Locating a Coolant Leak on Universal M25-XP Diesel Engine

 A Drip is still a Leak!

We keep a doggy pad under the engine to use an early indicator of any leaks, they fit nicely beneath the engine between the engine support stringers. Earlier this year the pad was wet, looking around the engine I noticed that the Heat Exchanger was showing signs of Corrosion around the hose connection from the Engine Coolant Pump. Checking my Service history ( there's a link near the top of this page ) shows that I serviced the Heat Exchanger in 2022. At that time I took it apart, cleaned everything and put it all back together. It lasted another 4 years.

So, I replaced the heat exchanger, and, of course, while at it, I replaced all of the hoses for the Coolant system, sure looks pretty now. BUT the leak, a persistent, barely a drip, but it's a Leak!

After a few days, there is a visible drip forming on the Port Rear Underside of the Oil pan, not easy to see but with my head on the cabin soul it's visible. Cleaned it up, wiped down as much of the engine as I could access, more Paper towel balls pushed into places where you might expect a leak. That included all of the hose connections, around the filler cap ( Radiator Cap ) below the coolant pump, around the bottom of the new heat exchanger.

Finally, we found that the source was somewhere higher up than the oil pan. Of course there is no coolant in the Oil pan, but finding trace markings where something was leaving a mark was a step in the right direction.

Using a Endoscope ( Boroscope ) connected to a mobile phone, we took dozens of pictures, there really was nothing to see but it did look like the Freeze Plugs were all ok, well, that's 'all' that we could see! There's one hidden behind the Injection pump and another behind the Starter, I think there is also one behind the Oil Filter.  Anyway, nothing to help pinpoint the leaky leaky.

While doing all of this, I recognized that the Thermostat Housing was looking rough, paint flaking off of it, and excessive dirt around the gasket between the Housing and the Mounting.  So, just a bit of Project Creep, I removed the Thermostat housing. We last changed out the Thermostat in 2016, it's due.

Removing the Thermostat housing also exposed the Gasket, not in good shape! ?A trip to Southern Power in Coconut Creek to get new Thermostat and Gasket nearly failed! I have purchased several engine related parts from them in the past, their staff were extremely helpful, I even promote them in an earlier blog post. BUT things have changed. They sold their Kabota supplies and focused on Phasor generators. No more Kabota support :(   However, the rep did go back and found both the Thermostat and the Gasket.

Back home I stripped down the Thermostat Housing, removed the Temperature Sensor, Temperature Switch and the Air Bleed valve ( that has never worked ). I removed all of the old flaky paint, polished the interface between the housing and the mounting. Cleaned the Air Bleed Valve, it works now, then applied 3 coats of high heat golden (color) spray paint.

Took the Thermostat housing and new Thermostat + Gasket down to the boat. Now the good news.

There was no Coolant Leak !!! The few drips from when we removed the housing the other day was all. 

But now the Bad News - Seems that the coolant leak was disguising a fuel leak!

We had put wads of paper towel in several parts of the engine, all were dry except the piece that was beneath the Injector Fuel Return hose from Injector #3 to connect to the return pipe.

Looking closely, fingers rubbing beneath the hose clamp were wet with fuel - we smelt it!

The hose is almost original and so is the clamp, it's the perforated type miniature hose clamp. It had cut into the surface of the 3/16" rubber hose.

It took an hour to find the correct hose for a replacement. It has to be SAE J1527 but I could not find 3/16" hose to that standard.

Further reading suggested that 1/4" hose would fit if the correct type of hose clamp is used.

I found the 1/4" hose online at West Marine and the hose clamps on Amazon. The clamps are on their way to arrive Monday, the hose was supposed to be delivered last night - didn't happen and I had paid shipping for same day delivery. Perhaps I misunderstood and perhaps it was same day shipping and not same day delivery.

If the hose arrives Monday, then we'll install on Tuesday, test run the engine and assuming all is ok, then we'll plan to do a dump run ( 3 miles out ) on Tuesday so that we can enjoy a trip Friday till Sunday for the 4th of July.

This is the Thermostat housing before removing for servioce.

The air bleed valve on the top does not work, there seems to be a leak between the housing and the mounting.

Note: The fitting to attach the Blue hose to the Housing has only one hose clamp. That's because the fitting is too short for 2 hose clamps.

With the housing removed, the condition of the Gasket is obvious.

Forgive me, but I didn't take any photos of the housing after cleaning it up, removing all of the paint and polishing the gasket surface. It looks like new now.

Also installed a new Thermostat and serviced the Bleed Valve. So it's in a lot better condition now.

This is the fitting that is from the Coolant pump, the hose to the Hot Water Tank is attached to it.

It has a Tapered thread. I removed it, it does have a handy slot in the end for a large flat screwdriver. That got a clean up too.

So the Coolant system should be a lot better now.

The Hose and Clamps arrived, just a couple of days late, but better late than .... :)

 Just in case you didn't know. Eximius is docked in Fort Lauderdale, that's South Florida - Freaking HOT! 

Here's a pic to prove it. The Cabin temperature was 103º F when we got down to the boat.

Of course, first thing - get the AC running, that dropped it to 93º F within an hour, barely bareable.

With the cabin cooler, off came the engine covers, again, and the serviced Thermostat cover was reinstalled. I use a Liquid Gasket to follow up with the new Gasket, that worked really well.

The Red Liquid Gasket is just visible where the Housing attached to the Mount. Looks pretty with the new paint job.

The Liquid Gasket has to be applied to both surfaces and allowed to air cure for a few minutes, then the two pieces are attached but only bolted down loosely. Then is is supposed to cure for an hour before tightening down the two bolts.

While waiting for that hour, I went ahead and replaced the fuel return hose from the 3rd Injector Banjo to the hose securing point before it connects to the 1/4" return hose that takes the unused fuel back to the Diesel Tank.

Still some time to go, so I climbed into the Aft Berth and replaced the bulkhead that separates the Fuel Tank from the Berth.

With that all done, I tightened the bolts of the Thermostat housing.

The final cure of the Liquid Gasket is 24 hours before running the engine.

So I placed a bunch of tell-tale paper towels around the areas where we previously saw the leak: Around and beneath the Fuel Return hose clamps.

Beneath the engine and around and beneath the Thermostat housing.

Then it was time to shut up shop and leave the boat for 24 hours.

Next Day! Woooooo Hoooooo !!! Fixed. I filled the Coolant Reservoir, installed the Pressure Cap, checked all of the paper tell-tales--- Nothing! I'll put that one in the Fixed column.

Time to start the engine.  Opened the Fuel cock on the top of the Fuel Tank, Opened the Raw Water thru-hull, Turned on the Engine Electrical supply switch on the Control panel above the Nav Station, Key in the 'Ignition' and let the fuel pump run for 10 minutes - I always do this after any service work on the fuel system - running the fuel pump effectively bleeds the Injector system.

Time for a break from the Sun. My Buddy Mike arrived to see how things were going and the get in some Chin Wag time which worked our great. After 10 minutes of curing the worlds problems, back on the boat and start the engine. It started 1st time after 20 seconds of Glow Plug heating. 

With the engine running, back in the cabin and there's a Coolant Leak !!!! Grrr. 

With my head down on the cabin sole, I could see the source - not a leak! but the Reservoir was overflowing. I may have put a tad too much coolant in the system.

After drying that up, I checked the engine temperature, it was starting to overhead - totally expected. We needed to Burp the coolant system.

Process is easy. Shut the engine down and it cool back below 160ºF then restart the engine and immediately go Wide Open Throttle. The Engine Temperature gauge needle dropped down to it's normal ( about 150ºF for us ) and all was well. We checked the Heart Beat: Engine Temperature, Fuel, Voltage, RPM and Exhaust + Water discharge. All was good.

Finally, move this one into the DONE Column.

We're actually planning to trip on the boat.

So, I can positively say " See you on the Water "

Thanks Peggy for keeping me Sane on this one.