cap replaceable? How much $? Thanks.

https://www.clublexus.com/forums/mem...body-33615.jpg

That's your diaphragm.

Do not seal the cap on too tight with the silicone it is suppose to vent some air out when actuating.

It acts as a cushion when you abruptly let off the throttle, keeping the plate from slamming shut all at once.