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7.4 Mercruiser freshwater conversion or not?

J

jtsailjt

New member
I've made an offer on a boat with a '92-'96 vintage 7.4L Mercruiser with Bravo 3 that's raw water cooled. The engine looks clean and brand new with no signs of rust anywhere (on the exterior) so I assumed it had recently been replaced but I checked the serial numbers and it's the same age as the '94 boat. Current owner has only had it for a couple years and is not very knowledgeable but says the guy he bought it from kept it on a lake and he's only used it in a lake as well. I plan to use the boat in both fresh and salt water.
So, my question is, should I convert it to a closed system or, since it's so old, should I just continue with raw water and then plan to re-engine when corrosion starts to cause problems? A local mechanic gave me a price quote for the conversion of $3750 using Mercruiser parts. I understand that if it's been run in salt water there's no sense in doing the conversion but if I pull off some hoses and find no signs of corrosion, is it worthwhile to spend that much money on an old, probably rebuilt or remanufactured engine? Is my price quote reasonable? Is there another good quality conversion kit that costs less and that I can install myself? About what would a whole new, closed system engine cost? I'm new to I/O world but pretty accustomed to being a do it yourselfer to maintain the diesel engine on my 47' sailboat so feel comfortable doing mechanical work if it's not too technical.
I'm brand new to this site and to I/O's so will appreciate any guidance some of you more experienced folks can provide!
 
Regardless of being run in salt water if that is where this boat with a long history of use in fresh water will be used on a regular basis, $4000.00 to convert to Closed coolong should be done. If not a new 7.4 replacement could cost $10,000.00 + to replace.

One suggestion, even converted to closed cooling the Elbows will still have raw water flowing thru them to keep exhaust cool.

If you plan to keep this boat for a long time, consider purchasing stainless steel elbows. It will add a bit to your cost but in the long run you will not have a corrosion failure point.


My opinion is any boat to be used primarily in salt water must be closed cooling.

Also look into crankshaft driven incoming water pump vs the current mercruiser belt driven one.
 
kghost said:
Regardless of being run in salt water if that is where this boat with a long history of use in fresh water will be used on a regular basis, $4000.00 to convert to Closed coolong should be done. If not a new 7.4 replacement could cost $10,000.00 + to replace.

One suggestion, even converted to closed cooling the Elbows will still have raw water flowing thru them to keep exhaust cool.

If you plan to keep this boat for a long time, consider purchasing stainless steel elbows. It will add a bit to your cost but in the long run you will not have a corrosion failure point.


My opinion is any boat to be used primarily in salt water must be closed cooling.

Also look into crankshaft driven incoming water pump vs the current mercruiser belt driven one.
Click to expand...

The Mercruiser belt driven setup has the pump installed under the engine oil pan.... On some boats, access to it requires pulling the engine. At best, it's a stretch.... pun intended.
 
sandkicker said:
The Mercruiser belt driven setup has the pump installed under the engine oil pan.... On some boats, access to it requires pulling the engine. At best, it's a stretch.... pun intended.
Click to expand...
??? On the engine in question, the seawater pump is on the side of the engine and incorporates the fuel lift pump.
 
It is a good idea but before going forward I would remove the thermostat housing and look inside the intake manifold & see what it looks like. Just because it’s been used in fresh water doesn’t mean there will be no corrosion; some fresh water can be quite corrosive as seen by what happened to some fresh water used B-3 drives before Merc improved them. If there is just light surface corrosion then yes fine but if there is a lot of flaking corrosion I’d think twice about it because that can clog up the heat exchanger. I do agree 100% with closed cooling on any inboard cast iron engine used in salt water. If I repower mine for sure that’s what I’d do. But do some investigating before spending that much $$$.
 
Thanks, yesterday I did as you suggest and took a close up picture of the surface rust that I sent to my mechanic, who I trust very much. He said it looked awesome for its age so I'm going ahead with the purchase and plan to pick up the boat tomorrow. The boat is a Formula 252 bowrider and I'm pretty excited to first get this conversion done and then having a few areas of upholstery that have come unstitched re-covered. Im in Maine so plenty of time to get it all done before I'll want to be on the water, but I'm sure it's going to be a difficult wait. It's going to be a fun boat!

Thanks to all who contributed to this thread!
 
You'll be fine.
On any new to me boat I do a complete base line rehab. That is, new rubber everything: hoses belts gaskets bellows impellers motor mounts, thermostat, blah blah blah. New tune up parts: plugs wires cap rotor (points condenser if equipped). I undo all the wire connections, clean, reattach and grease (dielectric or axle, depending). Oil, filter. Water sep filter. Clear out the fuel supply and new alcohol resistant supply hose.
This way I can log what's been done and know when it needs to be done again.
 
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