Nissan Repair: Pick Up over heating, engine temp, running the water

Question
Dear Von Sudderth,
The problem I have you may not as yet come across.
I live in a climate where our average all year climate is from 0 deg.c to approx. 35 deg.c. Our Winters are rainy & cool. But, the rest of the seasons it is hot,dry or moist & warm.
The problem is that in Spring & Summer,when I use my Nissan 1800,1 ton Pick-Up inland, regardless of the weight load, the engine temp.gauge shows running hot to very hot, if I travel between speeds of 80km to 100km.
However, if I travel between 60km to 80km the engine tem.gauge runs at "normal" needle pointing to centre of gauge towards "hot".
Is this normal? Someone told me that I should turn up the cabin heater and open wide all windows, that the heater will in effect have the cooling effect on the hot running engine. Is that fact or fiction. (Are the just joking?)
What would you do or recommend?
Thanks.

Answer
Lin,

Yes, the heater trick will work as it is like adding another small radiator to the cooling process.  It sounds like your timing is too far advanced and when you operate at higher speeds the engine gets hot.  Other options here are a stuck thermostat and the engine is running the water through the radiator so fast that it does not have time to cool off when the vehicle is at the faster speed.  I would check timing first and then check the pressure cap, thermostat and radiator for proper function.  If you have not done a coolant changout in two years I would change the fluid to a fifty/fifty mix of dionized water and antifreeze to boot.