350 MAG MPI sterndrive, low raw water pressure, does it need an extra through hull?

[c1steve]

I installed a moderately used 350 MAG MPI Horizon in my sterndrive boat. The Guardian system is triggered due to low water pressure, and I verified this with a manual pressure gauge. I am only getting 3 psi at all speeds, despite a new impeller.

The prior owner of the engine had a thru-hull going to a tee, as well as the water pickup in the Bravo 2 outdrive. However I do not know if he used a block off plate or other arrangement. Is this a common or acceptable arrangement for the raw water supply?

One slight complication is that my new MCM engine is bolted to a Volvo SX outdrive, but I doubt that this is an issue.

 

[hall832]

Does your water pressure senor mount in the back off the motor in your oil cooler? I had a similar issue with a 2013 with twin Mercruiser 350 Mags,

 

[Bt Doctur]

what drive is on your boat?

 

[NHGuy]

It's in th first post. He's got a bit of a frankendrive going on.

 

[alldodge]

The prior owner of the engine had a thru-hull going to a tee, as well as the water pickup in the Bravo 2 outdrive ... bolted to a Volvo SX outdrive
and only getting 3 psi at all speeds

You have a bit of a mix bag, and the tee from the drive and thru-hull is new but should not be an issue with a Horizon motor using a belt driven water pump. I would take the water pump off and inspect it.

What is you engine serial number?

 

[Bondo]

You have a bit of a mix bag, and the tee from the drive and thru-hull is new but should not be an issue

Ayuh,..... I like one, or the other,.... If I put in a thru-hull, the drive gets capped,.....

Mongrel or not,.....

 

[c1steve]

Sensor is on the power steering cooler, rear of the engine. Serial number OM 675521. Old owner had a 3/4" tee and seacock, but I am waiting to hear from him about the details. Manual gauge reads 3 psi, except it jumps to 6 psi for a second when throttle is increased. I suspect the pump is sucking hard, then the intake hose compresses and supplies a slight amount of additional water.

eThe salt water pump flange shows a fair amount of wear, but original owner had no problems. I suspect that the Volvo SX restricts the water flow more than the Mercruiser Bravo 2 drive that was original.​

 

[c1steve]

The serial number is OM 755521. The old owner had a 3/4 seacock and tee arrangement, but I am waiting to hear back on the details. I did open up

 

[c1steve]

The iboat forum keeps losing my typed post, so excuse the disjointed reply. The sensor is on the power steering cooler, rear of the engine. The salt water pump flange has a fair amount of wear. Old owner had no problem, so I doubt this is the cause.

The manual gauge shows 3 psi, but will momentarily show 6 psi when throttle is increased. I suspect this means it is sucking hard and temporarily uses the saltwater in the intake hose, until the hose partially collapses. Right now I am thinking that I should observe the intake hose and check for collapsing during running.

 

[alldodge]

The sensor maybe what is causing your issue but I'm not ready to say it would be a hose collapsing. All hoses after the pump is under pressure and the ones leading to the pump should be hard wall.

 

[c1steve]

Friday morning I am going up to the boat, install the Tee, and retest the water pressure. As I tested the pressure with an independent gauge, and the scan tool says the Guardian mode is coming on due to low water pressure, the solution should be in this area. I plan to run at the dock and test pressure, and if the tee works I will then install a high speed type of thru-hull so water will be picked up when planning.

If the water pressure does not go up to 10 psi or so, I will have to try another saltwater pump.
​

 

[c1steve]

I rigged a temporary tee and the gauge showed 5-10 psi at medium rpm. On Sunday I went fishing, running at 2,400 rpm. On the way back I looked the engine over while underway. A big surprise awaited, the salt water intake hose was fully collapsed for the last few inches before the salt water pump. Evidently the pump is sucking hard, but definitely needs more supply.

I also heard back from the original owner of the engine. He told me that he always used the outdrive leg as well as a seacock. One day he forgot to open the seacock, and underway the alarm came on. Later this week I will install a larger thru-hull and permanent tee. ​

 

[alldodge]

the salt water intake hose was fully collapsed for the last few inches before the salt water pump

Sounds to me you have the wrong hose. The intake hose is hard walled hose and cannot be collapsed