The Hunt for a Boat Diesel Coolant Leak

A few posts down in this feed there's one called Give Me Pressure. It had to do testing for a leak between the raw-water side and the coolant side of a heat exchanger. The leak itself was one of those creeping, intermittent, insidious little sneaks that puts white steam and the sweet smell of coolant into the exhaust with no sign of external leaks on the engine, and no bad gaskets. 

The new heat exchanger (left), built from plans over 20 years old, had to be pulled back off the engine when the header/expansion tank (right, marked Nor'Pro) proved to be the leaky culprit. That tank has an upper chamber that holds the coolant reservoir; its bottom chamber opens to the exhaust ports. That space is supposed to be dry. It wasn't. There was rust. And two pinhole leaks. Let's not call the new heat exchanger unnecessary. Let's call it a proactive purchase. No hard feelings. We'll keep the old one as a spare.

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Ha (I laugh bitterly), if only I'd known when writing that post what I know now... that there was no breach in that heat-exchanger; that parts for old, off-brand, marinized diesels are not to be found on any old shelf; that by the end of summer I'd be as fast as a Le Mans crew chief swapping out major parts, knowing the peculiar recalcitrance of every bolt, every hose clamp... well, I guess I'm glad I didn't know.

The good folks at Sen-Dure in Fort Lauderdale made me a new heat exchanger from plans dating back to the last century that I think even they were surprised they still had. I installed it, started up the old Isuzu, and the leak continued. This was... let's say it was disappointing.

So, having eliminated as culprits...

• The coolant-side heat exchanger end gasket (replaced)
• The head gasket (pressure-tested)
• The heat-exchanger itself (replaced)
• The transmission oil-cooler (doesn't circulate coolant in this engine)

... I figured the only thing left to leak was the big header/expansion tank, which in this engine contains coolant in a chamber directly above a dry chamber that receives exhaust gases from the cylinders.

Good view of the port side of the engine, and a chance to bust some more rust, touch up the paint, check the water-pump impeller...

So, out came the new heat exchanger, off came the header tank, and in the lower chamber where there should have been carbon but no rust, there was rust. In the dark of night I stuck a LED gooseneck light down into the fill hole on top of the tank, peered into the exhaust-elbow opening... and lo, there was a pinhole of light. I rust-busted around it and took the tank to a talented local metal fabricator, who welded a small steel plate over the pinhole. This whole procedure happened twice, because another pinhole appeared after the second rust-busting.

There was a lot of rust and a couple of small leaks from the coolant reservoir into the chamber of the header tank where the exhaust comes to mix with seawater on its way to the discharge.

And finally (never say finally) the tank, which I tested with both water and air, was leak-free. I reinstalled everything, filled up with fresh coolant, took the boat on a couple of late-season runs to double-check everything, then topped up the tanks and changed the oil. Now the boat is hauled out. The next steps are to winterize this carefully coddled cooling system, cover the boat, and call it quits for the year.

Most of the surfaces cleaned up pretty well with solvents, 3M kitchen scrubbers, rags, and wire brushes. WD40 is a good, relatively cheap cleaner for jobs like this. It works especially well sprayed on the threads of studs and bolts that have a lot of grime in them.

The chances that more pinholes won't appear over the winter or next summer are slim. Maybe the tank will last one run, one week, another whole season? But there's no replacement tank out there. When it leaks again it will be time for major surgery. With luck the shell and the welded-on exhaust couplings will survive, but it will have to be cut open for new channels and chambers to be custom-made and installed. Which is why I'll be dreaming this winter of repowering, of sailing, of rowing, of swimming.

-- DL