Car Stuck! Engine Coolant boiling over

[SJ]

90 100q, normally aspriated, manual

Wife drove car to a wedding shower. When she pulled into the parking lot,
the expansion tank boiled over. The parking guys waited till engine cooled
off and added water. No leaks.
Wife said temp guage read normal. She got a low water warning light when she
pulled into the parking lot.

I just came back from checking out the car. The water that was added has not
leaked out, so I can eliminate any big leakers.
Started up the car and let it idle. Temp came up normally. After about 10
minutes or so, temp guage read 1/3.
No leaks noted . .both hoses were hot.
After another few minutes, the expansion tank boiled over(I had the cap on
loose). About 2 quarts and lots of steam came out.
The radiator fan never came on.

The radiator is new, about 5k miles on it. All the hoses  are new.
Timing belt and water pump replaced 35k miles ago.
Thermostat replaced about 25k miles ago.

My hunch is a stuck thermostat.
I will pick up a new thermostat and gasket and go to where the car is stuck
and attempt replacing the thermostat in the parking lot.

As I recall, unbolting the hydraulic pump and moving it aside opens up a lot
of access room to the hoses and thermostat housing. After that, one has
access to the two thermostat housing bolts.

Any other opinions as to the nature of the problem?


SJ
85 Dodge PU, D-250, 318, auto
85 Audi 4k - - sold but still on the road
88 Audi 5kq
90 Audi 100q