I had coolant problems a few weeks ago with my '00 DiscoII overheating. After the help of this group, I tracked it down to the coolant pump and so that has been fixed.

Now, most of the time it runs nicely at the normal operating temperature (according to the gauge). However, occasionally the temperature rises and then drops without overheating (no red light). This can happen around town as well as after a long hill. When the temperature goes up, I can see the water boiling in the coolant tank and a little steam coming off the tank. I've change the coolant tank cap, so don't think it is that. However, if it was a fault in the coolant tank itself I would expect to see it overheating all the time rather than intermittently.

I used an OEM coolant tank cap, so one idea I had was that perhaps it was not up to specification for the truck. This would cause the cap to 'let off pressure' at too low a rating? What pressure rating are official caps supposed to have?

 

Thermostat is sticking. Replace it.

 

Make sure your radiator is clean as well. No dirt clogging up the exterior
Core. Fan works... Fan shroud is there... That you put the belt on right and that your water pump is spinning in the right direction.

 

And that you bled the system of air properly.

 

Have you even checked the coolant level, when it is stone cold, where is the level?
You most likely have air in the system either because you still have a coolant leak or you didn't bleed the air from the system

 

check the fan clutch too.