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Old 17-08-2018, 04:57   #1
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Yanmar 2gm20F Overheating/Part II...

I posted a thread on my 2gm that is overheating and got a lot of good feedback - mostly geared around the saltwater cooling side of things.

However, the problem is definitely on the freshwater side, and appears to be caused by a bubble in the system; when she overheats and the port on top of the heat exchanger is opened, there is no antifreeze in there. The expansion tank, at that point, is overflowing.

Opening the hose that comes from the expansion tank (which is the high point) to the thermostat housing, and filling thru the expansion tank, usually fixes it. But, eventually, we get another bubble.

There IS a hot water heater in the loop, which is lower than the expansion tank.

Any thoughts on where that bubble is coming from? I have replaced all the hoses, and it appears that there is no leakage...
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Old 17-08-2018, 05:04   #2
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Re: Yanmar 2gm20F Overheating/Part II...

You've tested the thermostat?
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Old 17-08-2018, 05:30   #3
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Re: Yanmar 2gm20F Overheating/Part II...

Yes - brand new one in there now.
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Old 17-08-2018, 06:02   #4
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Re: Yanmar 2gm20F Overheating/Part II...

Water heater is below tank, but how about the hoses to the heater?

A high spot can collect air.

Only have had this happen on a Camaro that I inadvertently rerouted heater hoses after a pump replacement. But I think it could apply to a marine heater install in a similar manner.
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Old 17-08-2018, 09:31   #5
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Re: Yanmar 2gm20F Overheating/Part II...

I donít want to get you in a state of panic but bubbles that lock the cooling system are very often caused by a leaking head gasket. I suggest that you have the circuit pressure tested. If the pressure does indeed drop then turn the engine by hand a number of revolutions before attempting to start.
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Old 17-08-2018, 09:46   #6
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Re: Yanmar 2gm20F Overheating/Part II...

Ah - that's a good thought! I rebuilt the head probably 4 years ago, but that doesn't mean it isn't the culprit. You are suggesting turning by hand first, to avoid a hydraulic lock in the cylinder?
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Old 17-08-2018, 10:19   #7
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Re: Yanmar 2gm20F Overheating/Part II...

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Ah - that's a good thought! I rebuilt the head probably 4 years ago, but that doesn't mean it isn't the culprit. You are suggesting turning by hand first, to avoid a hydraulic lock in the cylinder?
That is right. I have seen engine heads crack because of this.
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Old 18-08-2018, 03:56   #8
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Re: 2gm20F turning by hand first, to avoid a hydraulic lock in the cylinder?

Would releasing the compression release levels do the same thing?
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Old 18-08-2018, 08:04   #9
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Re: Yanmar 2gm20F Overheating/Part II...

It probably will but electric (high speed) cranking should be avoided, same reason as why a radial aero engine is cranked by hand prior to starting.
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Old 18-08-2018, 20:05   #10
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Re: Yanmar 2gm20F Overheating/Part II...

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same reason as why a radial aero engine is cranked by hand prior to starting.

Which is??
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Old 18-08-2018, 20:12   #11
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Re: Yanmar 2gm20F Overheating/Part II...

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Which is??


Hydraulic lock, bends connecting rods, blows heads off, bad things
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Old 18-08-2018, 20:23   #12
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Re: 2gm20F turning by hand first, to avoid a hydraulic lock in the cylinder?

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Would releasing the compression release levels do the same thing?
Yes it would work to avoid damage
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Old 19-08-2018, 17:38   #13
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Re: Yanmar 2gm20F Overheating/Part II...

Don't often see Wasp radials in boats, but could be interesting. Used to work at PWC (Pratt&Whitney Canada) in Montreal which started out repairing Wasp radials in the 1920's. I was there much later on...70's.
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