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post #1 of Old 07-25-2011 Thread Starter
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Yanmar 1GM10 Overheating

I have a (1993?) Yanmar 1GM10 (single cylinder diesel) that gives an overheat alarm after about 10 minutes running at mid rpm. This has been a persistent problem and I've already done the following:

1) Flushed the cooling system with RydLyme (electrolyte descaler / cleaning flush - amazing stuff that every sailor should know about) which removed alot of scale and rust. I've done this flush several times now, and have done just about everything to get a thorough flush including both forward and reverse flow, even flushing with the impeller and thermostat removed.

2) Replaced impeller - and subsequent inspection shows that it is installed properly and not damaged.

3) Replaced all of the cooling system hoses and clamps.

4) Thermostat has not been replaced, but has no "guts". In other words, it is open all the time (more about this in a minute).

5) Removed exaust elbow and flushed / thoroughly cleaned - no restrictions there.

6) Checked the inlet / strainer and that's all good, too - plenty of inlet water flow.

Engine starts and runs fine and water is exhausting through the transom - not a whole lot of water, but a steady flow (I'm told these engines don't use a lot of water and based on the tiny impeller, suspect this is normal flow). I haven't checked the temperature of the exhaust water.

The bottom is clean and the (recently rebuilt, properly functioning, folding) prop is the correct size - plus I'm not running hard - usually mid rpm +/- 2500 rpm.

When flushing the system, I get good unrestricted flow in either direction (with impeller removed).

Soooo, I've put up with this for a long time because I normally only motor a short distance before and after putting up sails. Each of the efforts listed above seems to help some, sometimes for months, but eventually the God awful screeching high temp alarm returns. I even (temporarily) disabled the alarm, but know there is a problem and won't rest until its resolved.

Here are my questions:

Does anybody know if NOT having the guts in the thermostat could cause this engine to run hot (I know it could be a problem running cold in cold water / weather, but that's not my issue)? In other words, does a properly functioning thermostat in this engine somehow re-direct the water flow to force it into the head, and not allow the water to simply bypass as it would when the thermostat is closed? Or, is having no guts in the thermostat effectively the same as having an open thermostat? (FYI - this gutting the thermostat trick was done by a "mechanic" a couple years ago and I just discovered it - will likely replace it soon either way).

How likely is it that my temperature sensor is off a few degrees and is sending me on this "wild goose chase"? Is there any way to inspect / clean the sensor - or should it just be replaced? Does anyone know what the proper operating temperature of this engine should be (using it in salt / brackish water)? I'm thinking about using a temperature gun to measure the head temperature to test the sensor - anybody ever tried that / would it work effectively?

Last, but not least... Is it just time to throw in the towel and call a (real) mechanic for major $urgery / pulling head? And is that a sure fix?

1981 Pearson Flyer, 30', if anyone is wondering.

Anyway, I'm stuck and reaching out to the cyber masses for help! Regret the long post, but trying to answer basic questions on the front end... Thanks in advance for any experiences, help, or advice!


Last edited by BayLAzr; 07-25-2011 at 04:32 PM.
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