High Water Pressure Merc 700 SCI
 

Helping a buddy troubleshoot his high water pressure issue, the boat has low water pickups and staggered Mercury 700 SCI’s. It has Hardin two stage sea water pumps, boat pumped water on the hose with no issue. Backed it in at the ramp and it wouldn’t pump any water. Pulled it out and pulled the pumps apart. Both impellers were fine and no missing vanes, replaced both impellers with new and put back in the water and pumped fine. Took the boat out for a spin and now it has high water pressure, both mechanical gauge and water pressure sensor readings are the same. About 45 mph the water pressure is pushing 35 psi and have read up to 45 psi on smartcraft. Checked the relief valve in the strainer and the dump lines, all is clear and relief valve appears to be working. Boat ran flawless in the past, any ideas?

Did this happen when water pumps were changed to the harden 2 stage? Also does the boat have any of the whipple tunes?

The Hardin 2 stage has been on the boat for some years with no issue, it has a Whipple stage 2 kit I think. It has stainless marine strainers with the relief valve built into the lid, he spoke to them yesterday and they recommend using a newer design lid they offer. Will see how that goes!

Thanks!

Have you tried back flushing from the headers? Cap stbd side at jumper outlet with a bar stool rubber foot or something similar, pump in from port header jumper outlet and watch what comes out the sea pump hose, disconnected from sea pump obviously.

Hey Jim, are the low water pickups for the intercooler ? how is the rest of it plumbed..........NXT drives ? or

Thanks! I’ll pass that on.


Sorry, I meant to say transom pickups. It goes from the transom to the sea strainer, strainer has a relief valve dumping out transom. Strainer feeds the two stage pump, pump is a single inlet/single outlet pump. Thinking the relief valve might be the problem.
Thanks!

Ok, my 2 stage pumps have a -20 inlet from the strainer then a -16 to the engine / headers and a -12 to the intercooler............why is there only 1 outlet on the two stage pump ? agree the valve might be the issue if all the other line are clear

I had to do some research and came up with this from Hardin Marines website (below). So, he might have found the issue! Looking at the adjustable water pickups, he noticed the motor giving issue was 3/8” lower than the other pickup. It had the original install mark, moved it back to that and now there both equal. Waiting on gaskets for coolers and he’s going to try it out. He pulled everything apart with no blockages found.


“One of many of Hardin Marines latest cooling innovations in a new two stage sea pump for the popular Mercury Racing HP525, 600SCI, 662SCI, and 700SCI engines. Engine builders have been pleading for a solution to the lack of water that the factory sea pump delivers and Hardin Marine rose to the occasion with the development of this product. Finally an answer to failed headers from water starvation and detonating engines from under supplied intercoolers.

This pump is designed to retrofit to all Mercury Generation 5/6 engines equipped with a sea pump mounted fuel pump. This pump accepts the OEM factory mechanical fuel pumps that are integrated components in the OEM fuel system and provide additional priming support to the factory EFI system. On the supercharged engines this unit is perfect to solve the cooling system short comings of the factory single stage sea water pump system that splits the single water supply to feed the factory intercooler.”