For the last couple of days I’ve been noticing the fluid level in my 1997 Honda Civic’s radiator reservoir has actually been rising! What’s with that? Now I periodically have to stick a rag down in the reservoir to absorb and pull out the additional coolant in order to avoid overflow.
Try replacing the radiator cap.
The cap may have failed where it allows hot coolant into the reservior, but isn’t allowing the coolant to return to the cooling system as it cools down.