After driving some time, is it normal that the plastic spillover coolant tank, usually attached to the fan housing, that takes boiling overflow, get so hot that you get bubbles boiling out of it even though the engine itself, according to the temperature gauge on the dashboard, does not get overheated? Is it an indicator that the engine borderlines overheating? Or is it a normal occurrence to have it boil?
I just recently had some overheating issues on my 4wd Tacoma and, after replacing the thermostat, water pump and radiator cap, the truck drives fine WITH JUST TAP WATER IN THE COOLANT SYSTEM (which I will replace with real coolant after I'm done testing) but, when I check the coolant spillover tank right after driving, I see it mildly boiling and I'm curious if that's a sign of potentially more overheating or is normal. So maybe I thought it was getting to a simmering point because it's just water and not 50/50 mix.