Coolant mystery

[Top Shelf]

So last weekend I replace that "fuel return cooler" that was leaking coolant into the raw water side. Add the DD Pro 2000 stuff and water, burb the turbos. Run her at the dock for while;add a little more water to get between the "eyeballs" and shut her down......life is good again.

Out of town all week, go to the boat this AM and there's coolant under the engine. I pop the cap on the expansion tank and I'm sure a gallon or so of coolant came out under pressure. This is a cold engine that hadn't been run in a week mind you.....

Cleaned up the mess, topped off. Ran the boat all day and everything seems fine.

What the heck happend?

[Genesis]

That's odd.

I always start up after service to the cooling system with the cap OFF, so I can see if there was an airlock somewhere that cleared and top it off immediately.

Any time I do cooling system service I also pull a pressure test with the system EMPTY (air will leak MUCH more so than water does if there's a place for it to go); I pump it up to rated pressure (7 psi) and let it sit for at least a half-hour. If the needle moves something's wrong.

BTW the kind of thing you had happen can be a compression leak into the cooling system. Hope its not, as that's an ugly scenario. Take the cap off and run the engine for a while at the dock until it gets warm (150F is about as hot as it will get) - if you see bubbles in the tank you've got a problem.

[MikeP]

I want to be sure I understand...The engine was cold-hadn't been run at all for several days. You opened the expansion tank cap (with the engine COLD) and it blew coolant out of the filler at that moment? Is that correct? Had you cranked - not necessarily started - the cold engine at all just before you opened the cap?

As Genesis said, this is decidedly odd. In fact, until now, I would have said it is impossible. A head gasket blown to the coolant passages will pressurize the coolant system. But over a fairly short period of time the pressure buildup will leak right back into the cylinder it came from, and then past the rings or any open valves. In fact, all the coolant pressure would quickly take this route, and with it, some coolant into the cylinder.

It could have simply been overfilled but if the coolant was topped off when the engine was warm, it can't get "fuller" as the engine cools. The only thing I've seen close to this was an engine that had coolant topped off up to the filler opening when completely cold in the winter. It was untouched for several months and, when the temperature warmed from the 40's to the 70's, coolant welled up out of the filler when the cap was removed. But this doesn't seem to fit the conditions you describe, does it?

With the engine cold now after your recent run has it repeated the behavior? Is it still full of coolant? Try Genesis suggestion. If no bubbles are apparent, I'd run the boat locally for several trips, ensuring that you run the engines at varying power levels, including 5 minutes at full power and see what happens to the coolant level when cold. During this, pay close attention to cylinder head temps with an IR gun. Keep in mind that if there is a serious problem with the coolant system, and somehow all the coolant is blown out of the exhaust or whatever, the engine will sieze up before the temperature gauge will register the overheat. So rely on the cyl head temps. I assume you have two engines, take some cyl head temp readings on the "good" one and use them as a benchmark for the other. Don't expect them to be the same but they shouldn't be dramatically different. I don't know if the service manual lists cyl head temps or exh temps - my copy is down on the boat.

Something keeps nagging at me re the fuel cooler but it's vague at the moment. I'll keep thinking (probably nothing will happen).

[Genesis]

Yeah, I was going to recommend a pressure test but thought the better of it, because if there IS a leak into a cylinder and you pressurize the system with the engine off you'll hydrolock the affected cylinder(s) and trash the engine as soon as you hit the starter.

(That's one reason I do those "dry" after cooling system service. Air won't do any harm if it leaks somewhere.....)

BTW if you had a large "gusher" like that you HAD TO HAVE trapped gas somewhere in the system under pressure. The caps on Detroits are only 7psi and the only way you could displace any significant amount of liquid is if there is a large bubble of gas under pressure somewhere. Liquids do not compress, of course - only gasses. As such to get a huge mess like this you had to have air in the system somewhere that was able to expel the coolant, AND it had to be somewhere OTHER THAN in the expansion tank.

This should not be possible unless you have a compression leak, and the bad hole stopped with the piston up (e.g. that is, the air bubble it produced was in the block and was unable to circulate to the tank as the pump was not running anymore) The other possibility is an airlock somewhere that got locally quite hot, but normally if that's going on when it cools down the pressure will go away as the gas returns to its former volume.

This definitely bears close investigation because if you DO have a compression leak you need to fix it IMMEDIATELY or you will wind up either hydrolocking a cylinder - or worse.

CHT monitoring is critical while running as was noted - this is NOT instrumented on Detroits, so you need to use the IR gun. Once the temperature probe is uncovered (which will happen real fast if the coolant is being blown out of the engine at speed) it will not register the overheat-in-progress.

Has that engine ever been overheated to your knowledge?

[Top Shelf]

Well, just came back from the boat.....after the run yesterday, everything seems fine. Coolant level where it should be, bilge dry, no water in the oil or vis versa near as I can tell.

Yesterday when running the boat and after this little gusher, hit both motors with the IR Temp gun (water is So Fl is still a lttle cool) and 172 and 175 on the thermostat housings, usually in August they're 180 and 183 at cruise.

Didn't think about each head.....

Went to WOT for about 10 minutes....no problem.

Genisis, your thoughts on trapped or compressed gas....I jacked around with a radiator pressure tester several times before I could really get the dang thing to seal (stupid little black rubber ballon in the collar of the filler neck). It'd start to pressurize then it'd let go. Had to do this several times before I finally got it pressurized. I don't know maybe I pushed air all over the system....at least that's what I'll tell myself when I try to sleep tonight.

But to answer your other question...yes, this is the motor that overheated in the new river. Temp guage was at 210 when the alarm went off and I shut her down. I've probably got 50 miles of running on her since then, lights right away, passes your 10 second "smoke test", runs fine, exhaust looks same o same o, no water in oil.....

So I'm out of town until next Thursday night. Friday AM heading to Lucaya. 78KM's.....

This should probably be on another thread...but if I do have to shut her down...what is the real story on the Allison gears and pin wheeling props?
In other words if I have to limp back on one motor, can the shaft pin wheel on the dead motor or do I need to block it? Thx.

[Genesis]

If you're going a long way I'd lock the shaft.

I don't know if you need to, but I would. Being wrong means a rebuild on a gear and its a royal bitch to get those out and back in due to their weight and wet flywheel design.

[Top Shelf]

Thanks much...hoping for a nice "un-eventful" trip.........