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Discussion Starter #1
Hello,

Some advice please for an overheating mini one 2003 petrol. The water/coolant is boiling in the expansion tank at 95 degrees. There is no bubbling until it reaches this temperature at all. Cant see any leaks at all and the level doesnt drop. Top and bottom hose are hot so im presuming the pump/stat and rad are circulating ok. Is this likely to be a blown head gasket causing the loss of pressure at high temperatures causing the lower boiling point ? There is no sign of oil in the coolant or vice versa but im guessing it could be between a water jacket and cylinder ? Anyone had similar issues like this ? Thanks
 

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A blown head gasket (or other failure between the combustion chamber and the water jacket) doesn't cause a drop in pressure in the coolant. The combustion pressures are far higher than that of the coolant system so causes excessive pressure in there. This can be felt in the hoses. It can't cause the coolant to boil at a lower temperature than normal either.

With the car running and hot do the hoses feel firm when you squeeze them?
 

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Discussion Starter #3
A blown head gasket (or other failure between the combustion chamber and the water jacket) doesn't cause a drop in pressure in the coolant. The combustion pressures are far higher than that of the coolant system so causes excessive pressure in there. This can be felt in the hoses. It can't cause the coolant to boil at a lower temperature than normal either.

With the car running and hot do the hoses feel firm when you squeeze them?
Ok thanks. Yes the hoses are firm to squeeze when its running and hot. What do you think could be the causes ?
 

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There's a good chance of a failure between the combustion chamber and the water jacket as you were suspecting. Does the exhaust have excessive whitish 'smoke' too? I'd be doing a sniff test of the coolant for combustion gasses at this point.
 

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Discussion Starter #5
There's a good chance of a failure between the combustion chamber and the water jacket as you were suspecting. Does the exhaust have excessive whitish 'smoke' too? I'd be doing a sniff test of the coolant for combustion gasses at this point.
Thanks Gary. No smoke. Ill keep my fingers crossed its just the gasket and not the head which is bad! I bought the car as a hobby project to do with my nephew so he will be happy we get to do more work.
 

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Discussion Starter #9
Small update as not had lots of time to work on it. Got a coolant pressure tester and it holds the pressure at 1.1 bar for over an hour. No visible leaks. Tried a " sniff tester" in the overflow tank and this was negative as well, so its not putting combustion gases into the coolant. Any ideas please guys ?
 

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Discussion Starter #12
Forgot to add....also did a compression test of all the cylinders for good measure. They were all 122 to 127
 

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Discussion Starter #14
275947

Well decided to go for removing it and getting a better look. The cooling jackets dont look corroded and nothing stood out with the gasket. Theres a lot of dry coolant but im guessing thats normal ? Ill be taking it to a machine shop this week for their opinion so fingers crossed. There is some discolouration on the inside at the exhaust manifold side. Should that be something to worry about? Cheers again for all the advice guys
 

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Discussion Starter #15
276030

Head skimmed and rebuilt. Put it all back together and ran nicely.....until it hit 95 degrees and started to boil again in the overflow tank.....noooooo. So after all that ( cant complain too much as i enjoyed doing the rebuild) it still has the same issue. Any ideas anyone ? Im refusing to give in haha
 

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Any word? I am having a similar issue on my 05 Cooper Base. Noticed the expansion tank is a basic wiper fluid bottle and seems too flimsy to hold any pressure. Actually there is a little notch in the rim which would allow air to escape by design. Every time mine overheats the snap on cap pops off. That seems unintuitive there and I don't know what to make of it.
 

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Discussion Starter #17
Unfortunately not. Since then we have replaced the water pump and radiator which showed signs of damage but still have the same problem. As it gets hot ( from about 90 degrees ) the coolant level rises in the overflow tank and eventually boils over. The part you mention is the overflow tank and isnt pressurised. The pressurised section finishes at the thermostat housing cap which is located between the airbox and engine head. If you have the same problem id suggest trying the housing cap and see it how it goes as they arent expensive. Ive tested mine on another mini and it works so im a dead end :cry:
 

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below link if its air trapped in system will clear it handy bit of kit to have with any future cars as well.
if this dont work then chance re something might be cracked some where not a common problem does happen, trouble with pressure testing a head when off is its a cold test and a lot of cracks dont open up until hot, i tend to look at it from a point where crack test will cost £60-£120 if can find a know good head for same money rather spend it repairing it than testing it if that makes sense, this all said those engines are a royal pain in arse to get all air out of the head and if dont will cause steam pockets where a hot spot is produced,,, i have a length of rubber pipe i use i fill the coolant then seal the pipe on to filler and then blow down pipe with bleed screw open until no more air comes out ,, this worlks better on cars with a screw top header tank rather that the cheap silly over flow tank early r50 had even worth changing them with the cooper s header tank and thermostat housing it also allows a good head water pressure that helps push the air out, will need a t-piece to heater pipe and small over flow pipe from top of header tank to new thermostat housing its so much better they really should of done this on all of them
the t piece bmw sell them £7 ish plus pipes
i done one a few weeks go just kept boiling up spent hours feeding it drips of water in end done this mod on it bled it in 20 minutes with a rubber pipe to blow through etc,
 

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Discussion Starter #19
I did consider a vacuum bleeder....if nothing else then i could persuade the missus to let me buy a compressor ! Think that could be the issue....simply trapped air ?
 

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thats why i also said rubber pipe and blow in to the system with bleed screws open i do that all the time as cant be bothered to go set the pressure bleeder up,, i've found if i dont write the in a perfect word way of dealing with it someone will come along and have a poke about lol
 
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