Install freshwater cooling old older engine?

jumpjets

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My new 6.2L mercruiser engine has run a great season. I’ve logged 70 hours on it. About half of them have been In salt water. I’d like to install a freshwater cooling system on the boat so I can run in salt water worry free.

I’ve heard rumors though, that if I install a freshwater kit on a used engine which already has some corrosion in the water jackets, it’ll clog and ruin my cooling kit, and overheat the engine.

I just tore down a small block with 127 hours of salt use, and the water jackets are very rusty. About a quart of sand and rust dust poured out of the engine block onto the floor.
 

Scott Danforth

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those are not rumors, that is the truth

you will get sand out of every raw water cooled motor. fresh water or salt water. every time you drive thru water with sand in it or bury the outdrive, the sand goes thru the cooling system and settles in the block. if you drive past a school of minnows, at least one of them is going to get sucked up in the inlets. same goes for floating weeds, etc.

you could put oxalic acid in the cooling water jacket to loosten up the rust, pull the motor apart and mechanically clean many of the cooling passages (a 2' long section of 1/4" steel cable chucked into a drill motor works)

however if you just bolt on a HX, the dirt and rust will plug the HX on the tube side (the side you cant clean)

you can try to put a strainer between the thermostat housing and the HX. I did that when I rebuilt my motor because my heads came off a raw water cooled boat.

I plugged the strainer screen every use for the first 10 times I ran the motor, and my heads were not only soaked in oxalic acid, they were baked at the machine shop when I had the machine work done. that means that every time I went out, 10 minutes later, I was anchored to let the motor cool, and then 2 hours later, pulling the cooling system apart to clean the strainer.

so your choice is to pull your motor apart to clean it, install a HX or leave it as is.

BTW, mercruiser wants about $3200 for the HX for the 6.2 liter (not including brackets, etc). that is a significant bit more than what they pay for it.
 

jumpjets

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Ok. I guess I’ll just run the dog **** out of this motor, and continue to flush it after every use.

I'm gonna build a freshwater cooled 383 for fun, and drop it in when the time is right.

I’m m curious if the saltwater block I just pulled apart can be cleaned up. Off to the machine shop. It looks horrible and rusty of the inside.
 

Lou C

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I’ve had some long term experience with running a raw water cooled cast iron engine in salt and this was moored all summer not flushed till the end. Winterized and back filled with the best PG marine antifreeze with corrosion inhibitors each fall. After about 15 years use this way the cooling passages in the cyl heads get eroded and are getting thin. However the blocks at least here rarely rust through. In 2017 I replaced the cyl heads on my old engine due to leaking head gaskets. The machine shop checked the heads and found cracks (previous overheat) and eroded cooling passages. So I replaced them with reman heads and just kept using it. The block has about 20 years of salt water use. Despite this it does not overheat; runs at 160 with a 160 stat. So I think you will get good life out of your engine leaving it as it is, just make sure to check the exhaust system at 4/5 years if running in salt. Even the Merc dry joint system can rot thru and put water in the engine. Save the $ you would have spent on the heat exchanger for timely exhaust replacements.....
 

Scott Danforth

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Ok. I guess I’ll just run the dog **** out of this motor, and continue to flush it after every use.

I'm gonna build a freshwater cooled 383 for fun, and drop it in when the time is right.

I’m m curious if the saltwater block I just pulled apart can be cleaned up. Off to the machine shop. It looks horrible and rusty of the inside.

when you do that, PM me and I will sketch up a proper cooling system for you and not the hyper complex mercruiser one with 15 hoses, however a proper 6 hose setup

i have 4 hoses in the raw water line from the drive to the HX (drive to transom shield/PS cooler. PS cooler to oil cooler. oil cooler to raw water pump, and raw water pump to HX

2 large hoses for the engine to HX and 2 discharge hoses from the HX to exhaust on my HX cooled big block using the exact HX for a mercruiser 6.2 with a modified elbow and modified expansion tank.

take a look at this thread on options of building a 6.2. In the end, my buddy was given a good deal on a new 6.2 bobtail

https://forums.iboats.com/forum/eng...utdrives/10786828-2004-mercruiser-6-2-repower
 

jumpjets

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Scott, will PM you for sure. Thanks! I’m pretty blown away by the amount of cheap, lightly used SBC and Merc parts in my area. I think I’ll build a semi-outrageous 383 with freshwater cooling over the next several months.
 

QBhoy

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Hi.

id think that it wouldn’t be financially viable to do this really.
mad someone on here once told me years ago...I’m not aware of any raw water GM marine engine like this, that’s ever had the block rust right through. He was likely right.
as for the risers and manifolds..they are consumable parts, either way you go.
but most importantly (and maybe controversial of all)...I truly believe that most of the internal corrosion that takes place...is due to the way the engine is laid up over winter...especially a sea water boat. In terms of flushing the engine and then leaving or not leaving it full of a liquid medium (antifreeze) to stop cold corrosion over that period. Some will argue..but it’s my strong opinion on the matter.

All the best.
 

Lou C

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I'm gonna agree with Qbhoy above about filling with good antifreeze. That's what I've done and even before I had to change the heads/head gaskets, my engine did not overheat. That's the concern with internal corrosion, eventually you get so much of it that it insulates the cast iron from the cooling water and you can get chronic overheating. Even then with the original heads before the HGs leaked it was not overheating at all. After I changed them it ran just a bit cooler with the new heads. A couple of mechanics I know were not big believers in closed cooling because they had never seen a block rust through. If I repowered with a new or reman engine I'd for sure add it because I can't stand the idea of running salt through a new engine and it makes winterizing much easier. Closed cooling and remote oil filter mount to save my back not having to remove the seats etc for winterizing.
 

Lou C

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PS I have heard of cyl heads rusting thru esp right behind the valve seats. So installing reman heads as I did at the 15 year point might well extend the life of the rest of the engine.
 

Scott Danforth

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Scott, will PM you for sure. Thanks! I’m pretty blown away by the amount of cheap, lightly used SBC and Merc parts in my area. I think I’ll build a semi-outrageous 383 with freshwater cooling over the next several months.

cant get too outrageous with a wet-exhaust marine motor running a stock drive. most your going to make is about 450hp at the crank

watch CR
get proper cylinders for the vortec heads
watch cam profile.
add a carb spacer to increase plenum volume
 
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