Is your thermostat right side up?

To the editor: Steve D’Antonio’s interesting piece in the latest issue (Elements of engine cool: raw-water pump and pressure cap Issue 148 Sept. 2005), with the pictures of thermostats on the article and table of contents pages, reminded me of a perplexing issue we had with our boat engine. Clairebuoyant is equipped with a Westerbeke model 58 engine.

Certain engines, like the W-58, are designed to use a thermostat like the one on the left in the picture, which has a disk on the bottom. As the thermostat opens, the disk moves down and closes off a passage in the engine cooling system that otherwise allows the coolant to recirculate back to the engine without going through the heat exchanger. A mechanic or prior owner had substituted a thermostat like the one in the picture on the right. The passage stayed open, and most of the coolant never left the engine. This resulted in a much-diminished cooling capacity. It would stay cool at idle, but with any significant load on the engine, the temperature gauge would head quickly into the overheat range. We didn’t figure it out until, by chance, I happened to look at the Westerbeke parts list, an exploded diagram of the cooling system, with the thermostat in my hand; then I realized the difference in the thermostats.

Obviously, the old mechanic trick of solving an overheating problem by removing the thermostat would not work with these engines.

Also, if I might comment on the impeller picture in the article: When you remove one like that with rubber missing, it is important to dismantle the raw-water part of the cooling system – including the heat exchanger – and chase down all the pieces. If you don’t, they will block the passages necessary for water flow to cool the engine. Chronic problems and shortened engine life will result.

-Quent Kinderman, a frequent contributor to Ocean Navigator, holds a 50-ton, inland waters masters license earned at ONSOS, and works on his Pearson 424 sailboat, while waiting for his bride, Claire, to come out and sail.

Contributing Editor Steve C. D’Antonio responds: The thermostats you are referring to are called bypass thermostats in that they allow coolant to flow one way or the other rather than through the thermostat itself. You also are correct in stating that this thermostat cannot be substituted with a flow-through-type thermostat, otherwise improper engine cooling almost certainly will be the result.

As far as hunting down the missing blades from a disintegrated impeller, indeed, this is mandatory for replacement of any impeller that is not removed intact. Most of the parts typically end up at the inlet side of the heat exchanger. However, I have removed small pieces of rubber impeller farther into the raw-water cooling system and sometimes as far as the injected exhaust elbow. Thus, if the old impeller cannot be pieced back together jigsaw-puzzle-like, then further disassembly and inspection is required.

By Ocean Navigator