i am new to the boating world and i have a water injection valve,at idle there is no water going throw the headers and the motor temp is getting high. about 210 degrees.if i take the ball and spring out the temp stays low about 180 degrees. is there something wrong with the valve or am i doing something wrong? thanks for any info

i have a 1979 tahiti with a 351 windsor with a omc 800 outbaord. its got over transome exhaust headers.there is a hose that comes out of the thermost housing that goes down to the header t valve that has water coming out of it only as long as the ball is taken out. now i havent put it in the lake yet, its running in my front yard with a garden hose attached. does the valve only open at high throttle?