Mystery Gasket Stuck In Thermostat!

[Lexus_Pray]

Anyone know what gasket this is from and how it could have traveled to the thermostat? (see attachment)

Temp gauge was maxing out after 15 minutes of driving, radiator was cool to the touch though.

Poured coolant into radiator, noticed leak. Ended up being water inlet pipe (aka thermostat housing) had a hairline fracture.

Removed broken inlet pipe, pulled out thermostat and saw this- wth??? Should I be checking/doing something else before installing new thermostat/thermostat housing? My guess is the rest of the gasket has to be either in the radiator or block coolant veins or both?...

Edited July 30 by Lexus_Pray
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[CoRifleman]

Hopefully you've received an answer for this elsewhere, but this is loose RTV from the valley plate.  It alone isn't an issue, doesn't necessarily mean your valley plate is leaking, but definitely something to look at.  The issue is the gasket material as it is smushed out when the plate is installed is just loose - and over time will break down and come loose like you found.  It can work its way into the water pump impeller blades, the tstat, etc.  If it holds the tstat open then the car will not heat up as quickly as if it was fully closed.

 

If it was my car I'd look closely at the valley plate area and see if there is any pink foam/brainy looking material signifying a leak.  A coolant flush might also remove more of this from the system.