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Losing water into the overflow bottle.


brian GT6.
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So i've just run in my rebuilt 2.0 litre engine, or at least got it to the stage where I need to re-torque the head down after a couple of hundred miles.

 

The only thing wrong at all with the car now is that I am losing water into the overflow bottle. There is no oil in the water it is still clear blue water/antifreeze. There is no water/mayo in the oil.

 

So is this likely to be a weak spring on radiator cap. It is a new one but seems really easy to squeeze together or maybe pressurisation from a head gasket? The car runs at the same temperature as ever before and doesn't overheat.

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Water should go into the overflow bottle when the car heats up, and then be sucked back as it cools. 

I have seen a lot of rad caps that do not have a good, or any, seal on the actual cap part, only on the bit that seals lower down. That stops the coolant being sucked back.

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Brian,

as Clive says the water is intended to overflow into the bottle and then as the water in the cooling system cools it contracts and sucks the water back out of the bottle.  For this to work though, the pipe into the bottle should extend all the way down to the bottom, and the bottle would typically be half full when cold, so that air cannot be drawn into the pipe and back into the radiator.

Pete.

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9 minutes ago, clive said:

Water should go into the overflow bottle when the car heats up, and then be sucked back as it cools. 

Clive is correct - it's designed that way - the bottle needs to be half full with the end of the overflow pipe covered; as the water expands it's pushed out into the bottle, but as it cools it's drawn back in. If you want, you can mark the level on the bottle, and this will help to indicate if there's a problem ie if it fills without going down again afterwards, or is constantly empty, and so on. 

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rad caps   a nightmare of mix and match   the cap main seal has to match the depth of the filler neck seating , ( quick measure will give a clue )

 the pressure be it 5 , 7 or 13 doesnt matter that much 

it raises the boiling point of coolant , of course hoses etc need to be of a spec to  hold the higher pressure  ok  .

raising a low psi to a higher may balloon some weak hoses 

on a semi sealed there must be a fibre or rubber flat seal to cap the top of the neck. when screwed down, it just sits under the top , there is normally a metal seal here on sealed they add the fibre/rubber to make it air tight 

if this is not present coolant will fill on heating/expanding but cannot be drawn back from the bottle 

Pete

 

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The other thing Ive found is that if the system has a small leak due to say a hose clamp not fully tightened I dont seem to lose coolant from it (possibly its in the form of vapour when hot so I dont see it) but it somehow manages to draw air in through it when cooling. This means the coolant isnt drawn back from the overflow bottle and the radiator level is low when I remove the cap which in turn usually means the heater stops working.....

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Dont know the TR4 arrangement but the Vitesse/GT6s always fill themselves automatically to the brim of the radiator because any air is pushed out on heating up and coolant drawn back in from the bottle (as long as this is kept filled to the correct level of course) when cooling down. It has to be like this because the heater is actually the highest point in the system and so if the overflow pipe isnt always full of coolant theres something wrong....

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