Hi all,
Uno 45 MK1 1986 with 999engine.
Changed the heater matrix to new one 2 weeks ago.
All worked fine until 2 days ago when I noticed the temp is 1 line above half after 20 min drive.

Head was rebuild 1 year ago, I changed everything, radiator, thermostat and new water pump since then.
After changing the heater matrix, I bleed all the system, opened the bleeding caps and filled the system until no bubbles.

All the hoses are hot at the same way when the car is running.
The weird thing is, when the car reaches to half temp, fan starts at exactly half and car is not overheating. the overheating occurs only after 25-30min driving.

I changed today both coolant and fan switch sensors but it's the same.
Fan looks to be working ok, goes on and off every 1-2 min or so.

NOTE
Coolant is not boiling.

Even ( almost) new thermostats can fail...

I changed the thermostat yesterday and gave the radiator a wash, temp is still the same.
I checked the the radiator with heat gun and it goes up to 87-90c then fan kicks in.
Hoses are hot and does not pass 90c.

Waterpump OK ?

Peter Pick-Up said:

Waterpump OK ?

Click to expand...

I changed it a while ago, it doesn't leak.
I think it's ok since I know if it wasn't ok, when hitting the gas pedal the temp would drop down but it's not.

You changed the fan switch sensor, but what about the temp. sensor on or near the thermostat ( if I remember correctly...)

Peter Pick-Up said:

You changed the fan switch sensor, but what about the temp. sensor on or near the thermostat ( if I remember correctly...)

Click to expand...

There's a red sensor on the head, I changed it as well.

Sometimes the propeller inside the waterpump comes loose from the shaft, that might be a possibility as well.
Or maybe the gauge itself is faulty...??

Hi there
I had exactly the same symptoms as yours and had tested and tries there was to try. Until with the help of this forum that realised that I had put the coolant straight without diluting it. So the radiator couldn't get rid of the heat. Could you have done the mistake I have done too?

50% dilution, coolant-destilled water indeed.

I am using 4L Paraflu which is already diluted.

From cold,have you checked what temp the thermostat opens,with your heat gun. My cheap thermostat opened after the cooling fan cut in,new thermostat then cured it.