I drive a 2003 Honda Civic Hybrid 1.3L with a manual transmission.
It has about 126.6K miles on it.
On Fri, I left work to pick up my younger brother so we could go to scouts.
For some reason, enough pressure built up in the coolant system to caused a rupture in the coolant reserve tank.
Fortunately, I carry extra everything with me,
So I used zip ties on the small hose between the radiator and the reserve tank in order to isolate the leak,
And refilled the radiator with fresh coolant.
I then picked up my younger brother and headed off to scout camp for the weekend.
Roughly a 150 mile trip there, 300mi back (All on 3/4 of a tank, yay hybrids!)

Somewhere on the way down,
The pressure built up enough to blow the hose off the radiator and sprayed out most of the coolant.
So I put the hose back on, used a zip tie to secure it, refilled the coolant, and carried on.
The engine temperature fluctuated from normal to dangerously close to the red line for the rest of the trip.
I assumed the coolant merely needed to be "burped" (is there a technical term for that?).
We arrived without further incident.

On Sunday as we were packing up camp I went to check the cars vitals.
I was surprised to find that the radiator was positively pressurized when I took the cap off nearly 36 hours later.
It sounded like opening a can of coke (pop/soda/whatever. I'm in Texas, remember?).
The coolant was low as I suspected, so I topped it off and we headed home.
The engine temperature continued to fluctuate from normal to dangerously close to the red line as it did on the way there.

Any ideas what could cause this?
I had something similar last year,
I ended up replacing the radiator and head gasket before i found out the problem was a bad thermostat.
So it has a new head gasket.
Any thoughts on what else might cause a build up of pressure in the coolant system?

 

A lot can happen in a year. The thermostat or the head gasket could be bad now.

It's hard to test a hybrid I suppose because it's not going to let you rev the engine with the car sitting still.

Blocking the hose is a bad idea, pressure will build out of control. That's what the reservoir is for. I would tend to think that if you have lots of pressure and lack of coolant when the engine is completely cold, it wasn't just steam, it's head gasket leakage.

 

I'd first try a new radiator cap. If that doesn't work it sounds like a bad HG.

A lot of shops dont do HG replacements correctly unfortunately....mainly because of the lack of prepping the block/head surfaces correctly.