Hi I've got a peugoet expert van which keeps chucking its coolant out the header tank cap.

I've had a look at it myself and changed the header tank and cap but it still doing the same.

I can leave the van to idle all day with the cap off and the temp never goes above half way if i put a weight on the accelerator still with no cap on then the temp will go up to around half way then the fans kick in and the temp goes back down and the coolant never moves.
Then i put the cap back on and drive it the temp gauge stays bellow half and the heaters are nice and hot.
Pull over have a look and coolant all over the header tank.

I've put a coolant pressure tester kit on and left it on for around half an hour and no loss of fluid.

Has anyone got any other ideas?

Cheers Bram