"If you are not getting water
"If you are not getting water from the plug, it's not comming out of the pump. Your maybe not going to like this, but take the lower off and stick it in a tub of water. Spin it with a big drill. I used some leather to protect the splines. If it won't come out of the pump, then the key to the impellor has vanished. If it does pump, put it back on and remove the exhaust side plate. Put the baffel back on (hold to sun light to see pin holes)and spin again. If you get water there, you probably have a blown side gasket or head gasket keeping the water from getting past the water jacket. If no water at the exhaust cover, pull the power head and find the blockage from the water tube to the exhaust jacket.
one final test is to spin the drive shaft while the powerhead is off. You should get water everywhere so put the drill in a rain coat and watch the plug. (might find a ground fault outlet too!)"