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.

coolant spillover tank

2 Answers 2


What you are describing is usually caused by a weak radiator cap. Weak meaning it is no longer holding the pressure which is needed inside the system. Since you said you just replaced the cap, having just water in the system will lower the boiling point and may cause over pressure in the system. You'll also want to ensure you have the entire system burped of all air, which also causes hot spots inside the engine. Easiest way to do this is to fill it from the radiator with the front of the truck pointing uphill.

When you do get this fixed, you'll need to ensure you drain the entire system to get the tap water out. Then you'll want to refill with 50/50 mix which utilizes distilled water instead of tap water, which will introduce minerals and contaminants into the system which degrades it over time.

  • It could also mean there is air in the coolant circuit and it's being purged into the tank. Especially since they did work on the circuit. Jun 1, 2015 at 11:33
  • @JuannStrauss - I would expect a small amount of air to purge this way, not for it to be boiling over, though. Jun 1, 2015 at 11:35
  • Which is why I put this as a comment to an existing answer for the sake of completeness. You would be surprised how violent the purging can be. Jun 1, 2015 at 11:37
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    @amphibient - Since the vehicle is not overheating, I don't think this is a concern. Jun 2, 2015 at 1:42
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    @barbecue - and I would highly disagree with you. While antifreeze contains corrosion inhibitors, the minerals in tap water will fall out of suspension and create sediment. These sediments builds up over time and lowers the ability of the cooling system to do its job. There's no reason to be adding sediment when you don't have to. You don't know what exactly is in your tap water. Distilled water is just that: water. Pure and simple. Sep 26, 2015 at 15:50

I had the same problem with my car and finally it was the Thermostat who was the culprit, it was not working properly and due to that even when the engine was heating up it did't start the Cooling fan and then ended up with the lots of heat and pressure and due to that coolalnt fluid overflows. Now I got the Thermostat replaced and its first day and i did't experience the same problem again and hoping it should run successfully now. ~Sanjiv

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