try replacing the thermostat.

Maybe you know this already, but to review: The coolant is
supposed to expand somewhat into the reservoir, under
certain conditions, from cold engine to warm engine. Upon
turning off the car and letting it cool, it should be sucked
back into the radiator, etc.

If the temperature rise is only slight, I wouldn't be too
concerned. To ensure optimal operation, you can always do a
thorough air purge of the cooling system, following your
manual's instructions (or see the free online manual for
your 90 Civic at www.autozone.com). Be aware as much as 45
minutes or so of idling may be necessary for the rad fan to
come on twice, while performing the purge steps.

Also inspect the coolant for signs of engine oil. Also, Look
under the oil filler cap into the valve region for signs of
coolant.

Lastly, how old (miles and years) are the thermostat and
radiator cap? Is the current thermostat OEM?

Elle
Original owner, 1991 Civic, 175k miles.

Replace the cap. The pressure seal has failed. You want a 13lb (or 0.9 bar)
cap.