Problem which needs sorting if anyone can help pls?

Right i was driving my 89 gti cabby the other day when i notice a funny smell coming from under the bonnet. when i got home i lifted the bonnet to find water had forced its way out of the expansion tank.

Straight away i thought it was the head gasket but there was no oil in the water and no water in the oil so i took it to the garage.

They have done a compression test and said it was fine, it started on 3 cylinders but then the other kicked in. They also had it running all day and it never overheated and the fan kicked in and out when it was needed.

has anyone got any ideas what this could be, my thoughts was the thermostat but im not sure.

any help would be much appreciated.

cheers

Faulty cap , or incorrect type .