Mk5 reading over temp

[PeteD]

Need a little advice please

I have a problem which I believe to be electrical rather than mechanical.

Background - Leak from the water pump and tapping from the head. Head sent to be refurbished/skimmed /valves set.
Engine rebuilt with the fully reconditioned head with new head gasket, cam belt etc., new pump, thermostat, New condenser and radiator as originals had damage and a new expansion tank cap
Coolant system had no airlocks as far as we could check.

First drive saw the temperature gauge rise and the overtemp light illuminate. Rad inlet pipe hot outlet pipe cold. Suspect that the new thermostat was faulty and not opening, so 2nd new Thermostat fitted.

Same problem.

Connected OBDI unit.

Car runs on the drive fine revving to 3K ( 1 hour check). Temp in the normal on the cluster. OBDI reads up to 105 deg C fan cuts in and temp drops to 96. At idle temp rises to around 96 and drops to 91ish. Using a digital thermometer the readings from the Rad inlet and outlet pipes read around 6 deg C lower than the corresponding OBDI reading.

Driving, with the OBDI attached it indicates that the temperature rises to a high temperature (+120 Deg C) gauge reads high and light on. Checked the temperature of rad inlet and outlet pipes which read around 50 Deg C.

Suspect that the water system is functioning correctly and the indication is at fault.

No help from the Haynes manual.

Does have A/C. Heater matrix not changed but appears to be in good state with good flow.

Temp sensor on the head is my next thing to change but could it be something else in the electrical system??

Any help appreciated. Thanks

 

[LionsTooth]

"Driving, with the OBDI attached it indicates that the temperature rises to a high temperature (+120 Deg C) gauge reads high and light on. Checked the temperature of rad inlet and outlet pipes which read around 50 Deg C."

This seems like the temp sensor is broken. That is a big difference in temps. So logically changing the sensor seems like the next option. Could it be electrical not the sensor? If the wire is conducting electricity with little or no resistance then it must be the senor. Most of these older sensors are bi-metallic, so as they get hotter they send a varying signal to the gauge or computer. It could be the sensor just is not reacting anymore or is stuck or...? All of the is basic mechanical knowledge and may not apply to your Fiesta....I don't know your car at all.