WR250R boiling coolant

Hey everyone,
I bought this bike with 6600 miles, and over the last month, put 1500 miles on it. I have not seen an over temp light not once.

I decided to check my coolant and reservoir level today, the reservoir was empty, and the radiator was pretty much empty. Emptying my system gave me about 400ml of blue coolant. Filling the system, my bike started to purge it all through boiling, right out the reservoir drain hose at 100*C. Which makes sense to me if the system is holding 0 air. However it was closed, and in my eyes should be holding pressure. The PO had a "1.1 Bar radiator cap" with one of those fancy thermostats on it, which I have not changed yet. Can someone point me in the right direction? Fan relay works using the diagnostic mode, but I don't think it turns on below 100C.

I'm leaning towards the cap being faulty before it being a head gasket or anything like that. Running the bike with the cap off, I do not see the pressure inside the radiator that would need to be there for it to be a head gasket issue.

I can't quite follow what happened and what the problem is. You seem to be saying that somehow the radiator had lost almost all coolant, and you tried to fill it back up while the engine was hot and running?

Once the engine cooled down, did you try filling up the radiator and the reservoir, and then start up the bike to observe what is happening?

Yes, I waited for it to cool down, then refilled the radiator. It was not holding pressure.

The radiator cap was to blame..Unfortunately after solving this issue, After owning this bike for a month and putting 2000 miles on it, it has been stolen and reclaimed by the streets of NYC. If I buy a WR again, I will come back. Thanks for the interesting reading, too bad I did not get to know you guys well.