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Discussion Starter #1
hi all, I have suspicions I might need a replacement yellow coolant tank cap. after a while of driving I lifted up the bonnet and could hear this hissing and it was coming from the top of the expansion tank. The system seems to be pressurising OK because i very gingerly opened the cap and the pressure released. can anyone back up my suspicions please, i have been having to top up a lot lately - maybe the coolant is evaporating through the cap?
 

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Never open the cap on a hot engine. The system is pressurised to raise the boiling point of the coolant. If you release that pressure on a hot engine then the coolant can instantly boil which will cause it to erupt out of the expansion tank faster than you can react and screw the cap back again.

If the cap is worn it can leak.

But one thing to bear in mind is that (and this is worst case scenario - I'm not saying it is the case) if the headgasket is leaking between a combustion chamber and a coolant passageway then the cylinder will pressurise the cooling system above its usual level. And that can cause gases to escape from the cap.

If you are losing significant amounts of coolant you may want to get your coolant tested for signs of combustion gases. Or take your spark plugs out - if one looks a different colour from the rest (eg very clean) that's a worrying sign.
 

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Discussion Starter #3
Never open the cap on a hot engine. The system is pressurised to raise the boiling point of the coolant. If you release that pressure on a hot engine then the coolant can instantly boil which will cause it to erupt out of the expansion tank faster than you can react and screw the cap back again.

If the cap is worn it can leak.

But one thing to bear in mind is that (and this is worst case scenario - I'm not saying it is the case) if the headgasket is leaking between a combustion chamber and a coolant passageway then the cylinder will pressurise the cooling system above its usual level. And that can cause gases to escape from the cap.

If you are losing significant amounts of coolant you may want to get your coolant tested for signs of combustion gases. Or take your spark plugs out - if one looks a different colour from the rest (eg very clean) that's a worrying sign.
thanks for the advice, I never remove the cap in one go I always release a little air then tighten up then release a bit more till is safe to take off the cap fully - as a matter of fact as I was opening up I personally felt that there should have been more pressure in the bottle, I reckon I am loosing pressure somewhere and the temp gauge doesnt get much off cold, am hoping its not HG but will whip out all the plugs today and compare. cheers.
 

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I used to undo cap a little at a time, until the pressure was so high, it smashed cap past the threads, and left my hand and face badly burnt... Luckily it healed.

Was cap already a little loose? Just buy new cap and find out?
 

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ive noticed some pressure in the system its only when it gets up to full running temperature that it will nearly come out the bottle when undoing it rest of the time its fine undoing it while the engine is running

hissing does sound like its leaking get another from scrappy should solve it
 

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should use a thick rag over the cap and stand as far away as possible if you need to open it. check hoses on top and the one underneath too. make sure they have a tight fit, may have come loose or got brittle and cracked?
 

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Discussion Starter #7
I used to undo cap a little at a time, until the pressure was so high, it smashed cap past the threads, and left my hand and face badly burnt... Luckily it healed.
wow thats real bad news - yeah for the sake of a couple of quid i am gonna get one of ebay and see how it goes
 
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