After it cooled off it would start up and run from anywhere from all day to just a few minutes. Calls to Volvo service centers and Polar Power didn't provide any helpful information so I just continued to document the conditions around the overheating to find out what the problem might be.
The most interesting measurement was the exhaust water temperature. Under normal operating conditions it was about 10 degrees F warmer than the sea water, but just prior to overheat shutdown the raw water exhaust was the same temperature of the sea water. This indicated that the problem was with the heat exchanger i.e. the sea water was passing through the engine without picking up any heat in the heat exchanger. This could be caused by two situations: either the thermostat was causing the coolant to bypass the heat exchanger or the coolant was being blocked from going through the heat exchanger.
When we got to Jacksonville FL we had a Volvo service mechanic check out the genset, and he found no problems, but when researching the problem decided that the problem was the high temperature cut-out was set too low. We changed the cut-out temperature and that seemed to solve the problem. That is until we got to Miami.
In Miami I flushed out the engine and got out about half a teaspoon of metal shavings (which the folks at Polar Power said was normal for a new engine) and replaced the thermostat. Again that seemed to solve the problem, but two weeks later, in the Bahamas, on the way to Nassau, it overheated again, and would not restart for more than a minute.
In Nassau I flushed out the coolant and raw water lines very thoroughly and got out some more metal shavings. Again this seemed to solve the problem for a month. But as we were leaving Georgetown, Exhauma it overheated again. This was the second time it overheated just as I was thinking that since the wind had piped up it was time to shut the genset off and I suspected that healing might be the problem.
It wasn't until a week later when we were sailing from Eleuthera to the Abacos across the NE Providence Chanel that I really narrowed the problem down so it could be solved. We were motor sailing into the wind. Ruthie was down below watching the temperature. We were on starboard tack which is where the reservoir for the coolant is below the heat exchanger. After a few minutes the temperature began to rise and just before the overheat temperature Ruthie told me to tack. As soon as I tacked the temperature went down within less than a minute. We repeated this many times. We also tried keeping the boat on starboard tack and let it cool off then, when we restarted the engine it overheated immediately.
Here is an email I sent to Polar Power after these tests:
I have now identified a very specific and repeatable way to initiate
and correct the overheating of the Genset.
When the boat heals 10 degrees to port (toward the oil dipstick) the
temp goes from a stable 180 degrees F to about 220 F. Further healing
to about 15-20 degrees will cause the temp to fluctuate into the 230 F
range until one of the spikes goes to 239 F and it shuts off.
At any time when the temperature is high, leveling the boat or healing
to starboard will bring the temperature into the 180 F range. The
temperature drop is immediate and dependable.
The raw water exhaust volume remains constant at about 6 gal. per
minute. The coolant level in the expansion tank is at the minimum
line when the engine is cool and at the max line when the engine is
hot. The oil level is nearly full.
The specifications allow for a 30 degree angle of operation (operation
manual p. 68). The overheating usually occurs within 15 minutes of
healing at 15 degrees and within 5 minutes at 20 degrees. The cooling
starts immediately upon tacking and is back to the 180 degree level
within 2 minutes.
My working hypothesis is that an air bubble is being sucked in from the reservoir when the boat is healing and as the air fills the tubes in the heat exchanger the cooling is reduced until it finally overheats. The solution then would be to move the reservoir above the engine so that it is always above the heat exchanger at any angle of heal.
It wasn't until we got to Charleston SC that I was able to find the small diameter heater hose necessary to move the reservoir. When I finally relocated it to the bulkhead above the engine again the problem seemed to be solved. This is the email I wrote after testing the new location:
I believe the genset overheating problem is solved!
I moved the reservoir to a location two feet directly above the genset. The temperature was rock steady at 187 F while we were in gusty conditions on starboard tack with a minimum 10 degree heel and up to 20 degrees during gusts for about 20 minutes.
I hope my celebration isn't premature, I won't really know for sure until I have until I have a sustained rough heal to port, but I don't believe I will have these testing conditions before I put the boat on the hard for the summer, and I wanted to share this good news.
Two other changes that I've noticed:
The temperature does not fluctuate it stays at 187-190 F (it used to range from 170 to 220).
The cooling fan shuts off shortly after the engine stops and does not come back on. (Previously it would come back on within a minute and stay on for nearly an hour. The "Auto Vent Temp" is set to 195 F.)
As of this writing Prestissimo is in a boatyard until October, and further testing won't happen until this fall, but I believe the problem is finally solved.
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