2GM20 Engine Overheating

[a64pilot]

While you may need to replace the elbow, there is no way that the raw and fresh water are combining in the exhaust or getting into the coolant from elbow. There isn’t any fresh water there.
You can flood an engine from an elbow that has rotted through though.

[Captn_Black]

Originally Posted by Frankly

A lot of years back (like early70s) a mechanic told me to remove the mixing elbows every 4 or so years and simply toss them in the trash. He said to not even look inside as that might tempt me to clean them out and reuse them. I have followed that advice pretty closely over the years as mixing elbows are typically not really expensive in the boat ownership game.


Four years back I purchased and installed a 316 SS mixing elbow for my 3GM off ebay. If I can ever get over Hurricane Michael, I plan to see how SS fares in that application. Planning on at least looking before I toss it.


Frankly

I also had problems with my Yanmar's mixing elbow on my previous boat, although I was advised against a stainless replacement (here I think) as the sea water and high temperatures negate the stainless' effectiveness.
I'd be interested in opinions and/or data on that.

[a64pilot]

SS is better than iron, which is better than aluminum. However if possible what you want is bronze which supposedly will last the life of an engine.

[Columbia65guy]

Thanks again. I'll report back next week once all my parts get here and I can install and clean everything.

[Frankly]

It would be a bit of a stretch, but if the exhaust elbow is fouled, and the seals between the tube bank and the HE casing are deteriorated, might get some blow back into the FW cooling system. Wouldn't take much to show up in the closed cooling system, particularly if the water/AF are not changed regurarly. Over the years I have run radiator cleaner through mine a couple of times between changes.


As to the SS elbow, it will be interesting to see if there is any difference. Looks better on the outside at least. In the past (I do really look before I toss them), there has been considerable carbon build up along with the rust ect. On ebay the SS copy was high quality and was less than $25 more than the standard cast Yanmar. She is still running normal temp and I have other priorities so the look see will have to wait.

For two boats with GMs, the elbow replacement, new impellor every 3 or 4 years, and 32 Cal gun cleaning brush for the tubes, have kept my temperatures under control. These are great little engines. The 2GM from my Hunter 30 (Michael left perched on my dock) is sitting in my garage. Was running fine before the storm, but i decided to do some maintenance on it before i put it on Craigs list. Lot easier on a dolly than working in an engine box.


Frankly

[Columbia65guy]

Hi guys,

A quick update, I replaced the mixing elbow which was definitely very clogged. I also flushed the coolant and used the rifle cleaner to clean out the heat exchanger.

I went for a long motor over the weekend, and it seemed to behave. However, on the way home after a short while my temp alarm went off again. I'm at a loss on next steps to troubleshoot, any advice is appreciated. Thanks!

[Jahwork]

Greetings all,

I hope this finds you well and that you are enjoying time on the water.

OP, Just some food for thought on your engine issue. I would take A64’s advice and test for combustion gases in the jacket water(coolant). A very common early sign of head gasket failure is intermittent overheating. This is caused by the head gasket allowing a small amount of exhaust gas, which is under a much higher pressure than is the jacket water system, to enter the cooling system. This pressure then prevents the jacket water from circulating as it normally should and consequently it cannot absorb and shed heat as intended. Even though the raw water side is functioning perfectly, and raw water is free flowing through the heat exchanger, this is only one side of the needed liquid flow as the engine coolant side is either not moving, or moving very slowly, due to the above described issue. This problem is often intermittent as the heating/cooling/expanding/contracting of the engine and it associated parts can cause the exhaust gas leak to open and close during engine operation. As you can imagine this issue never gets better on its own and will only continue to worsen over time. In my previous days of a mechanic in the Coast Guard, and then years working on commercial and private vessels I have seen the above description play out many times.

Lastly, depending on the amount pressure/duration of exhaust gas entering the jacket water side of the cooling loop, you could see loss of coolant through the overflow, the “radiator” cap is usually rated around 12-18 PSI, sorry I can’t covert to metric for you non imperial measuring type this early in the morning until I have another cup of coffee. If you have an overflow container check it for contents and watch it’s volume during the overheating periods. If not check the bilge as the older Yanmars often just had an overflow tube run down the side of the engine and into bilge/engine sump area.

What is telling to me and my backyard, amateur, poorly trained, low-skilled, non-textbook following style of repair is that you have “black” film in the engine water coolant. This is a telltale of combustion gas infiltration.

Again, this is all a best guess on my part. I hope I am wrong and that it is something much more elementary, such as maybe a sticky or broken thermostat, which is also worth checking or simply replacing due to its low cost and the need to remove and clean the housing anyway for testing. If you do decide to test just remove it and put into pan of water on the stove with a thermometer then simply check temperature of water when the thermostat opens, if it does. Also ensure it opens fully.

Hope any or all of the above helps you solve your problem. If it doesn’t........well as the saying goes you got what you paid for. 🤣

Safe journeys,
~Jake

[Columbia65guy]

Thanks for the quick reply! I did replace the thermostat last year as a first crack at the issue. I will work on testing the coolant next, this was the first day under motor since getting the repair taken care of. Thanks for the reminder on this point! Where does the coolant need to be sampled from? I do have an overflow tank, what should I be looking for in the overflow during overheating?

[Jahwork]

Hola,

If the thermostat was replaced a year ago it is unlikely the culprit but still might be worth checking as it is one of the easier diagnostic steps. Some have, In the past, and many more likely will, in the future, been known to remove the thermostat from the system completely and run engine without it installed to test the “cooling” system. I am sure that will wrangle the ire of many who read this and start a narrative based argument as to the pros and cons of this idea. As such, I am not “recommending” the practice but rest assured in many far far away lands, in out of the way locations, off the beaten path, and in areas travelled by few, these types of non traditional techniques are oft employed in and effort to either stay or become operational. Sometimes non-traditional methods which don’t appear in the manuals or textbooks are useful.

As to your other question. If sufficient pressure is infiltrating the jacket water system it will need to find a way out. This is usually by way of the “radiator” pressure cap lifting and purging excess pressure. I have seen on rare occasions the “leak” appear at or around hose clamps and even have seen a split coolant hose as a result. Essentially wherever the weakest link in the jacket water side of the cooling system is located is the place where the pressure will escape. This is usually fairly easy to find by the presence of antifreeze, or whatever coolant your system is filled with, leaking out. To check the most likely location for coolant to escape, simply evaluate your overflow container. If it is dry and empty at rest, check again after engine overheat to see if coolant is now present. If so then obviously pressure caused the cap to “lift” and expel coolant into the container. If it has coolant in it while engine is cold and at rest, either empty it or mark the liquid level on the container’s exterior with a pen, pencil or a piece of tape then run engine until overheat conditions arise and compare the cold coolant level to the hot coolant level in the overflow tank using your above mark as a reference point.

The best idea is what A64 suggested. Simply test for the presence of exhaust/combustion gases in the coolant. It will be much more accutare and indicative of the potential problem.

Safe Journeys,
~Jake

[Compass790]

If you have black slimy stuff in the coolant it's very likely you have a blown head gasket.
Best to do the coolant test as A64 & jahwork suggested. It doesn't mean that much that your head was rebuilt four years ago as the head gasket can easily blow if engine was overheated say by a raw water blockage.
We are all assuming you have sufficient raw water flow as well. Please post back if you find the smoking gun.

[Columbia65guy]

Hi there, thanks for all the suggestions. I finally got into the engine and there are clear signs the head gasket blew through to the coolant circuit. I will be cleaning everything and checking for flatness next.

What do people recommend for cleaning the carbon deposits on the valves? How abrasive can you go for?

[Compass790]

Originally Posted by Columbia65guy

Hi there, thanks for all the suggestions. I finally got into the engine and there are clear signs the head gasket blew through to the coolant circuit. I will be cleaning everything and checking for flatness next.

What do people recommend for cleaning the carbon deposits on the valves? How abrasive can you go for?

Wow this is a long running thread. I just use a wire buff in a drill or 4-5" grinder to clean carbon off valves. The valves are pretty hard so no worries there about damaging them.
If you have blown the head gasket when installing the new one I recommend using Loctite 518 or 515 or Permatex51831 which is a cheaper copy. The reason I say this is IME it's much more reliable than putting the head gasket on dry. Others may scream about this as they believe advice from 60 years ago but I'm sticking to my method. 518 seals up to a 0.25mm gap frm memory so is great at blocking any imperfections.

[Columbia65guy]

Thanks! Do you apply it to both sides, near the edges? Let me know what you ended up doing, I'm ready for a tried solution so I don't have to do this again anytime soon.

[Compass790]

Originally Posted by Columbia65guy

Thanks! Do you apply it to both sides, near the edges? Let me know what you ended up doing, I'm ready for a tried solution so I don't have to do this again anytime soon.

Apply both sides & everywhere, you only need a thin even coat & it stays liquid outside the joint. I have used the stuff for decades, everyone should have a tube in the toolkit as you can use it virtually everywhere except on exhausts before water injection. Used it even in injection pump but I dont recommend that as you have to pick where to use it in an injection pump.
Dont be scared of it, still no trouble getting a head off, it just stops it from leaking, doesn't glue it on. It just works. I just spread it all over with my fingers but there is a warning about dermatitis on those anaerobic sealants IIRC
I sell head gaskets locally for little YS series Yanmar singles & they are known for head gasket leaks but I give the head gasket buyers this tip. Shooting myself in the foot really as I'd sell more head gaskets if I kept my mouth shut lol.

[Columbia65guy]

Cool thanks so much for the advice!