I need to find out the water pressure of the sea water pump
on a 496 mag ho. After replacing the impeller and water pressure sensor before the ps cooler, it now only reads 8.5 psi at 5000 rpm. If I now opens the above water exhausts, the pressure drops to 4.7 psi and the power then gets limited by the Gaurdian. Please help as this is driving me mad and I can not enjoy the boat

Many 496 owners just throw a new impeller into the Merc sea water pump and still have pressure problems with the engine and associated overheating and Guardian alarms.
This usually occurs because the end plates of the pump body housing are scored with groves and even with a new impeller the pressure will not come back to stock adequate levels.
Your sea pump pressure at idle should be in the 3-7 psi range and should go up to 22-30 psi at 5000 rpms with the pump and the system working properly.
There are some kits on the market that add some stainless end plates and a special impeller in their kit that can many times save having to buy a whole new pump. Search threads here on OSO for 496 raw water pump rebuild kits.

Best Regards,
Ray @ Raylar

I just orderd 2 sea pump impellers for my 496 and the first thing the guy ask, is the back plate groved as they have fitted a few new impellers and the motors over heated at idle due to sucking in air ,you just have to have your back plate planned flat which i did myself useing hi speed polisher but with a sanding disc on it mine is brass not sure they are all the same?? good as new paul from England hope this helps