Mariner 150HP Magnum III Impeller replacement

[Domeyhead]

Hi folks, first post from a newbie and novice boat owner
I have the Mariner III engine on a 6 meter RIB so there's plenty of power. However when I push up above 3.2k rpm for more than a minute the overheat alarm sounds so I figure it's time to look at the impeller.
WHen I look at the lower half of the Mercury Magnum engine to get to the pump I can see three forward facing bolts that are easy to remove but remioving these doesn't completely free the lower assembly.
However at the back of the engine directly above the prop is a pitted circular disk which I guess is the anode, and I think there must be a dourth bolt hidden somewhere behind the anode disk so I also need to remove it first. Question is, how is the anode plate secured and removed? Directly above the plate is an access hold protected by a plastic grommit plug which I can remove, then there is a void inside the casing of about 6 inches down to the other side of the anode plate I can't see what is securing it. Is there a screw, a nut? Is this the right way to remove it? Or have I got it all wrong?
All impeller change help welcome! THanks guys.

 

[j_martin]

Re: Mariner 150HP Magnum III Impeller replacement

Under the plastic cap is bolt, probably allen head that holds the zinc on. Under the zinc is another allen head bolt that goes up into the mid.

Forward from there are 2 nuts on studs that go upward into the mid. Remove them. Forward from there, between the ribs on the sides are 2 nuts on studs that go downward into the lower. Those have to be removed. Back them off to the end of the stud, and finish removing them after you get the LU to start dropping. At the very front is a nut on a stud going downward into the LU. 6 fasteners total hold the LU on.

You are setting yourself up for a fall if you're tackling this without the manual.

hope it helps
John
John.

 

[Domeyhead]

Re: Mariner 150HP Magnum III Impeller replacement

Under the plastic cap is bolt, probably allen head that holds the zinc on. Under the zinc is another allen head bolt that goes up into the mid.

Forward from there are 2 nuts on studs that go upward into the mid. Remove them. Forward from there, between the ribs on the sides are 2 nuts on studs that go downward into the lower. Those have to be removed. Back them off to the end of the stud, and finish removing them after you get the LU to start dropping. At the very front is a nut on a stud going downward into the LU. 6 fasteners total hold the LU on.

You are setting yourself up for a fall if you're tackling this without the manual.

hope it helps
John
John.

Hi John, thanks for the advice and I hear you. I looked out for the maintenance manual on the net but I have yet to find a site where I can download a decent manual ( I mean one that explains the process steps and tools needed as well as the parts assembly). I don't mind paying so long as it's like a "Mariner Magnum III for Dummies". Perhaps there's someone out there could write it. I have seen a coupla posts from people saying impeller changes are easy if you take care. Maybe not.

 

[ufm82]

Re: Mariner 150HP Magnum III Impeller replacement

Domey- if the engine maintains a normal temp at idle and at lower rpm's and then overheats at higher rpm/speed, it may not be the water pump. Water pressure at the water intakes should be sufficient to push water through the engine at speed. You may want to check the poppet valve while you are at it. This requires removing the lower cowling- screws hold the two halves together. Split the cowling and the poppet valve cover is on the lower part of the engine on the starboard side. Remove the cover and pull the valve assembly- check for crud that is causing the poppet to stick. Check in that cavity for debris/sand/sediment as well. If you've ever run through muck it can get stuck in there. The kit for that is cheap too. I have a Merc 150 and our engines are very similar. Been there, done that.

UFM82

 

[Domeyhead]

Re: Mariner 150HP Magnum III Impeller replacement

Domey- if the engine maintains a normal temp at idle and at lower rpm's and then overheats at higher rpm/speed, it may not be the water pump. Water pressure at the water intakes should be sufficient to push water through the engine at speed. You may want to check the poppet valve while you are at it. This requires removing the lower cowling- screws hold the two halves together. Split the cowling and the poppet valve cover is on the lower part of the engine on the starboard side. Remove the cover and pull the valve assembly- check for crud that is causing the poppet to stick. Check in that cavity for debris/sand/sediment as well. If you've ever run through muck it can get stuck in there. The kit for that is cheap too. I have a Merc 150 and our engines are very similar. Been there, done that.

UFM82

Thanks ufm that sounds like good advice. In fact I noticed that although the engine was steaming slightlly after the overheat the telltale was still running so you could well be right. I reckon the first time I get the poppet checked I might lean over the shoulder of someone who's done it before - and then I'll report back. Thanks again.