It's all come down to suction. Have you tried to drink your glass of water with a defective straw? You can if you work hard on it but if the straw works 100% it'll be much easier and with less effort.
Unfortunately your radiator cap is set to constant pressure of 1.1 atmosphere and cannot overcome the loss of pressure like you would do while drinking with a straw.
Coolant going to the overfill tank uses gravity and pressure built up from the radiator however the radiator needs to work against gravity in order to bring back the coolant inside the radiator.
I got a similar issue with my 2001 Civic (187,000 miles) and my overfill tank was way over the max line I set it up while my radiator was getting empty.
A lot of people don't diagnostic correctly and don't go over the cap swap and if it didn't work, well too bad, you have to change your head gasket (very expensive). Your description of the problem doesn't make me lean toward a head gasket issue but know that I could be wrong.
I fixed my issue for just about $5 dollars. When I emptied the overfill reservoir in a clean bucket (water jug), I've noticed some black specks in it, which made me think that some rubber is deteriorating (AKA hoses).
On my Civic, I have 2 hoses joined with a plastic coupler. One side goes from the radiator to the coupler, the other side goes from the coupler to the reservoir tank nipple.
I decided that the lower hose was potentially the issue and decided to swap it (I'm cheap even though the hose, 5/8th ID was only $2.19). I also got some worm type clamp (don't use zip ties) and put them strategically.
I put a clamp where the hose connects to the overfill tank nipple, 2 on the coupler on both sides where the hose meet the coupler.
I filled up the overfill tank to the MAX line, made sure the radiator was full and went for a ride.
After 20 min driving I opened the hood and as expected the coolant was about 1/4 inch above the MAX line (coolant expanded in the radiator and it went to the overfill tank). I let the car sit overnight and in the morning I checked it again. The coolant was back to the Max line, meaning the system works as expected again.
Bottom line, don't always believe what you read on the Internet and stay logical, sometime the solution is as simple as it sounds.
Sorry for the long thread, hope it'll work out for you the same it did for me saving you money on the way.
Tuesday, May 31st, 2016 AT 11:14 AM