2003 Honda Civic hybrid

Around town no issues

On highway overflows from reservoir and if gets too low it will overheat

Did a NAPA block test came negative
Changed thermostat and rad cap

Flushed twice

Oil and spark plugs clean

No smoke

I recently put supercoolant in to make the car temp lower and the car has not overheated since then but still overflowing coolant

This video is 35 min of me holding the car at 2.5 rpm that's when it started to bubble and foam

 

Can't say for sure, but it does look like a leaking HG. Very common on these cars. make sure you replace with a real Honda part or you will be doing it again next year. :-)