OK:

a) When the engine and radiator cool, the coolant contracts which reduces the pressure in the system. If your radiator cap has a return valve (the 'recovery' type) and an overflow bottle, coolant will be "sucked" back into the radiator keeping it full. Alternatively, if you have a pressurised overflow then the radiator has a flat cap (i.e. no spring and 'tongue' which seals against the radiator neck).

b) With a pressurised overflow system, position of the overflow tank isn't important -- mine for example is underneath the guard, below the level of the radiator. I've never fitted a non-pressurised system, so I don't know whether the overflow needs to be above the radiator; I'd guess that it probably does, though in order for gravity to keep fluid up to the return valve on the radiator cap.

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Simon

The adventures of an owner builder in the Tallarook Ranges

I agree. I drove a Cooper S from Sydney to Melbourne with a short-necked radiator cap fitted instead of the correct long-necked one. The previous owner had fitted it just before I picked it up and I didn't notice initially. It looked OK from above but the cap didn't seal at all - obviously. No problem cruising along at 100 kmh. Just needed to let it cool and then top it up at each break. Certainly not recommended, but it got there.

Thanks for the reply SGC, your answer helps me and also makes me feel better....After I posted question I thought about it some more and had a brain wave..."How will I check and top up coolant in overflow if it is under guard" So the proof is there it can be done and is accessible. I will be fitting a pressurised overflow system tomorrow.
Thanks again.

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1971 Mini K

Considering how much fluid is being lost, I still say that there is another problem here. I have driven a mini from Newcastle to Adelaide and back with no radiator cap and it only lost the top inch or so of coolant from the radiator from expansion. And its the old bushman trick if you've got a leak to loosen the cap to stop pressurisation of the system. To be spitting out about 1+ litres (? is that correct) is excessive. As you have said 'not seen one loose that much from expansion'.

I am not sure is it the same issue but I had one of those ebay alloy radiators as well. Last week I did realize am loosing some water check everything (just like you) and nothing visible then a day later I see that each time I give it a bit of gas water gashing from wright hand side of the car. in front of the wheel to be exact. There was a small scratch on first row from wright side of the radiator cowling cower so can`t see properly. Have look is it the same you might be able to see some discoloration.

Coincidentally I was looking at my Uk rad earlier today... It has a tab about an inch and a bit down from the base of the filler neck that is embossed "fill to this level". Obviously you're not supposed the fill the tank right up.

Mine used to spit water out until the tank was almost empty and then would sit at that for ages.

Since then i've bought a "recycling" type cap from a local auto shop and installed an overflow tank made from a used 250ml brake fluid container. It sits neatly on the cross member between the wiper motor and the inner wing. Never needed topping up again.

Cheers, Ian

No worries!

As for checking the level and topping it up, I've done this maybe twice in six months at the most -- you really, really don't have to worry about it unless you have a leak. I've taken the flat cap off a couple of times to check the radiator level, and it's been full to the brim every time.. it really does just work.

When adding coolant to the expansion tank for the first time, all I did was fill the radiator up to the top of the neck so it was over the overflow pipe, and allow it to drain slowly into the expansion tank (with the pressure cap off the expansion tank, naturally ).

As for checking the level, all I do is take the cap off the expansion tank and stick a finger in -- if I can just feel the coolant, it's full enough.

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Simon

The adventures of an owner builder in the Tallarook Ranges

I would not recommend the above statement/fact(?) unless desperate messures are called for as this can cause more damage than first thought especially over such a long distance . . . +1000 kms. The reason being that this can cause hot spots in the cylinder walls, over heat the oil which in turn over heats crank bearings and the list goes on . . . If the coolant is able to flow, the water pump is only able to push so much coolant/water around the cooling system considering the size of the cooling system and the size of the impeller on the water pump and the amount of load put on the water pump from engine speed as well as the air it is trying to pump throught the radiator.

A cooling system not only uses a water pump to aid circulation to the cooling system but also uses "convection" when the coolant/water is heated. When the water is heated it becomes less dense the hotter it gets therfore rises to the top of the system . . . the radiator is a heat exchanger (radiator) that is cooled by a mechanical fan (we are talking classic mini) attached to the water pump. The radiator top tank/cap are the highest point in the engine allowing the coolant/water due to convection (or another term thermo syphon) to go to the highest point away from the engine (as heat is a big killer of any engine, it also increases wear on the softer metals used due to friction).
Convection is more efficient when the cooling system is under pressure as it is able to take the heated coolant to the heat exchanger faster more effectivly allowing the fan to cool the coolant/water making it more dense, when it becomes more dense it drops to the bottom of the heat exchanger/radiator and then makes its way back throught the engine removing the heat from the walls of the cylinders (as this is where the heat is created) and circulating back to the radiator. there are small cooling veins through out the head, block and inlet manifold as some parts actually require heating rather than cooling to work more efficiently/effectivly eg. inlet manifold . . . but thats another story
I hope this hasn't bored most people and was easy to understand



As you have said with no cap there is no pressure then you will lose little coolant in the process as the water/coolant that has turned to steam when it has reached its boiling point does not return to the cooling system, it evaporates into the atmosphere.
What seems to be happening with the short cap (because they are not what they used to be and neither are most of the radiators) on the radiator is it does seal to its full pressure, so the cooling system is making pressure but it is over coming the spring tension on the cap earlier than it should and the pressure (coolant) is being lost.
If you are driving for a longer period of time you and the engine reaches its normal operating temp (or more) it will lose more coolant than if you make a short trip and the cooling system doesnt get as hot therefore you will have a difference in coolant loss.

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http://www.youtube.com/watch?v=nDMkVq1jRGU

Scott, you've missed the point.

I was losing a bit of coolant out of Slidey when hot. So I bought a plastic recovery bottle set up from Supercheap Auto but the thing is though you can't buy a recovery cap to suit Mini radiators. So I took the radiator to Red Devil and they cut the neck off and put a Commodore neck on it.
Now my radiator is always full.

Not really as was said, it takes 4 days for it to get that low and i am only guessing that everytime it is topped up the cooling system is not bled so therefore there is also trapped air, hence why more coolant/water will be pushed out of the cooling system especially if the thermostat hasnt opened.
The only way to find out if it is more (eg . . . headgasket) than a cap not sealing correctly is to do a coolant pressure test and also a compression test.
Personally I do very much the same as TK with most of the radiators I have fitted . . . change the neck to HQ or early Commode as the recovery caps are redily available from almost anywhere and fitting an overflow bottle cant hurt especially if you are not sure you have bled the cooling system properly as the air will expand and evecuate from the cooling system and when the cooling system recovers when cooling down that air will be replaced with the coolant it has in the overflow bottle.

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http://www.youtube.com/watch?v=nDMkVq1jRGU

You have a leak somewhere. Look for it on the car itself. You will not find it looking on here! Good luck.

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DID I MENTION THAT I AM THE BEST SUPPLIER FOR MINI ELECTRICAL PARTS AND REPAIRS?
"Let us remove your "SHORTS"

The smartest and most correct answer so far.

"the thing is though you can't buy a recovery cap to suit Mini radiators. "

Not true - I've not had any trouble buying Nippon Denso items that fit. (no part No unfortunately - its all in Japanese). Maybe the rad shop saw you coming

Easy to check if it coming out the overflow though - just stick the end of the overflow hose in a small container. It won't get sucked back to the rad without the proper cap but at least you'll know where the missing water is going....

Cheers, ian

After reading the replies I went ahead and fitted a pressurised overflow system. (Mounted somewhat temporarily until I assessed if it would work as advertised). So far the Mini has been driven every day to / from work and every night I have checked the coolant levels and it has been perfect. No loss, nothing spewing out of overflow from overflow reservoir, and temps seem to recover quicker after she heats up in those long idle times in Sydney traffic.

I am confident that the fitment of the overflow system has solved my loss of coolant issues and wanted to thank those who offered the good advice.

Tim

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1971 Mini K