02 1.6 erratic coolant - marcopolo
I have a 2002 Fiesta Zetec-S and had the thermostat housing changed by a mechanic (mate of a mate) because of a coolant leak and I had no previous temperature issues.

The temperature seems to be going up and down like a yo-yo. It seems to be ok for the 1st part of the journey but as soon as I get to higher speeds and then reduce the temperature rises, one way to make it go down is to accelerate in a lower gear. If I rev out of gear it makes no difference.

I've done the water pump test buy putting heating on full but this made no difference

On one occasion I left the overflow cap off and this seemed to help the problem, but I'm wondering if this is an air pocket and how to sort?

Any advise would be most appreciated.