Cummins 6BT losing coolant at rest

Hi all.
Doing a routine check on my engines about 6 weeks ago and I noticed the plastic coolant reservoir was higher than usual. When I removed the radiator cap I was down about 4 litres of coolant. The coolant was real murky but when it sat for a while a rust sediment settled leaving clear red coolant on top. I git. Mechanic to come and pressure test it at rest and with the engine running and he couldn’t find any problems. I topped it up with water and gad to go away fir a month,
I went down to day to drain flush and replace the coolant and found it had lost another 3 litres of coolant while I gad been away.
There are no apparent leaks in the bilge or on engine and such large volumes of coolant loss should be detectable I would think.
The oil looks good and level has not risen.
I started the engine before I left and it ran fine sbut will bump it with the stop solenoid engaged to ensure It will turn over.
I disconnected the exhaust elbow from the turbo and it seemed dry.
My thinking at present is if it’s not in the bilge and it’s not in the engine and it’s not leaked out the dry part if the exhaust then it must be draining into the heat exchanger and into the raw water side. The header rank fir the coolant is higher than anything else.
It may also be drawing some saltwater back into the coolant system when it cools causing the rusting,
Appreciate any advice and suggestions.
Thanks?

I would join the Seaboard marine forums and ask there https://www.sbmar.com/community/

Check water flow coming out the exhaust too see if you have reduced flow. Also, some B engines have vent lines going to the expansion tank. Check to see if you have them and they are clear. They vent trapped gas pockets to the expansion tank.

Ted

Next time you leave the boat, take either the zinc out of the heat exchanger or the sea water inlet hose (aft, toward the bottom) and leave HX sea water side opened up while gone. Put a container under the open port to catch any dribbles. See if any coolant shows up. Might have a pinhole in a tube in HX.

Of course close sea cock and hide key for safety.

And yes if there is a leak in HX, likely some sea water in there so coolant is now corrosive. If coolant looks bad, this is a hint against HX as well. You want to do a bunch of flushes after any repair to get the salt and gunk out. Fortunately the B is pretty tolerant of crappy coolant so I would not freak out over this.

Any past history of overheats? Any crap built up around pressure cap seals? Could point to head gasket or exh manifold leaks.

Do you have a hot water heater connected to that engine?

What year is your 6bt?

i will drain the raw water from the heat exchanger today. There has been that much coolant loss into the HX then I think I should be able to see it.will leave a bucket under to collect any further leakage and see what we find.
Thanks for all the good advice?

Went to drain the raw water side of the Hx today and found it had lost litres more coolant. Opened the raw water inlet and out poured pink water. So it’s losing coolant through the HX fast, must have a big leak. Does this mean a new HX or are there seals inside that may have failed or repairable leaks?
Many thanks for all the help.
???

Probably a new heat exchanger. The separation between raw water and fresh water are all soldered connections. A good radiator shop can test it and find the leaks. If the heat exchanger is leaking, I'd rather have the peace of mind with a new one.

Ted

Yep, if one tube is leaking it can be plugged, but likely another not far behind. Replace whole thing.

Over zealous in place acid cleaning maybe corroding out non compatible solders?