Loosing Coolant 1.8T

[shavermcspud]

Hi all, had this car for 4 weeks now, all has been well up until a week ago when the temp gauge went right over to 120, stopped and no water in the expansion tank.
I have since filled it up but every 50-60 miles its empty again, its not leaking from anywhere visible as if i leave over the weekend it still has the same coolant level, only when driven does it loose coolant.

When i purchased the car i did the usual checks and there is no visable signs of head gasket failure, oil does not look contaminated.

Im not really sure of the next stage, its an old beast 187k but fsh on time every time and in superb condition.

Any ideas appeciated, cylinder compression test is ok on all 4.

[shavermcspud]

I have just noticed that the thread on the expansion tank cap is totally shafted and doesnt seal properly, only one ring of thread left.
Is a complete seal of this system i.e pressure required, water isnt comming out but im guessing without pressure the boiling point is lowered?

[iwaters]

Yes replace the cap, the system is pressurised to about 1.5 Bar so if that cap isn't sealed you will loose a lot. It only costs about £3 for a new one from ECP. Is the expansion tank threads ok?

[yaman]

I have just noticed that the thread on the expansion tank cap is totally shafted and doesnt seal properly

These caps do give trouble, only a few pounds to buy new.

Regards
Jim