I changed the inpeller, housing, and end plate. Running 5 Ibs pressure at 3000rpm and steam is coming out of the exhuast. I was running 15 to 20 pound before. Engine is not running Hot.
Any Ideas? or what to look for?

Were there chunks missing from the old impeller. If so you need to find them.

Impeller was 2 season old but all there.

Sounds like you may be sucking air. The merc pump housings are crap. I usually spend 1/2 an hour smoothing out the casting marks, and of course porting it.

I was in a similar sitch. Low pressure, lots of water out the pipes, not getting hot. Ended up being sand, or small rocks blocking the sender at the block.

If i understood you said, steam but not water, humm? Maybe the system needs to be back flushed, coolers possibly blocked up. usually in that sitch they need to removed and cleaned thoroughly. Ive had that problem also. After that, I installed strainers.

I will back flush but did not have the proplem before rebuilding sea pump, Sucking Air, would'nt the gasket take care of that. Suggestions on porting. I have the old style pump with the 90 degree outlets. Any one ever got bad parts from Mercury like housing spec to loose. I only changed one wear plate would changing the other one help?