Coolant leak.

[Cliffhanger]

I have a catamaran with two Volvo Penta MD2020 engines. One of which has developed a coolant leak from the overflow tube. Eventually, the header tank empties completely and then of course overheats (after the temp gauge drops). I have cleaned out the heat exchanger tubes and been really careful to clean and reassemble the rubber end caps. There is plenty of water coming out the exhaust so I think the elbow is OK. I thought the cap could be a problem so I swapped caps between the two engines. Any ideas what the problem could be?

[Hesti]

Is the overflow tube controlled by the valve in the filler cap? In that case it might be the seal on this item. Otherwise, it just might be the head gasket.

[a64pilot]

You have a coolant leak, on the fresh water side, not the raw water side. Although don’t rule out a leak from the fresh to the raw from where you took the heat ex apart. That is I suspect the most likely.
It could be the head gasket, if you suspect the head gasket there are kits that will detect combustion gasses in the coolant, even if the water is leaking out through the head gasket, most often some combustion gasses are leaking into the coolant as well.

If not the head gasket or heat exchanger, coolant almost always leaks externally to the engine and you should see it in the pan under the engine, the fresh water pump and of course any hose are also suspect if you see evidence of an external leak.
You need to get this fixed as soon as possible, especially if it’s a head gasket.

You can also buy a radiator pressure test kit that will pressurize the cooling system with the engine off, you can confirm a leak that way and often find the leak.

I suspect the heat exchanger, mostly because you have had it apart

[Bill O]

Swapping out the caps was a very good first approach and usually something over looked in motor maintenance. Don't know how old they are, but may not be a bad idea to get new ones. Unless the size is odd, you should be able to get a replacement at the auto store w/ the same psi rating.

While you are at the auto store pick up coolant expansion tanks. Not sure why they aren't standard issue, but probably will fix your coolant loss issue. The expansion tank is also good way to monitor your coolant levels to see if you have any leaks that need attention.

The blown/leaking head gasket is also a possibility as mentioned by Hesti. A simple check (w/the expansion tanks in place) for a blown head gasket would be if you have continuous stream of bubbles coming out of the overflow tube. They also make a chemical test to detect exhaust gasses coming through the coolant system if you are still having problems or are uncertain.

Loosing all of your coolant is significant and as mentioned a bigger problem than a minor leak, still expansion tanks are helpful. Would agree to check the gaskets on the exchanger to rule out that possibility.



Bill O.

[sailah]

I just went through this on my Westerbeke so not same engine but seemingly same problem. I have an expansion tank connected to exhaust manifold, and the expansion tank is vented to atmosphere

Symptoms would be:

Could run for about 5 hours, then temp would climb rapidly. Esp when I idled down like coming into a dock. Usually would get high temp alarm and shut down myself.

Expansion tank was full, and coolant was dark and oily.

I could crack rad cap, and coolant would drain back to exhaust manifold and I could run engine for a number of hours until it happened again.

Starting engine cold, with cap off, I could see vigorous bubbling in exhaust manifold. And if I put the cap on and pressurized the system the bubbling transitioned to the expansion tank and I could clearly see it. Looked like tank was "boiling". Try this first it's free to do and if you see bubbling there I'd do the combustion gas analyzer test next.


I suspected head gasket, and I had just put on a fresh one last season due to a valve issue when I had to remove head anyways.


I bought a Stant cooling system pressurizing checker. Held 14 PSI for 3 minutes and passed that check.

Changed rad cap, no change, also thermostat.

I bought a Leakdown tester which showed failure in cyls 2-3

The conclusive proof was the combustion gas analyzer tester. They aren't very expensive and easy to use. Basically fill a "turkey baster" thing with some special fluid that turns green in presence of combustion gases. Mine changed in 2 seconds.

I removed head and checked flatness with feeler gauges and it was out 0.002". Machine shop confirmed and they magnafluxed it for cracks as well and blued the head before skim cut to show me where is was leaking.

I suspect previous owner had an overheat issue and head was warped. Put it all back together with a new head gasket and ran it for awhile last fall on the hard, seems to be fine now.

This is the combustion gas tester

https://www.amazon.com/UView-560000-...ion+gas+tester

Coolant system pressure tester

https://www.amazon.com/Stant-12270-C...ressure+tester

Leak down tester

https://www.amazon.com/Stant-12270-C...ressure+tester

[billknny]

As usual, people have jumped to the head gasket without actually reading (or understanding) what you wrote. Kind of like going to the doctor with a headache, and he skips the aspirin and wants to operate on a brain tumor.

You say you are leaking through the overflow tube. I assume when you say that, you see coolant coming out that at least approximates the volume you are missing from the engine. That eliminates the head gasket as the leak.

You have swapped the caps, that eliminates the cap as the cause. It is possible, but unlikely, that the top rim where the cap seals is damaged. Look closely and be sure.

Get a pressure gauge on the coolant system. If coolant leaks out the overflow at pressures below the cap rating, it is the cap or the rim. If the coolant system pressurizes BEFORE the temperature rises, it is POSSIBLE exhaust gases from a head gasket leak are causing it, but check these other things before you rip the head off the engine...


Check the coolant water pump. If that is not circulating coolant efficiently, the water in the block could boil, and push coolant out the overflow without showing a high temperature on the gauge. It is unlikely, but it happens.

It is less likely, but also possible, that there is a blocked passage inside the engine, again allowing more or less stagnant coolant to heat to the boiling point, raising the internal pressure without hot coolant reaching the temperature gauge. You cleaned the tubes, but did you give the block a good chemical flush?

[Suijin]

I would start by pulling the heat exchanger and pressure testing it. It may look clean but if the tubes have failed you’re going to get raw water in your coolant and overflow.

If the exchanger is good reinstall and test the whole system. Probably worth doing an acid flush if you don’t know when it was last done.

[Thumbs Up]

Can you see the coolant coming out of the overflow constantly, or are you just assuming that is where the "leak" is? If you overheat for any reason at all the cap should do its job and pop open at about 14 psi. You will then lose the rest of your coolant through the overflow tube. Your heat exchanger housing could have a hole in it allowing coolant to escape into the exhaust. When the level of coolant drops below the heat exchanger, the remaining coolant will overheat, popping the cap and resulting in a "boilover". If you have constant dripping from the overflow tube, you have a bad cap, or a damaged flange. It could also be a head gasket issue but there is no reason to jump to that conclusion without first eliminating the other possibilities.

[sailah]

A $60 combustion test kit and 5 min would tell you whether it's a head gasket or not. I'm not saying it is, I'm just explaining what I went thru. In my case the overflow was contained in my expansion tank so I let that drain back into exhaust manifold and knew I wasn't losing coolant through HX.

$60 and 5 min is a lot faster and cheaper than removing and testing a HX.

And the coolant pressure test kit is a very quick way to determine overall coolant system integrity without removing a bunch of stuff. In my mind if you pass the coolant system pressure test you can probably save yourself from testing the HX at a shop and move to more likely candidates.

Both those tests are very simple and non invasive to do and can be done by anyone, highly recommended before you start taking things apart. In my case, the combustion test allowed me to zero in on head gasket and then this was confirmed in leak down test. I had already bought the coolant system pressure tester so I took a few minutes to pressurize it anyways to ensure integrity.

The fluid in the tester turned to green in about 2 seconds and you can see my pressurizing tester holding after 3 min.