Why does my discovery radiator boil after 10 miles?have replaced water pump ,thermostat,andhad rad flushed .visco fan is working as is electric fan.car runs heads have beentested

check thermostat in boiling water make sure it opens. make sure it is not fitted up side down, check the radiator pressure with a gauge there may be a leaking head gasket. also check the relief valve on the radiator cap.

How did you check thermostat? Did you check coolant level. Is the auxiliary water pump leakinh

There a several reasons why an engine over heats; Low coolant level in the radiator; stuck thermostat, bad water pump, cooling fans that are not working properly. clogged radiator. All of these possible faults need to eliminated one by one.

Step_1 disconnect inlet (top hose) & outlet bottom) hose. I'm assuming you're replacing a corroded radiator. They now offer copper or plastic radiators that have high resistance to corrosion. If you replace with alluminum or steel radiator, do a flush job yearly with replace with the green radiiator solution or the red one if you replaced with aluminum radiator.
Step_2 disconnect the electric fan. first unhook the wire connection, then loosen the 4 screws that's holding the fan assembly against the radiator.
Step_3 Loosen the bolts holding the radiator unit. When re-attaching the new radiiator, install new hoses as well. Fill the radiator through the pressure relief cap with 60-40 radiator to water mixture. The instructions say 50-50, but 60-40 is a stronger solution that prevents freezing, boiling, & corrosion buildup. It there's a coolant filter, replace this unit as well. It looks like an oil filter cartridge.
Preventive miantainance: Perform a coolant flush every 50K miles. Most car manufacturers recommend 30K miles or 2 years, which ever comes first, but that's to boo$t revenue for car dealership service centers. IMO the max is 70K or 2 years. When you notice the temperature is overheating in the summer months, an engine radiator flush AND replacing the thermostat is the solution. Most shops will say "you need a new water pump" and the scammer had not even performed a diagnostic test. The water pump will last the life of the engine (140K+ miles) unless the car had run dry without radiator fluid. The radiator fluid lubricates the water pump. Running the engine dry (or low of) without radiator fluid damages the water pump. Running tap water instead of 50-50 or 40-60 radiator fluid mixture will also cause premature wear on the water pump because water by itself is insufficient lubricant on the water pump.

sounds like head gasket, shes putting pressure out into your rad, check your big hose when idling see if they swell up with pressure,another way to check is to leave rad cap loose.

is your water pump circulating the water nissans used to have a problem with impellers on pump
leave rad cap off and run enginge should see water circulating when stat opens

Might be blown head gasket. Is there oil in the coolant or coolant in the oil? Either of these would tell you that.
Also the exhust can get into the coolant and make the coolant low all the time.
I had two Chrysler products that the exhaust forced coolant out and had a bad head gasket.

Remove and raise upper hose at thermostat end and remove lower rad hose.
Fill rad with a hose and watch for flow from bottom rad hose and/ or bottom rad hose connecton.......water should fall though rad easily.
Honda rads are cheap.....any restriction....replace rad.

Remove thermostat....fill thermostat housing with hose and watch for water flow at bottom rad hose.
Flush good.

Remove heater hoses.......flush back and forth until a good flow is present "With heater control set to hot" if no or poor flow....replace heater core.

Stop leak can plug everything.

Also.......not likely causing over heating....but ....at your mileage,if you do not know if the timing belt has been changed.....CHANGE IT NOW!
Water pump is optional at this mileage.

When it gets hot feel the 2 pipes comming out the radiator.if both are hot then the visco fan is not working.if only one is hot and other warm then the thermostat or water pump is not working

i agree water pump , but thermostat can go bad very fast some times they will have a defect when u buy them for that you can do a simple test ,,,put the thermostat in boiling water if it expands that means its working