expansion tank boiling after use

[ANDYMC_TQS]

hi iv searched for hours to find a similar prolem but havent found one yet.
basically whenever i use my car it drives fine at the temp guage sits nice and steady at 90 as it should and never passes that but when i turn the car off i can hear the water in the expansion tank boiling like a kettle and its obviously very hot there too. i checked the thermostat and that is working perfectly so not the problem,
i checked the water pump and although it was in perfectly good order i changed it to a new one with a metal impellar.
i also checked the rad and big hoses for blocks by running a hose through thm but good flow.
i did find a load of browm sludge in the expansion tank so cleaned that thoroughly

but once i re-assembled the car the promblem still remains
does anyone have any ideas they would be greatly apreciated
andy

 

[s3dave]

Maybe a faulty seal around the expansion tank?

 

[ANDYMC_TQS]

i did find half an oring in the tank actually the size of a 10p but i dont know where its come from or where the other half is

 

[colicabcadam]

change the lid of the coolant chamber, chances are it's lost its air tight seal - no pressure = lower boiling point

with an air tight system, i believe the boiling point of water / coolant is something like 125c

 

[ANDYMC_TQS]

ok thank you i will give it a go

 

[colicabcadam]

they are only like £5 too

 

[Smokey_clark]

change the lid of the coolant chamber, chances are it's lost its air tight seal - no pressure = lower boiling point

with an air tight system, i believe the boiling point of water / coolant is something like 125c

What he said..