Merc 350 MPI low raw water psi and cool engine temps

[Leardriver]

I am trying to figure out a weird cooling problem. 2004 350 MPI, ow 011398, bravo three drive ow 222650. The boat found some sand in shallow water, and the smartcraft gauges showed raw water psi at .5 idle, 5.2 cruise rpm, which was previously 7 psi idle, 9.8 cruise.

So, I figured that it sucked up some sand, degrading the performance of the impeller.

But... the engine now registers temps of 135-140, down from 165 previously. I once again was trying to guess why it was running cooler. Chunk of impeller or a rock wedged in the thermostat, blocking it open? Yes, the cooling water pressure was less, but the engine ran cooler, not hotter.

I replaced the impeller. It was all there, and in pretty good shape, but the raw water pump is a drag to get out, and a new impeller is never the wrong thing to start with.

I replaced the thermostat and gaskets, even though the old one boil tested perfectly at 160 degrees.

The cooling system looked perfect, and I ran hose water through the hose to the outdrive pickup, and full water pressure came out. I was testing for an obstruction.

Now, the water psi on the gauge shows 0. I tried it on the hose, dragged it to the lake to drown it in water, and no reading at all. I shut it down after 10 seconds, but I fear that I have to take it all apart again, which is a pain in this heat.

Any ideas? Pressure indicating problem? Where is the pressure sensor located?

The raw water pump had plenty of resistance when I spun it by hand, just like a new impeller in the housing should be. The two raw water hoses went back on like they came off. I didn't get them mixed up. I took pics before and after.

Update-took it apart again. It's amazing how the first time takes 2 hours, and the second time took 17 minutes. The impeller and pump internal housing looks brand new, just like I left it. Some water in there. I flushed the red top hose from the raw water pump again. It is a gusher out the pickup on the outdrive. Great flow. I hooked the ears up to the outdrive, a gusher inside the boat now. I think the water intake path is unobstructed.

Why no psi reading from the inlet water pressure on the gauges?

 

[Don S]

Re: Merc 350 MPI low raw water psi and cool engine temps

You probably have a thermostat that is now stuck open (gritty sand is like that).
I would bet a new impeller and thermostat and you will be back where you were temp and pressure wise.
Might want to flush the block and manifolds out too while you're at it.

 

[Leardriver]

Re: Merc 350 MPI low raw water psi and cool engine temps

I thought the same thing. I replaced the thermostat as a precaution, though. It was sterile clean in there. I have flushed every hose I could find, and they were all clean looking.

Where does the bottom hose on the raw water pump go? The smaller hose, not the top red one that picks up lake water.

Where does the raw water pressure get measured, so that it can send that info to the smartcraft gauges?

Thanks!

 

[Don S]

Re: Merc 350 MPI low raw water psi and cool engine temps

What drain system do you have on your engine? single point, 3 point, 7 point, air actuated, manual ??????

 

[Leardriver]

Re: Merc 350 MPI low raw water psi and cool engine temps

It has the 4 blue plug manual drain.

The red hose on the back of the raw water pump was on the top hose fitting, but I saw a diagram that showed the sea water inlet coming off of the bottom hose fitting. I put it back like I found it, red on top. Hopefully the diagram was wrong.

 

[Don S]

Re: Merc 350 MPI low raw water psi and cool engine temps

Cool, Merc shows a 3 and a 7 point, no 4 point drain.

Not sure about the hose color, but the intake hose on that pump is the upper hose, the outlet from the pump is the lower hose. The sender for the water pressure is in the power steering cooler.

 

[Leardriver]

Re: Merc 350 MPI low raw water psi and cool engine temps

Good news.

There are four blue plugs. I have seen the Merc descriptions also, and they don't make sense to me. Much of the manifolds drain into a central drain bulging unit on the port front. Maybe there is another name for that system?

Any idea where the cooler is? I am starting to suspect that the cooling system is perfectly functioning, and the sender has a bad connection.

 

[Don S]

Re: Merc 350 MPI low raw water psi and cool engine temps

Let me look and see if I can find a cooling system diagram to fit your engine.

 

[Leardriver]

Re: Merc 350 MPI low raw water psi and cool engine temps

You are more awesome than people will admit.

 

[Don S]

Re: Merc 350 MPI low raw water psi and cool engine temps

Much of the manifolds drain into a central drain bulging unit on the port front

That is called a distribution housing, there should be a blue plug in the bottom of it, and 2 blue plugs on the raw water pump. Where is the 4th blue plug?

The power steering cooler should be mounted on the back of the engine. Follow the power steering hoses, the return line should lead to it.

 

[Leardriver]

Re: Merc 350 MPI low raw water psi and cool engine temps

The fourth plug is on the front port side of the thermostat housing.

I will crawl back in. Access can be a little snug, as you know.

Thanks a bunch for your help.

 

[Don S]

Re: Merc 350 MPI low raw water psi and cool engine temps

Thermostat housing ?????????????????? Ok, must be an air vent to assist in draining. Never heard of one there before.

Is your blue plug located where one of the brass plugs shown here (item #2) is located? If so, it's just an air vent, not a drain and probably explains why they say 3 point and not 4.



Here is what I assume is your cooling system.

View attachment 350 mpi cooling.pdf

 

[Leardriver]

Re: Merc 350 MPI low raw water psi and cool engine temps

Ok, I found the power steering cooler, pulled both hoses off of each end, flushed it with water in the cooler and both hose ends. The electrical connector, which I assume is where the raw water psi gets its reading from, looks fine. I unplugged it, inspected, etc, and plugged it back in. Tight fit laying on the engine.

I reinstalled the sea water pump and put the front of the engine brackets back on, serpentine belt, etc. Started it on the muffs, and still no water pressure reading. Grrr!.

Is there a hose I can disconnect from the thermostat housing to check for water flow even though I am getting a zero reading? Maybe by the rarest of coincidences, the sensor failed, undisturbed, as I was correcting a deficiency in that system. I know, unlikely.

That blue plug on the thermo housing is exactly what I am referring to. Just a vent, huh? It looked identical to the other blue plugs.

 

[Don S]

Re: Merc 350 MPI low raw water psi and cool engine temps

Do you have the hoses on the raw water pump right?? Did you have them backwards at one time?

 

[Leardriver]

Re: Merc 350 MPI low raw water psi and cool engine temps

They have always been on correctly. Red on top, which I traced to where it comes out of the outdrive. I back flushed it with the hose, and water poured out of the outdrive pickups on the outside.

It looks like I can disconnect the starboard power steering cooler hose again, start the motor on the muffs, and watch for flow from the raw water pump. If I am getting flow, I have a bad sensing or indication problem, not an actual water flow problem. Solid logic?

 

[Don S]

Re: Merc 350 MPI low raw water psi and cool engine temps

Well, if your going to test it, might as well do it right. DO NOT try to do the test on muffs, it must be in the water.

View attachment Bravo pump test.pdf

 

[Leardriver]

Re: Merc 350 MPI low raw water psi and cool engine temps

I just tried what I was inquiring about.

Disconnected the hose at the far side of the PS cooler, turned on the water to the muffs. No flow. Started the engine, and even at idle the raw water pump is pumping huge amounts. So, an indicating problem? What a weird time for it to pop up. I can't believe what I am seeing.
Is there a test of the sensor that screws into the PS cooler?

I just saw your test instructions. I have determined that I have big flow from the raw water pump. That is good. What I don't know is exactly where it is measured, so I can fix my indicating problem.