Water Pressure, A diffrent question....
 

I know that water pressure has been talked about tons of times but I have a twist on my question. So here goes I have high water pressure on a 1999 single step 35ft Fountain with stock 502 mpi's. I just had pressure relief valves installed on the motors. They are from Eddie marine and have check valves also. They are installed on the hosed b/4 the raw water pump. Here is the problem I ran the boat for the 1st time this week-end with they on and the never opened?? Any ideas as to why. I know folks out there have them so where did people install them in the cooling system???

Re: Water Pressure, A diffrent question....
 

I have mine after the water pump. It is made to open at about 25 psi. If you go before the pump you could have say 20psi there but you motor could see way more pressure then 25. You are wanting to limit the pressure at the motor. Mine only opens up when I get past 80 mph.

Re: Water Pressure, A diffrent question....
 

They have to be installed after the discharge of the raw water pump, preferrably after the oil cooler. The suction side of the raw water pump uses the heavy wall hose to prevent it from sucking in. This side of the pump most likely is in a slight vacuum. It may be in a positive pressure when the boat is under way, but I don't know. I would think that if it is in a positive pressure, it wouldn't be more than a few pounds of pressure.

Re: Water Pressure, A diffrent question....
 

What 88F said.
I would bet you are sucking air while running in that setup. Better change it

Re: Water Pressure, A diffrent question....
 

I will try that but why then is there still tons of water pressure with the current configuration????

Re: Water Pressure, A diffrent question....
 

My water pressure is measured at the block itself. If you have high pressure with a normal MPI setup then you more then likely have a blockage of water exiting the block to the exhaust or at the exhaust itself. I had a similar problem with one motor, a couple impeller teeth came off and were wedged in the thermostat housing on the exit side. My guess is you have a similar problem. Have you pulled your sea pumps apart and checked the impeller for missing parts? Mine runs 12-15 lbs at 3000 rpm, with the blockage my right motor burried the gauges at 2000 rpm.

Re: Water Pressure, A diffrent question....
 

Because where the relief valve is currently plumbed in, there isn't tons of water pressure. It's after the discharge of the pump.

Re: Water Pressure, A diffrent question....
 

Bob, you are getting high water pressure cause the sea water pump is making it. Your reliefe valve wont work if you have it mounted before the pump becuase its sucking water into itself. The only way it woudl ever open would be if you happen to have a water spike like if you caught some air and came down just right.

We actually tested the merc sea water pump here at work on our test tank. We have a 5000 gal tank and a bunch of cool electronics to see all sorts of parameters on a working pump......... We hooked the Merc pump up straight shaft through a collar to a 3hp elec motor. One of the first tests we did was to see how much power it would take to trun this pump over.....The 3hp couldnt do by itself without some help getting the pump turning....unfortunatly we did have any of our bigger pump motors avail at that time. Anyway at 3000rpm this pump is capable of pushing over 100' of head height!!! Now this is not real word testing as we dont have the water pressure force feeding the pump like a boat does when its up on plane. And i dont remember off hand how much water this thing can do but it floored me at that time (This was about 3yrs ago) Point being to all this is that the merc seawater pump can do HUGE volumes of water.....and if theres restriction in the system then it can build some big pressure as well! So its either plumb the relief valves betwen the engine and the pump or you can play with having some straight full time water dumps letting excess water psi out the boat.....just a matter of having an adjustable valve to open and close the amount of water or figure out the right size hose. :)

Re: Water Pressure, A diffrent question....
 

so we all agree that the have to be plumed after the raw water pump....because we are doing that now.....i will let you know.....BZ

Re: Water Pressure, A diffrent question....
 

BZ, it's also best to plumb the relief valve =after= all of the coolers, if possible, especially after the oil cooler. This way, the coolers will benefit from the added volume up to the relief valve.

Re: Water Pressure, A diffrent question....
 

Thanx for all of the input we moved them to after the raw water pump and b/4 all of the coolers and they worked great...same oil tem water temp and the poped off at 15psi and maxed out at around 19-20 psi hope this helps the motors over the long run......BZ

Re: Water Pressure, A diffrent question....
 

It doesn't matter where the relief valve is plumbed as long as it is after the seawater pump discharge and before the inlet to the engine.
If you read Wally's post it states that there is more water (volume) than could be imagined. Do not forget that you are controlling, PRESSURE. There is more than adequate volume to cool the oil and engine even after the relief valve opens. :cool: