Seawater in closed coolant circuit. 2003 5.7 Carb Bravo

Hello,

The coolant overflow bottle on my 2003 5.7 carb bravo with a closed cooling system has been slowly filling with each heat cycle. Several times, ive set the coolant level at the cold mark, run the engine several times over a weekend, and after all has cooled down i have more coolant than i began with. When i check the heat exchanger for coolant it is full and under pressure. This must be seawater displacing coolant and mixing in the system. I don't have overheating issues, and the engine runs smoothly. I suspect an internal heat exchanger leak that somehow only leaks from the salt water side into the coolant, but not the other way around. I say this because i don't have overheating issues, the system holds pressure, and ive pressure tested the coolant circuit to 15-20 pounds. I have a dryjoint manifold exhaust system and wonder if that could be a leak path also. Has anyone had a similar experience? Thank you for your time.

Usually, the pressure in the antifreeze side is higher than the raw water side, so seawater into the antifreeze is unlikely... However, a "vacuum" leak may allow sea water into the antifreeze.... On average, the coolant level in the overflow bottle rarely nicely fluctuates between hot and cold levels and especially after the cap is removed from the heat exchanger and/or coolant added/removed the levels are... funky for a few cycles... That said, the best test ( after several heat/cool cycles) is if you remove the cap from the heat exchanger and
1) Observe the level... it should be up to the bottom of the flange on the radiator cap "riser".
2) Check the coolant in the exchanger with an antifreeze tester. If seawater is indeed intruding, you should see a degradation in the "protected to" temperature... if you have kept it at 50/50, it should be protected to the specified temperature for the antifreeze product you are using. One should note that BOTH higher and lower concentrations of straight antifreeze will RAISE the temperature you are protected to... Modern antifreeze products are "calibrated" to give the lowest temperature "protected to" at a 50/50 concentration (with tap water).

If in fact you are gaining sea water in the heat exchanger, places to look...
1) Gasket in the ends of the heat exchanger.... This often just a circle of neoprene, separates the coolant flow from the sea water flow. Could be a simple ( on some exchanger designs) as a loose bolt in the end cap... or... someone took cap off, replaced it and did not line up the depressions in the neoprene end gasket correctly.
2) Head gasket leak... compression test time.
3) Heat exchanger tube leak... possible but not likely unless engine has serious hours in seawater and raw water side never flushed/properly or correctly winterized.

Hello and thanks for reading Bob,

Yes, as you describe the coolant is diluted. It only tests to about +5 and would have been at -40 at the beginning of my boating season. The only fluid available to displace the glycol is seawater.

1) The gaskets at the ends of my heat exchanger only keep the raw water in, they don't separate raw water from coolant. If they leaked raw water would simply leak out into the bilge or air would be sucked into the raw water circuit.
2)If i understand it correctly, only coolant is circulated through my block, so a leaking head gasket would displace coolant with exhaust gases. The exhuast gases would likely gather in the top of the heat exchanger, but my exchanger is always full. I've tested my coolant for traces of combustion gas with a chemical tester and it was negative. The engine runs smoothly, and i wouldn't expect that with a blown gasket.
3)I have no idea how many hours the engine has as I'm not the original owner.

That leaves me with a heat exchanger leak by process of elimination. Does anyone know of any other locations seawater and coolant come in close proximity that a small vacuum on the coolant side as the engine cools would suck seawater into the coolant?

Pull the caps on the heat exchanger and pressure test the system. You will see what is leaking.

I pressure tested the system and couldn't get it to leak. Lacking a better idea I removed the heat exchanger and took it to a radiator shop that was able to pressure test from both coolant and saltwater sides. The exchanger would only leak from the saltwater into the coolant circuit. Because of this, they were unable to determine what tube was leaking and plug it. I'll be shopping for a new heat exchanger in the spring. Hope this helps someone else.