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Mercruiser 350 mag strange overheating issue.

N

nigb

New member
After a while idling my 350 mag mpi it starts overheating. Revving engine higher in neutral stops alarm & drops temperature down. What i have noticed is if i raise the leg up when it is idling temperature drops right down & doesn't overheat. It is as though having the leg down restricts the water flow. Leg last serviced about 18 months ago. Boat moored in seawater & we do have a problem in the area with small barnacles.
 
Back at the outdrive (Leg), with engine and water off (out of water on trailer).

Trim drive up and look on passanger side of drive, you will see a water hose about 1" in diameter.

This hose transfers water from impeller in drive thru transom to engine.

With the drive up, it should be a nice smooth curve in the hose.

Now trim the drive down and look at hose (you may have to turn drive all the way towards drivers side of boat)
Check to see if hose is kinked.
If hose is kinked/bent and not smooth in shape then that is your issue.
If the hose was replaced when maintenance was done they may have used a non OEM hose or possibly installed incorrectly.

If Non Oem hose it may not be as thick (re-enforced) as a OEM hose. And has now weakened and is collapsing when drive is trimmed down.
 
kghost said:
Back at the outdrive (Leg), with engine and water off (out of water on trailer).

Trim drive up and look on passanger side of drive, you will see a water hose about 1" in diameter.

This hose transfers water from impeller in drive thru transom to engine.

With the drive up, it should be a nice smooth curve in the hose.

Now trim the drive down and look at hose (you may have to turn drive all the way towards drivers side of boat)
Check to see if hose is kinked.
If hose is kinked/bent and not smooth in shape then that is your issue.
If the hose was replaced when maintenance was done they may have used a non OEM hose or possibly installed incorrectly.

If Non Oem hose it may not be as thick (re-enforced) as a OEM hose. And has now weakened and is collapsing when drive is trimmed down.
Click to expand...
Thanks for reply & I was thinking along same lines but wasn't sure of the location of hose & its shape. Unfortunately boat is on a mooring & i have no trailer. Transom was fitted new in 2021 all oem & drive was serviced by same mechanic in 2023. I assume he would have had to disconnect hose to drive before removing drive. My son suggested turning drive fully left & right whilst idling to see if that makes a difference. Based on that & what you have suggested I will try that tomorrow.
 
If your boat is in Salt water maybe there is build up like barnicles or is it possible ghe water level at moring is low and you may be in mud or grassy area?
 
So glad you replied to my post & confirmed what I was thinking but was not exactly sure. Got under boat today with a mask & I couldn't believe what I saw. The seawater pickup hose had come off the drive! In the down position hose was being crushed & also was being sucked onto external parts of drive further restricting flow. In the drive up position the hose was uncrushed & open which is why it wasn't overheating in that position. Can only assume my mechanic didn't tighten properly.
My next question is how hard is it to get hose back on & orientation of drive? I presume with drive up & over to right. Looks too hard to do underwater on mooring? Where we live is tidal & there are sand & mudflats when tide is out so I could beach the boat.
 
Just been having a look online re this hose. I assumed it was held on by a hose clamp but from what I can ascertain the hose is attached by a special insert that has to have drive off to do. Is this correct?
 
First,

What year is your boat/outdrive?
Is it an Alpha drive or a Bravo?
Does your engine have a watee pump driven by a belt? (Typically located at the lower drivers side of engine) (typically this would mean you have a Bravo drive)
Or does the incoming water hose go to the thermostat housing ( this would mean you have a Alpha Drive)
 
Servicing of the drive does not mean the water hose was touched unless it was replaced. Outdrive service is, gear oil, grease and gimbal bearing check and over all visual inspection.

If you had a repower in 2021 I would expect all hoses and bellows were new at that time. The losseness of a hose clamp/connection after 4 years may not be the mechanics fualt.
 
kghost said:
First,

What year is your boat/outdrive?
Is it an Alpha drive or a Bravo?
Does your engine have a watee pump driven by a belt? (Typically located at the lower drivers side of engine) (typically this would mean you have a Bravo drive)
Or does the incoming water hose go to the thermostat housing ( this would mean you have a Alpha Drive)
Click to expand...
Bravo 3 2006 belt driven pump. Yes I realised later not mechanics fault. I can't believe hose installed at factory has come off after a couple of years.
 
The Bravo hose connection at the transom is NOT the same as the Alpha.

In my opinion it is a very bad design.

The transom pass through for the water is suceptable to corrosion and it is the kind that grows and squeezes the hose which restricts water flow.
 
I personally do not have direct experience with your year transom assembly so I am unsure how the hose is attached to the transom side or the Gimbal Housing side. I would imagine the Gimbal housing side uses a hose clamp, the transom side I am not sure if it uses a hose clamp or the horrible, Pinch fitting that is exclusive to Bravo....

Either way, The boat most likely will need to be pulled and have all that inspected and corrected.
 
kghost said:
I personally do not have direct experience with your year transom assembly so I am unsure how the hose is attached to the transom side or the Gimbal Housing side. I would imagine the Gimbal housing side uses a hose clamp, the transom side I am not sure if it uses a hose clamp or the horrible, Pinch fitting that is exclusive to Bravo....

Either way, The boat most likely will need to be pulled and have all that inspected and corrected.
Click to expand...
Yes I think it is the pinch fitting from what I can see online. Leg is coming up for a service anyway. Problem is now it has started overheating under power getting on to plane. I might have to travel a bit to a mercruiser mechanic. I was wondering as a temporary fix I could connect sea pump to the sea cock in the hull that is currently connected to the pump for the toilet flush? Not sure if bore size of sea cock is big enough for flow required.
 
The toilet seacock will be OK except to caution you about a couple points. First, that the toilet seacock inlet isn't just as fouled as the other. That its flow is equal to the one in the drive leg. That there is a strainer to keep any sort of large particles (marine growth, etc) out of the seawater supply. Lastly that the hose you attach to it is wire wound so it won't collapse under suction. On my big block with B3, the inlet hose on the leg is, in my opinion, too small. I have a 1.25 (bronze) seacock feeding a sea strainer that then goes to the seawater pump on the engine.
 
Ok thanks with heads up re the hose & strainer. I can get 32mm suction hose easily & strainers readily available. Appreciate the help from you guys I will let you know how I go.
 
Update boat is currently out of water getting drive service, new hose, antifoul & other work done. Will report back when it is in the water again.
 
Got boat back & apparently the hose was connected to drive with a conventional hose clamp. Now new hose retained with newer internal clamp. Boat running fine at normal temp.
 
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