Focus 1.8 TDCI overheating

[Taki-Juve]

i have recently bought a focus 1.8 TDCI (year 2007). everything in the cooling system seems to be fine : coolant liquid is not leaking (or maybe leaks just a little), thermostat is fine, water pump seems to be working well too, fan is working. the car when in slow traffic doesn't overheats, but when go over 70 km/h (4th or especially 5th gear) it overheats 1 or 2 mins later, same happens when I'm going uphill.

i don't know if this is related but i have a small turbo charge leakage (can hear it hissing).

the mechanic that checked it had no idea what the problem could be, he suggested i change the temperature sensor and to clean the radiator, i did but no change.

if anyone has another suggestion that would be helpful and thanks.

[unofix]

5 minutes ago, Taki-Juve said:

the mechanic that checked it had no idea what the problem could be, he suggested i change the temperature sensor

You are correct the mechanic has no idea. There is nothing wrong with the temperature sensor.

What is the mileage on the car ?

Does it seem to use much oil ?

[Taki-Juve]

the mileage is 350000km, the engine has been overhauled newly and seems to be fine.

one mechanic just told me the radiator belongs to focus 1.6 petrol and its original radiator should be thicker, it's not 100% the cause but it can be.

[unofix]

So in UK mileage your engine has done 217479 Miles

I surprised it's still running so well.

2 hours ago, Taki-Juve said:

the engine has been overhauled newly and seems to be fine.

What work has been done ?

[Tizer]

If it is the wrong Radiator then that could be the problem. I have had to change a Radiator for one from a bigger engined car in an old high mileage car that I once owned because it was just not good enough any more. 

That engine had been Re-bored and had different Pistons fitted as well giving it a higher compression ratio.

[Taki-Juve]

16 hours ago, unofix said:

So in UK mileage your engine has done 217479 Miles

I surprised it's still running so well.

What work has been done ?

everything, crankshaft, pistons, piston cylinders...etc