Wifes uncle had a Fiesta fitted with the same OHV engine as the KA. Ran sweet enough for years then it started drinking fuel like it was free and stank like a 60's Anglia at the back end. All gauges read normal, it started fine and ran well until it had done about 10 miles then it rapidly went down hill. Local garage checked the obvious with the exhaust probe and as expected the readings were off the scale for CO's and HC's, no way it should have even have been on the road. So their young mechanic whipped the connector off the ECU temp sensor and the car ran perfectly. His explanation was the ECU was getting a reading that indicated that the engine was overheating and to prevent piston damage the ECU overfueled to cool them down. From cold the readings were probably in range. By disconnecting the sensor it reverted to a default figure that was within range and all was OK. He later fitted a new sensor simply because in winter you need some enrichment on a cold morning.

Coolant level should make no difference unless its either empty or blowing out of the expansion.

I'd be more concerned about where the coolant is going, if its ending up in the inlet or combustion chamber(s) then wetting the plugs at certain times, then that could explain poor performance, but whatever is causing the loss of coolant isn't likely to fix itself.

Thanks.

It seems to be a relatively piddling amount - perhaps a cup a week... certainly much less than I've been used to losing from a variety of s***heaps since my first Anglia more than 40 years ago. ;-)

It's been happening so long, and without other signs of a head gasket or other failure that my view has been 'leaky hose'. Having sent it for check and repair a week or two ago, the water pump was replaced, since which he yesterday added probably just a cup and is once again concerned. (I told him it's probably just settling and losing air - but I don't pretend to be an expert and may simply be talking nonsense.)