Firstly, these coolant systems are crap to gauge if its low on level.
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What you'll probably find is its probably not lost any on the filler neck, but the rad is partly empty.
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crap I know.
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so when you check, do a good few bleeding exercises first.
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ie.
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Cap off.
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goto the front and bleed off the radiator.
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coolant level should drop.
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Tell tale signs of engine integrity problems, is carry out all the bleeding correctly first, leave the heater pointing to hot and take it for an inspired drive.
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the early signs of overheating is the heater will go cold.
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when it does this, it means there is no coolant circulating around the block.
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so get ready to ditch the car on the side of the road.
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Then its a process of elemination
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Check the caps seating, the rad cap is a two way valve.
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blowing out is not a problem, but its sucking it back in when it cools down.
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the system creates a vacuum.
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if the cap doesn't seal properly on the filler neck, this wont happen, so its constantly pushing the coolant out, but not refilling the system.
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Check the filler neck is not corroded excessively.
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take the cap off and look down at where the cap should seat.
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