" top rad has and radiator are cold - bottom os warm. "


Three year old Nissans radiator with the metal or the plastic side tanks?

If you used 50/50 premix, there may you may not have an issue with the radiator.

If you mixed antifreeze with water, you use at least distilled or best, deionized water.

If you used tap water, the minerals precipitate out of the coolant clogging the radiator and heater core from the lowest level horizontal tube upward.

Your location and water treatment to make potable water will have an effect on the rate of clogging the fluid to air heat exchangers radiator, heater core).

If an all-metal radiator, you can use a radiator service to 'boil out' the precipitate inside the radiator. With the radiator safely drained and recycled, remove it and use sun or a spot light and peer into the large bottom radiator hose port. The horizontal tube ends will have an off-white material clogging the end.

Ensure a new thermostat is installed in correct orientation. Wahler (Borg Warner) Vernet make quanily thermostats.

On the Volvo OEM water pump gasket I use silicone grease and thoroughly baste the gasket so it darkens and swells and slides as you lever up the water pump to compress the top seal. Also inspect the steel tube end for the coolant return for corrosion or may not seal well on the small ring gasket.

Verify all coolant carrying hoses are recent. Unsure how old they can be.

As for coolant, maybe 24o can use organic acid technology, as Arty B. mentioned before. Else, phosphorus-free green prestone, the usual collection of quality blue coolant (Xerex G-05, I forget now.). I use Walmart distilled water and a gallon of antifreeze. Pour in all antifreeze and fill with distilled or (more $$) deionized water.

Bear in the mind the trick with the 240 water pump where you lever up the water pump to compress the top sealing ring or gasket. Use Volvo OEM gaskets. If the block surface is without pitting or other sealing issue causing defects

Inspect the thermostat housing interior. That housing is an aluminium alloy that is the least noble allor in the system so when coolant protection fails the metal inside that house cavitates to form at least pits and worse. Next is the cylinder head water jacket interior.

So, in the FAQ, among the best resources for 700-900 owners anywhere, and replicated on other sites, has information you can use. Same engine under that hood as is on 900-700.

https://www.brickboard.com/FAQ/700-900/Cooling.htm

https://www.brickboard.com/FAQ/700-900/WaterPumpChange.htm

And I hope your brake fluid reservoir is not back and is clear or honey color.
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