I’ve owned this Civic since new, now at 153K miles. This is a recurring problem that I’ve been having for over a year. First symptom was there would be no heat coming from the heater at idle with the motor well warmed up. Add some rpm’s and the heat returned. The reason was not enough coolant in the system, filled up radiator and all OK. Then it happened again and there is coolant in the overflow tank, but pull the radiator cap and no fluid under cap. Fill radiator and all OK, but it kept happening. I replaced the OEM radiator cap with a new “Slant” brand for an '03 Civic. No better, no worse.
Last week I had a significant overheating event while on a highway. I pulled into a “Dollar Store” parking lot, went into the store and bought 50/50 coolant and shop rags and let the car cool off for 30 min. When I pulled the cap, it was a steamy mess and I let it cool some more. Eventually I put in the coolant and finished up the drive with no more overheating. The overflow coolant container was at the “full” mark all during the overheating event.
At this point I took it to my trusted mechanic. My mechanic determined that the overflow tank was clogged and he blew out the line. So, I hoped all was fixed. But I don’t think so. With the car cool this morning I pulled the radiator cap and got a swish noise indicating some pressure release. The level of coolant in the overflow contain hasn’t moved. It still seems coolant is not being pulled from the overflow container back in to the radiator when the car cools off. I blew into the radiator top (where the cap goes) and that did produce lots of bubbles in the overflow tank. It seems it is no longer plugged - but why isn’t the coolant behaving properly? I just ordered a genuine Honda brand radiator cap and I will try that next. Any other ideas?