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Old 07-20-2018, 09:01 PM   #1
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Troubleshooting a coolant leak

Mainship 400, twin Yanmar 4LHA-STP.


Here's the scenario.


Have done quite a bit of cooling system preventative maintenance in the last 24 months, here's a rundown:



24 months ago, sent raw water pumps off for rebuild.
16 months ago, replaced both elbows and mixers
12 months ago, flushed with barnacle buster.
10 months ago, had intercoolers serviced.
4 months ago, replaced coolant, and flushed.
2 months ago, replaced impellers.


Engine has never run hot, 173 to 180 degrees, under load.

Coolant in overflow canister always is below "full cold" mark.


Put about 100 hours on the motor(s) after the coolant change, including a 8 hour run across the stream and banks to the Bahamas at 2,800 Rpm. Engine never ran hot, right at 180 degrees. No coolant leaks.


During a routine engine check, I added a little (a few ounces) of coolant to the overflow canister even though the level was right where it always is. Not sure why I did this, it's never been a problem before, I do dumb things sometimes. Ran the engine quite a bit (15 hours) after this, but all at lower speeds, none at fast cruise. No leaks.


Crossed the banks to West End, 3 hours at 2,800 RPM. Go to the marina and there was what I think is watery coolant under the tranny end of the port motor, hard to say how much as it flows into the bilge and the bilge pump pumps it out. At least a quart. There is a sump of sorts under the aft end of the motor that is seperated from the rest of the area under the motor by a stringer. The fluid was almost all in this area, but some appeared to have run into the bilge.



The engine room smelled of coolant. No fuel or oil or coolant or transmission fluid leaks on the diapers under the engine. The level in the overflow canister is back where it always is, just below "full cold" and is a nice bright pink. Stuck my finger in the reservoir (it's not easy to see in there) coolant is up to the top. No milky oil or tranny fluid.


Cleaned up the mess and ran the engine while I watched. Couldn't see any leak.


Crossed the stream the next day at 2,800 RPM, 5 hour run. Get home and there is a mess again. Clean it up, watch the motor run again, this time 15 minutes, it gets up to 150 degrees at fast idle as it is still warm from the run. No noticeable leak.


Again, the engine didn't run hot, 180 degrees. Perhaps 2 to 3 degrees hotter than normal, but nothing that concerned me.


I also noticed that there was some coolant in the lazarette, the genset is in there, but it's not running hot either. I have not had a chance to inspect it for leaks. It ran about an hour during both crossings.



No jugs of coolant leaking in either the lazarette or the ER.


I'm kind of at a loss, I'll be back at the boat in a week or so. Where should I start?
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Old 07-20-2018, 09:24 PM   #2
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First thing I would do is follow the coolant line from the coolant tank all the way back. You found coolant in your bilge so there is a leak somewhere and it could be as simple as a loose collar on the line.

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Old 07-20-2018, 10:08 PM   #3
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I had a similar problem and never had a leak running at 7 knots, 2,000 rpm and about 170 deg F but as soon as I would kick it up to 7.5 to 8 knots, 180 deg F, I would always have a small puddle under the engine. I ended up leaving the motor compartment open and watching it running at 8 knots. Found the very small leak to be on the thermostat housing. Check while running hard and you are likely to find it.
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Old 07-21-2018, 04:53 AM   #4
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My Yanmar exacts exactly the same way.

You probably have the coolant overflow over filled. The coolant heats up / expands and is overflowing out of the bottle. If you look at the overflow bottle you will see that the full line on the bottle is down at the bottom of the bottle so as to leave room for expansion as it heats up. . Ck the level when cold and only fill to that line......enjoy boat
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Old 07-21-2018, 05:09 AM   #5
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My Yanmar exacts exactly the same way.

You probably have the coolant overflow over filled. The coolant heats up / expands and is overflowing out of the bottle. If you look at the overflow bottle you will see that the full line on the bottle is down at the bottom of the bottle so as to leave room for expansion as it heats up. . Ck the level when cold and only fill to that line......enjoy boat
He specifically says his level is always below the full cold mark.
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Old 07-21-2018, 05:37 AM   #6
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I would try a new pressure cap.
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Old 07-21-2018, 07:38 AM   #7
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He specifically says his level is always below the full cold mark.
Sorry. Should have read the entire post
In that case I agree with trying new pressure cap.
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Old 07-21-2018, 08:00 AM   #8
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If the coolant level in the expansion bottle is *always* below the full cold (when cold or fully up to operating temp) then the coolant expansion system is not working. And this same thing has happened to me before. If the tank on the engine is full to the top as it should be, when the engine warms up that expanding coolant has to go someplace. If it can't get into the expansion bottle then the pressure will increase until the cap lets it out.

I would check the whole circuit from the tank on the engine to the expansion bottle for a clog. It could be the small hole where hose to expansion bottle connects or maybe in the hose or even the cap.

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Old 07-21-2018, 08:45 AM   #9
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It could be as simple as a loose hose clamp or pin hole leak in a hose near the tranny that doesn't leak until you get the engine fully warmed up and the cooling system to maximum pressure. I would look over all the hoses in the cooling system and check all the clamps. Also as suggested above you might want to consider replacing the coolant cap on the engine. It may not be releasing at low enough pressure to allow your coolant overflow to work which coupled with a loose hose clamp causes the leaks observed.
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Old 08-02-2018, 09:25 PM   #10
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Hey everyone, just an update to this thread.


I got down to the boat this Monday night after leaving it for about 12 days. I opened the engine room and found that even though I left it perfectly dry, there was now a good bit of fluid under the aft end of the engine and in the bilge. This is without the motor running at all.


Upon further inspection I figured out it was diesel and not coolant. I was thrown off initially because I took on a lot of diesel during our Bahamas trip, they don't dye diesel there and the "clear" Bahamas diesel mixed with the remaining red diesel in my tanks resulted in a fluid that was the same pink as my coolant. Oddly, the smell didn't key me in at first.


I've traced the leak to the aft end of the fuel tank. There is a valve there, and it appears that when the valve is closed it doesn't leak, but when it is open it does. I'm not 100% sure of this yet though, it will take me another 24 to 48 hours to verify as it is a slow leak.


I'm praying it is not the tank. They are aluminum, not very old and the visible areas, for whatever that is worth, are in perfect shape. I realize it could be a pinhole on the bottom or a leaking weld, but I sure hope not.


I've got a fuel diaper under the valve and next to the tank where the fuel seems to be showing up. I'll fill you in as I get more data.


Anyone know the procedure for pressure testing a tank? Do the have to be empty?
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Old 08-03-2018, 12:30 AM   #11
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Hold on. How would the valve being open cause the tank to leak but with the valve closed not leak? The position of the valve shouldn`t affect a leak in the tank itself.

Isn`t the valve the more likely culprit? Without knowing what kind of valve, could fuel be leaking out of a spindle,or some other moving part of the valve?
Or the tube the valve feeds,or the connection of valve to the tube.
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Old 08-03-2018, 02:40 AM   #12
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Those were my thoughts too Bruce, and what I am verifying. If it stops when the valve is closed, the leak has to be downstream of the valve and the leak is in a hose or fitting very near the tank but not on the tank itself. I didn't do a good job of wording my first post.


I just checked the diaper though, and now it appears the leak is coming from the tank. There is a 18" x 18" rubber pad glued to the side of the tank just above where the fuel is seeping onto the diaper. I'm hoping that the leak is underneath it as it is a very accessible spot and perhaps it could be repaired in place. Every part of the tank I can see looks absolutely perfect, so it is kind of crazy to me that the tank is leaking.


It seems like very, very bad news.
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Old 08-03-2018, 06:39 AM   #13
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If you can isolate and close all the lines, pump 2 psi of air into the tank and speed things up. I wouldn't allow anymore than 3 psi for an air test.



A look behind the rubber pad glued to the tank might be needed. Poultice corrosion can happen in aluminum where aluminum comes into contact with other materials.


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Old 08-03-2018, 08:07 AM   #14
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Hmmm. A rubber pad glued to the tank. Someone may have tried a quick fix for a leak. But if it accessible, a proper fix should be simple.

Peel it off, clean it up and inspect. If theres a leak there, the simplest solution may be to cut a hole where the leak is and install an inspection plate.

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Old 08-03-2018, 09:39 AM   #15
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Hmmm. A rubber pad glued to the tank. Someone may have tried a quick fix for a leak. But if it accessible, a proper fix should be simple.

Peel it off, clean it up and inspect. If theres a leak there, the simplest solution may be to cut a hole where the leak is and install an inspection plate.

Seabuilt - Access Plate Systems


After draining the tank.
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Old 08-03-2018, 09:44 AM   #16
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I've got a fuel tank guy coming out on Tuesday morning to drain the tank and pressure test it to find the leak. Hopefully it will be accessible.


I checked the fuel diaper that I put in last night around 2:30 am (couldn't sleep) and after 4 hours it didn't have any fuel on it, or very, very little. The pad, which is more off a neoprene feel that regular rubber, was saturated with fuel about halfway up from the bottom. I peeled it off (it came off easily, spot glued in place) one small discolored spot under it near the bottom aft corner but no obvious leaks or holes. No corrosion underneath it other than the spot.
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Old 08-03-2018, 08:20 PM   #17
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Got back to the boat a little while ago and checked the diaper after 14 hours with the valve closed. It was saturated.


Pretty sure I found the leak, it is on the bottom edge, right where that discoloration is. It was just barely covered by the neoprene pad. The edge of the pad touched the shelf right there and I think it was wicking moisture up onto the tank. It is very close to my house bank. I had a catastrophic charger failure about three years ago and it cooked my house bank, boiling them over. Most of the acid was caught by the tray, but it is quite possible that some got onto that pad and held against the tank.


The leak is accessible from the outside. Hopefully it is the only one. Pressure test should tell the story, right?
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Old 08-07-2018, 02:38 PM   #18
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Tank/Fuel guy left a few hours ago. He verified, as best he could from the outside, that the leak appears to be isolated to one very small spot. After that he pumped out all of the fuel.



He is going to cut an access plate in the side of the tank close to the leak, open it up and take a look inside. He will then clean the entire tank, and scrub the area within 18 inches or so of the leak with a stainless brush. Clean the tank again. Then put down a diesel proof epoxy that he has had good success with for about 5 years.


He feels very confident that he can get it fixed and that it will last long term, which makes me feel better. He said my tanks are well installed and in great shape except for the one spot where that stupid neoprene pad was stuck on the side. It wicked moisture up onto the edge of the tank and held it there.


He is also going to install a sight tube on the tank for me. I'm excited about that.


Don't have an estimate yet, but he said 1 1/2 day of work plus a couple hundred in materials.
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Old 08-07-2018, 03:47 PM   #19
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That's sounds pretty good.
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