hey i just graduated from college and stuffed about with the mates for a while then i took my mate back to his house in the mini. on the way home i started to smell soem thing odd and then steam started coming from under the leyland vans bonnet. So i pulled over on a side road off the high way as soon as i could opened up the bonnet and to my surprise the radiator cap was half off. And just my luck i was about a 6 hour walk (estimation) home and out of spare water. so i unscrewed the windscreen wiper water bottle off and filled the radiator back up as much as i could. then the 1st car to coem by pulls over, then drives off and comes back with 4 litres of water and drives behind me for half the length of the way home. what a champ. but what i dotn get is why the radiator would mysteriously coem off, cause i always check the radiator cap is on secure and the mini specialists said the cap was fine? ne suggestions why it would come off?

cheers luke

Back pressure in the cooling system ?

Possible blown h/g ? Although you would probably notice that by -
- Increase in engine temp.
- Loss of power
- Radiator emptying itself into your engine bay.
- STEAM...and lots of it.

_________________
No more Minis!......For now.

well i hope its not a blown head gasket, only got it replaced a month ago and i had problems with the temperature for a while before it blew but ever since i got the head gasket replaced and flushed the cooling system out it seemed to be fine untill the the radiator came off, what would cause the back pressure in the radiator?

What caused the back pressure in mine was a blown h/g. It blew in between a water gallery and cylinder. Emptied my radiator very quickly on the Sydney to central Coast cruise #2.

Other than that, I couldn't possibly say what else would cause back pressure.

I'm sure someone else can though!

_________________
No more Minis!......For now.

If the cap was half off then someone has left it that way because no matter how much pressure you build up it wont turn the cap. If you're confident that you didn't have a lapse of concentration the last time you checked the coolant level then I would look at some of your college friends that may have been having an end of exams joke on you - we all know how secure a Mini bonnet is

mayb i didnt put it on properly but i drove this far since i checked it and there is a scale of distance bottom left of picture, pretty long distance and whereis.com estimates it being around a 27 km drive so it was a fair distance for it to be off and most of my friends dont know how to open the bonet let alone in the dark haha, oh well lets just hope it doesnt do it again if it wasnt my fault

a well tuned engine that isn't left idling for a long time with no air flow or driven too harshly will go a long way before it reaches 100C and start to boil unpressurised water out. It's only after it starts to boil some out that it enters the downward spiral because there is less water for cooling, so it boils more, spilling more.

I've deliberately left the cap loose when I've developed small leaks that get worse under pressure so that I can get home. If it's cool and you don't drive hard you can go a long way before it overheats.