How do I clean a sticky dirty heat exchanger on the coolant fresh water side?

No way core can be removed. Sen-Dure silver brazed it all together. It would have to be cut apart.
End caps can be removed, but all that does is expose the raw water side of things.

One end cap is bolted on with a center bolt, other is a thin copper cap lead soldered into the shell.
 
I just tried spraying oven cleaner in the outlet tube and it did nothing to dissolve it.
I rinsed with hose.
Then tried 90% rubbing alcohol and a toothbrush, did not seem to do anything either.

I am thinking of sealing the outlets and filling it gasoline and let it sit. Our gas is up to 10% ethanol.

If that does not work, will look into getting a strong engine coolant flush and will have to clean it when boat is back in the water.
 
Guys, the white vinegar is working.
I figure soak it for at least a day.
Stuck my finger in the fill, and rubbed my finger on the sidewall and it is dissolving slowly.

I wonder if you could do this another way, like flush out the antifreeze. Then refill system with vinegar and run it up to temp and let it soak a while.
That would clean the whole engine.
 
Zep Purple cleaner/degreaser is awesome stuff when used at high concentrations.
 
I am thinking it is less greasy and more like rust deposited in some kind of gooey matrix.
Whatever it is, it's an odd thing.
I would never have know it was in there unless I had taken it off the boat and looked inside that bottom exit tube.
I will leave it soak overnight.
Eventually if it does not come clean, I will try filling it with gasoline. Our gas has 10% alcohol, it is a good solvent.

I also like the idea of doing that coolant flush on a running engine using Irontite product.

Thoro-Flush — Irontite Products Inc.
 
The muck is probably through your engine's coolant galleries, but don't fill engine with anything but new clean coolant. Vinegar is acid, lye is caustic (base), both can ruin your water / coolant pump and seals. As montero says, brake cleaner may work, as may carby cleaner, both dissolve many substances but are usually highly flammable. If after your tests there is still some brown stuff remaining, don't worry about it; the cooler will be cleaner than it was, and it was initially causing no issues anyway. If really concerned, install an in line strainer in the coolant circuit, which is easily accessible and easily cleaned at sea. Then use your boat for having fun.
 
Sea squirt is correct, the gunk will be distributed throughout the system, so just cleaning the heat exchanger will not fix the problem.
Rydlime flush, follow the instructions carefully, and flush thoroughly after.
 
I may also try bleach. It takes a gallon to fill the heat exchanger.
Bleach dissolves some things.
So vinegar, gasoline, bleach in that order.
If I can't get it clean, taking it to rad shop is an option.
 
I put a spin on coolant filter on my Ram diesel truck. Makes me think could be an idea for the boat.
It only needs to go in a smaller coolant line, not needing to filter all at once like the large hose.
 
This am I dumped out the vinegar, it got dirty a little
upload_2025-7-16_9-23-37.png


Then I put in a gallon of gasoline E10
Here is my setup
upload_2025-7-16_9-25-8.png


I scraped off some brown gunk and put it in the gas, it dissolved and made funny looking loose particles that look like stop leak.
It may have gotten stop leak a long time ago. I have no memory of using any. I have had the boat since 1998. When I first got it, I had to silver braze 3 tubes shut as it leaked between the raw and coolant side.
The is an original to the part part, made in 1970.

upload_2025-7-16_9-26-53.png
 
Oh yes, the gasoline is very much cleaning off the coating. I put my finger in the opening and is all soft now wipes right off the metal.
Plan to soak a day, then flush with water and see how good it is.
It takes about a gallon to fill the whole thing to the filler opening.

Makes me wonder if the engine block is also coated, but you know maybe it can help with rust.

On my engines the exhaust manifolds and risers are FWC. The only raw water in the riser is in the tail end. But because engines pulse exhaust in waves and the cam profile, you always get some suck back when valves open between intake and exhaust, so it gets slightly wet with raw water on the inner surface of the riser.
 
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