Strange Overheating Issues

[gman83]

Hi all, sorry if this has already been addressed elsewhere, but I can't seem to find a solution.

I drive a 2009 Ford Fiesta 1.6 Ambiente.

If I put the car into diagnostic mode and monitor the temperature then idling and driving around small streets the temp never rises above 239°F (115° C). The car only seems to overheat when driving at highway speeds (75 - 90 MPH) then it raises and lowers to the highest 239°F (115° C) and the lowest of 221°F (105° C).

Here are some symptoms that I have noticed, and what I have done to try and resolve the overheating issue in general. So far nothing has worked.

Symptom: Heater Doesn't get very hot.
Tried flushing the heater core - I figured that this might be related to the car overheating, like maybe there is a blockage in there? Some gunk came out but it didn't make much of a difference to the heater or the engine temps.
Symptom: When the Engine Starts overheating, the air conditioner gets warm.
I drained the whole cooling system and flushed it out, and replaced the coolant.

The actual overheating lamp only seems to come on when the engine is extremely hot, but it disappears quite quickly.

Does anyone know what could be causing this?

 

[scotman]

Given the age of the car, I would have to look very closely at the radiator being plugged up as a probable cause. Overheating at higher speeds denote a flow issue.

 

[gman83]

Thanks Scotman, that makes sense. I have also been hearing a water pump squeak on its bearing, so the water flow diagnosis seems to fit. I wonder if I should just change out both at the same time and see if that helps. I'm also way over due on a cam belt replacement as it has never been done before as far as I know. Which means I need a tensioner and all the cam locking tools as well.
Eeeek