I have a 2001 Civic EX with 183000 mi. 1500 mi ago the overflow bottle was full and the radiator was 1-2 inches low on fluid. I sucked the excess fluid out from the overflow bottle and put it back into the radiator, then I replaced the radiator cap with a new Honda cap. Since then it has done it twice and it looks like antifreeze has been sprayed from the top of the overflow bottle leaving the radiator 2 inches low on fluid. Any ideas what this may be. Thanks in advance for your time and thoughts.
Head gasket failure. Compression and exhaust gasses get into the cooling system, the excess pressure opens the radiator cap, etc., etc. Some coolant may be going out the exhaust. Is your oil milky? That means coolant is there, too.
I have heard of test strips that detect unburned hydrocarbons (exhaust gasses) in the coolant. Your car is very clean burning and that test may not be definitive. Many shops have a “sniffer” to detect unburned HC. That may be more accurate.
Good luck. Please keep us informed.
I’d have a mechanic check for a leaking head gasket.
Thanks for your input. The oil looks clean, any idea how long that would last?