Boiling coolant

[Jono 3]

Hi folks

I'm here to ask a few questions that I hope you can help me with. My technical knowledge of cars is very limited.

Yesterday I noticed steam coming from my engine (Car is a 1.7 Puma). Checked the temp gauge and it was sitting right in the middle on normal. Opened bonnet to find steam coming from the coolant as it was boiling. Turned out the cap of the coolant wasn't on tight enough (I've just got the car, I presume previous owner topped up and didn't tighten the cap)

I've since learned that the reason this happened is when the cap is loose/off there is less pressure in the system therefore the coolant boils at a lower temp.

What I'm wondering is, could any damage have been done to any part of the engine or cooling system by the coolant boiling? Even though the engine temp gauge was still in the middle. In fact the engine never even got hot enough for the fan to come on (the fan still kicks in when it should as I checked it after tightening the coolant cap).

Am I just worrying about nothing? I hope so.

Thanks folks

[Pumaracing]

The temperature shown on the gauge is the coolant temperature not the engine components temperature. The coolant can never get hotter than its boiling point so if the cap is off that will correspond with about "normal" on the gauge i.e. 100c (or a bit more than that because of the antifreeze mix) even when the engine itself is starting to melt. With the cap on the boiling point rises by about 1.5 degrees C for every lb of pressure so a 15 psi cap will raise the boiling point of water to about 121C (maybe 125C with antifreeze) which is about what corresponds to max on the temp gauge.

In fact if the engine has lost enough coolant for the temperature sender to be uncovered the gauge will happily continue to read stone cold while the engine seizes solid and grinds to a halt. A very cold reading gauge is probably more worrying than a hot one.

However, while there is still plenty of coolant in the system it will continue to keep the engine itself below damaging temperatures for a while as engine heat turns into latent heat of vapourisation of the coolant. Steam is good. It means the engine hasn't melted yet.

So you're probably ok. If the engine runs normally with the cap back on properly and doesn't lose coolant then you definitely are.

[Jono 3]

Thanks for the response there pumaracing.

Well the cap was on but just needed tightened up a bit.

I tightened the cap as soon as the steam started and then turned the engine off. I waited about a minute then started it again and temp was still only sitting in the middle. Coolant level hadn't dropped at all, was still inbetween min and max.

Had the car out a run this morning for 30 miles or so and ran fine.

From what you have said, it sounds like the car is ok?

Thanks again.

[Nick3point2]

Pumaracing said:

(or a bit more than that because of the antifreeze mix)

Its been a few years since I did GCSE chemistry but I was always told impure mixtures melt and boil at lower temperatures as the bonds between water molecules are weaker thanks to the other substance in the water. Hence why salt water doesn't freeze until a lower temperature: you have to remove more energy from the mixture before the substance will solidify. And IIRC the same works for boiling: the other substance prevent the bonds between water molecules from being as strong so the temperature at which the liquid becomes a gas is lower.

I think....

[Pumaracing]

http://en.wikipedia.org/wiki/Ethylene_glycol

[Nick3point2]

Pumaracing said:

http://en.wikipedia.org/wiki/Ethylene_glycol

I withdraw my objection.