Since a prior mechanic had done a fair amount of work on the cooling system of that engine, I decided I should have another mechanic (an unbiased third party) come help determine what had happened.
Now, before I left I knew that the coolant was down quite a bit and suspect that the missing coolant was the cause of the steam in engine room. The plastic overflow bottle was empty and the cap was found elsewhere in the engine room. The secondary coolant reservoir (this is the engine that is plumbed into the hot water heater) was empty and I couldn't see coolant in the engine when I removed that radiator cap either. These are not good signs. I had looked around and didn't see any loose or broken hoses or missing clamps either. On the bright side, there did not appear to be any contamination in the oil.
The mechanic arrived and did similar inspections and could find nothing wrong either. Then we checked the impeller. For those not familiar with a boat impeller, it is the working part of the raw water pump. Unlike your car that uses air to cool the engine coolant that then cools the engine, a boat uses sea water (raw water) instead of air to cool the engine coolant. Since sea water can contain contaminants, the pump needs to be more forgiving of the liquid it pumps and so the working part of the pump is a series of neoprene rubber fins attached to a hub kind of like a water wheel..this is the impeller. When the impeller rotates, it moves water and if you restrict the clearance on one side of the impeller chamber, water will flow in one direction and you have a pump.
|Impeller conceptual drawing - found on the internet.|
So, you might have guessed that this little piece of rubber is fairly important. When we opened up the impeller housing, we found that the impeller had self destructed. As a result the engine wasn't getting cooling water to cool the engine coolant and this was the direct cause of the overheat (just as if you had completely blocked the radiator in your car).
|A new impeller on the left, the one from the engine on the right.|
As the impeller spins, it rubs against the metal housing of the pump body. As you can imagine this causes quite a bit of friction. When it is pumping water, the water lubricates and cools the impeller, but if the water supply is blocked, the impeller will generate heat and rip itself apart. It is our best guess that this is what happened. Maybe a piece of plastic or other trash was sucked up against the intake and blocked cooling water. The truth is we will probably never know.
We removed the pump, dug out all the parts of the impeller (as shown in the above picture), did our best to confirm we had all the pieces of the impeller and checked the raw water entrance to the heat exchanger (radiator) for any additional impeller pieces. We checked the strainer and the hoses to ensure there was no blockage anywhere in the system (and we found none) and then put it all back together. When we added coolant we found that it took about half the amount of an empty engine (so at least we didn't boil it all out).
We then tried to start the engine. It was a little hard to start but did start up and didn't appear to smoke or do anything out of the ordinary. We carefully watched the temperature as the engine ran and it came up to it's prior operating temperature. Watching the needle, we could see when the thermostat opened and the temperature stabilized. Since the temperature needle stopped at 188°F and the prior mechanic indicated that the gauge was indicating 15~20 degrees high, we again checked the temperature of the engine using a laser thermometer on a number of engine locations. At the temperature sensor, the temperature was actually 168°F and the hottest temperature we could find was 172°F. So we did confirm that the gauge was reading a bit high as was previously reported. Guess I'll need to see what options I have to make this system more accurate.
The other issue was why the high temperature alarm didn't go off. We tested the circuit by grounding the wire and the alarm did sound...albeit not as loud as I would like. I'll have to pull the sensor and put it in some boiling water to see if it is working at some point.
But the good news is that the engine is alive and that is a relief...particularly to my bank account.