New to Merc 600sci, all guages are perfect and motors are running excellent unitl 4600rpm or higher. This is when the water pressure drops from 25-30psi down to 15psi or under and the guardian kicks in right away for "Low Block Pressure". I have back flushed the intercooler, oil, fuel, and powersteering coolers, and water seems to be flowing very good. Also new impellers/housings have been installed. Boat is an OL Quattro w/ XRs and IMCO sportmaster lowers (2 inch shorty). Thoughts????

guardian thinks 15 psi is too low ? the pump volume vs rpm is essentially linear. double the revs and you double the flow rate... it isn't precisely but close enough for this discussion... the point is that , since the water exit is the same, then there should be no reason at all for the pressure to drop unless :

a) the water supply is disappearing like the supply hose is being sucked closed or is restricted or the sand filters are clogged

b) the inlets on the hull / drives are occluded

c) somebody did something wrong when they rebuilt your pumps and the impellors are wrong or something else bizzare.

realistically the only thing that would cause that is lack of inlet water flow at that rpm...

I agree Stevesxm. The motors are absolutely "dead on", & the IMCO's are suppose to produce more water psi than the merc sportmasters. The IMCOs are -2 so the X dimension is quite high on the boat, & I have heard of these issues with other 600sci owners who are cooling their engines & intercoolers thru the Bravo setup. At this point I am thinking of going to Hardin external water pickups for the motors for extra/steady water pressure, and cooling the intercoolers thru the Bravos and adding a strainer/sand filter for the intercoolers. Other than that I have no idea where to go next!?!?!

Have you tryd lowering the drive all the way back down, Once you loose the water PSI to see if you could get the PSI back up some as a test? Could be getting air bubbles around the water pick up's right at that speed. Have you added any thing new to the bottom of the boat inline with the water pick up area?

Stepped hulls aireate the water at the hull interface that can cause this condition with sea water pickups at that depth. Drive water pickups are usually below that level. Are yours located below that level at that speed?

Nothing new to the boat, I have even added a 3/8" hole to the nose of the imco speedmaster lower to gain water pressure with no success. Will test today with drive lowered.

The Quattro does not have external pickups for the motors or intercoolers. The drives are a 2" shorty, and I would say the drive shaft is level at number 5/6 on my indicator. Boat does not like a whole lot of trim above that mark, so yes it could be an "aireation" issue. Is their is a fix for that condition?

Most likely you will need to switch to pick-ups at the transom or thru the bottom. OL should be able to recommend the best pick-up point. This might be best as a good winter project because it could be difficult to rig with the engines installed. I really like my thru the bottom pick-ups...might also give you an extra 1-2 MPH. It is also very easy to adjust you water pressure with thru the bottom PU's.

If the stepped hull is causing aireated sea water at the water pick up depth you are running resulting in your low sea water pressure problem the only solution is to relocate the sea water pickup(s) to a location where the water is not aireated. Easy to say but....... Have you discussed your problem with the Outer Limits technical people. They should know what will and will not work with their boats.

X 2 they should know what to do, And if they do not know then at least you are giving them some new info they may have have missed in there testing and hopefully they will help you look more in to it with you, So they are ready for the next boat they build. Good luck with it, I hope you get it running right.