2001 500efi water temps on test drive was around 175 -185 on just 3500 rpm, and at idle ,manuel said 143 thermastats ???

thats about what my 7.4 mpi runs at, the thermostat rating is just when it opens to let water flow (after the motor warms up)

The 500 EFI normal operating temp. is 160 degree. not 170+. If you are operating above the 165 you need to check and replace the raw water pump. If there are any parts of the impeller missing, diconnect the cooling line going to the oil cooler at the rear of the engine and remove the impeller parts.

According to my gauges my 500EFI's run anywhere between 140-155 a little higher when running hard and the lake is warm.
Steve