Ive done a stupid thing, topped up coolant (was in a hurry and late for work but the light came on!) got in car and forgot to put the cap back on:eek: boiled it dry on way to work. Garage says Ive put holes in head gasket and going to cost

How far and fast did you drive?

Problem with running without water is that the coolant temperature sensor ends up not reading properly because there's no coolant so it can measure normal or even low when the engine itself is roasting!

The cap keeps the coolant under pressure, (15psi I think), to get the pressure cooker effect, meaning that the 'boiling point' of the coolant is raised by 25 degrees C, so leaving the cap off will loose the ability to do this and I would think that the temp sensor could give you a false reading.

Or something along those lines I think? :confused:

You ,may have had the problem going to happen if you were putting water in the headder tank anyway.

Hope you get it fixed ok?

You ,may have had the problem going to happen if you were putting water in the headder tank anyway.

That's a good point - you shouldn't be losing coolant in any great quantity - certainly not enough that you'd need to top up regularly, which is what it sounds like. Since it's going in to get all those repairs done, I'd suggest seeing if the garage can find any leaks as well (there are come lame reasons like coolant pipes rubbing through, as well as the more expensive-to-fix ones like water pump seals...)

HTH

the temp sensor will read the same temperature irrespective of wether the cap is on or or not, but you are right about the water boiling at a higher temperature when it's under pressure. that's why the boil over when you open them when the engine is hot because the release of the 'pressure' allows it to boil.

my guess is that the cylinder head will be warped too, it really does depend on how far and how hotis was

Of course the initial coolant loss could have been down to the head gasket in the first place - especially if it's been done before.

Garage should be able to pressure test head and advise if it's cracked or warped.