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Old 06-06-2020, 12:46   #1
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2GM20 Engine Overheating

Hello,

I have crawled this forum for help in the past, and now I created an account to seek your help. My sailboat has a 2GM20 that continues to overheat, even after antifreeze changes. Some days it will be ok, others it will overheat very quickly. Sometimes it will be idling and overheat after a while, mostly it's under cruise at around 1700RPM.

My coolant has been filling with black slimy stuff, I'm assuming the exhaust is making its way into the coolant somehow. My going theory is that the soot is coating the heat exchanger tubes and causing the overtemp, but no real proof other than observing the black stuff.

I took the front and back of the heat exchanger off and noticed that both ends have cut outs between the fresh water side and the coolant side, allowing mixing. I can't imagine this is the factory design, but they look too nicely cut to imagine someone did this themselves.

Can anyone comment on where I should go with this issue next? Thank you in advance for your input.

[IMG][/IMG]
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Old 06-06-2020, 13:21   #2
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Re: 2GM20 Engine Overheating

Not sure you have got your thinking straight on how this heat exchanger is set up. The partitions are to provide a 3 pass flow by the salt water though the tube set. The engine coolant is on the other side of the tube bank. Mixing elbow would be my first shot at the heating problem, obviously a new gasket set is in order. I have kept both a 2 and 3 GM cleaned by using a 32 Cal brass brush on an extension with a battery powered drill. Early on pulled both caps more recently just pull the fwd.


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Old 06-06-2020, 13:21   #3
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Re: 2GM20 Engine Overheating

Welcome to the forum, Columbia65

I won't speak to the possibility of exhaust in your coolant - that's scary. OK, I will comment: That sounds like a leaky head gasket.

I can't ID the part in the picture. It's design says it's a drain hole in an area that has splatter - hense the dog-leg shield above the drain hole. The slight crusting suggests that was the water level when it was in place. I don't think that it is intended to allow coolant into the raw water - there's just no purpose for that. Maybe someone else will recognize it.

If coolant really was going through that hole it would be gone very quickly. I would expect that you would have salt crusting (You're in seawater, right?).

Slide the core out of the heat exchanger and take a look. One possibility is that the bronze tubing is crusted with scale, preventing heat transfer. It can be disolved off, if that's the problem, with vinegar.
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Old 06-06-2020, 13:48   #4
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Re: 2GM20 Engine Overheating

That is a heat exchanger cap. Frank is correct.
Only interface between exhaust gasses and fresh water coolant is the head gasket that I can think of. There is a relatively inexpensive kit sold at car parts stores that will confirm or deny the presence of combustion gasses in the coolant.
If it’s there, I’d pull the head, if I pulled the head I’d take it to a machine shop and have it checked to see if it’s flat and magnafuxed, magnafluxing should not cost much.
If it’s good then I’d have the valves done and new oil seals.
Replace the head gasket and torque the head IAW the manual, retorque if manual specifies it.
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Old 06-06-2020, 16:59   #5
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Re: 2GM20 Engine Overheating

Sorry I should have specified, I'm in freshwater. The engine was fully rebuilt, including head gasket, about 4 years ago so I can't imagine it's blown already.

My theory was that if these caps are allowing the coolant to mix with the water circulating the block and there was enough back-pressure in the wet exhaust to allow some of it to sneak back upstream via turbulence, that would explain the slow degradation of the coolant. Typing it out makes it sounds crazy and very unlikely. I'll check out that analysis kit, thanks for the recommendation!

For now I'll check the elbow and clean the tubes to get her launched and see how that goes. Are there any additives that can be placed into the coolant to help maintain a seal if there is a micro-leak in the gasket?
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Old 06-06-2020, 17:33   #6
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Re: 2GM20 Engine Overheating

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.


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Old 06-06-2020, 18:10   #7
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Re: 2GM20 Engine Overheating

[QUOTE=Frankly;3157786]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.


I'd definitely give the 316ss elbow another shot if its not made of tube.i.e 1.5mm thick.
You will need to clean the buildup of carbon out though.
I've made 316L sched 10 water injected exhausts that have lasted 15 years & maybe longer as sold the boat.
Mechanics advice was sound for cast-iron probably.
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Old 07-06-2020, 03:13   #8
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Re: 2GM20 Engine Overheating

Quote:
Originally Posted by Columbia65guy View Post
My sailboat has a 2GM20 that continues to overheat, even after antifreeze changes. Some days it will be ok, others it will overheat very quickly. Sometimes it will be idling and overheat after a while, mostly it's under cruise at around 1700RPM.
Hey Guys - Amateur Marine Mechanic (LittleWing77), here.

Is it significant that the overheating is occurring in a variety of modes? (i.e. right away, idling, and /or under load) Wouldn't that point to a blockage of some sort...?
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Old 08-06-2020, 06:37   #9
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Re: 2GM20 Engine Overheating

Can someone point out any resources to better explain the heat exchanger? My understanding is that the coolant passes through the inner tubes and the raw water (fresh, in my case) flow through the case out the outside of the tubes. If the caps have the semi-circular cut outs, it's allowing coolant to mix with raw water, right?
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Old 08-06-2020, 06:52   #10
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Re: 2GM20 Engine Overheating

"My understanding is that the coolant passes through the inner tubes and the raw water (fresh, in my case) flow through the case out the outside of the tubes. If the caps have the semi-circular cut outs, it's allowing coolant to mix with raw water, right?"

Shortly after an overhaul is a good time for a head gasket to fail if it was not installed correctly.

You understand the role of a heat exchanger, but your last sentance cannot be correct, because the entire point of a heat exchanger is to exchange heat without mixing the raw water with the coolant. Back 40 years various inboards, largely automotive conversions, used raw water for direct cooling. In salt water environments, those engines rotted out in a hurry.
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Old 08-06-2020, 06:56   #11
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Re: 2GM20 Engine Overheating

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"You understand the role of a heat exchanger, but your last sentance cannot be correct, because the entire point of a heat exchanger is to exchange heat without mixing the raw water with the coolant. Back 40 years various inboards, largely automotive conversions, used raw water for direct cooling. In salt water environments, those engines rotted out in a hurry.
Thank you, I think you are confirming my point. I am noticing those semi-circle cuts into the separator flange. It seems to defeat the whole purpose of having a seal there. I think I will use some JB weld high temp to fill in those gaps.
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Old 08-06-2020, 07:01   #12
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Re: 2GM20 Engine Overheating

Here's a better picture of the gap. The motor did see salt a number of years ago, so that is why there is evidence of corrosion.

[IMG][/IMG]
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Old 08-06-2020, 07:09   #13
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Re: 2GM20 Engine Overheating

If you nose around on the net you can find both parts and service manuals on the GM series of engines.


The sea water (fresh or salt) passes through the inside of the tubes. The partitions that you are seeing are configured so that the sea water passes through 1/3 of the tubes front to back, back to front through a second third of the tubes, and finally front to back through the third set of tubes where it is plumbed to the mixing elbow to mix with and cool the exhaust gases.


The actual engine coolant is circulated through the block and the outside of the tube bank via the water pump mounted on the front of the engine and driven by a belt that also drives the alternator. The sea water pump is the small lower pump on its own dedicated belt. It takes sea water from the ocean/ lalke through the strainer and passes it to the front of the heat exchanger assy through a rubber hose on the front of the engine.


Not sure about the small cuts in the cap partitions but the flow through these is going to be small compared to the flow through 7 or 8 3/8" dia tubes in parallel. Might be there to aid in purging air from the system.


If you decide to tear into the mixing elbow, remove the elbow and exhaust manifold coupling/ adaptor as a single unit. There is a left/ right threaded coupling connecting the two parts together that is a bear to remove.


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Old 08-06-2020, 07:11   #14
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Re: 2GM20 Engine Overheating

Great, thanks so much for the run down! I purchased just the elbow, maybe it's worth buying the whole assembly to save the frustration!
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Old 08-06-2020, 07:37   #15
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Re: 2GM20 Engine Overheating

Over 35 boat years of messing with both 2 and 3 GM engines, never needed to replace the adaptor on the aft end of the manifold(do replace gasket), but don't think I ever replaced an elbow without replacing that threaded coupling. One end is left handed thread and the other right. Pay close attention to that point or a difficult operation will be made worse.


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