Steam coming from radiator cap after coolant not draining properly

While replacing my thermostat today in my 1995 Honda Civic, I drained out all of the old coolant through the drain plug. By the time all of the coolant had been emptied there was about 4 litres of it in the drain pan. After installing the new thermostat I filled up the cooling system with distilled water and a 500ml coolant flush additive. After taking the car for a drive to get the fluid to circulate through the system and then letting it cool, I drained out the new fluid in preparation for adding the new coolant. However only about 2 litres drained out this time (I turned the engine on to force more of it out but this didn’t do much). Since nothing else came out I added the new coolant. The system was full after I added about 2 litres of the new coolant.

After starting the car again, steam started coming out of the radiator cap and the check engine light came on. In hindsight I probably shouldn’t have put the new coolant in since there was probably still water and additive in the system (I assume the steam is mainly distilled water).

My question is why wouldn’t all of the fluid drain out the second time if it did the first? And how would I go about getting all of what’s in there out now so that I can add a proper antifreeze/water mix?

If you didn’t open the bleed bolt when adding coolant air got trapped in the cooling system.

This prevents completely filling the cooling system and air gets trapped.


Your Honda may or may not have the bleed bolt. It probably does have a valve controlling flow to the heater core. You should have the heat set to Hot when emptying and filling. After filling the radiator and the expansion tank, leave the cap off and let the engine warm up, adding coolant as needed, then put on the cap and shut it down. After a cooldown open the cap and put more in if needed.