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Running engine best before changing oil

Jan 1, 2003
From Ocean Navigator #62
July/August 1994
I'd like to respond to Mr. Black's use of a hot plate for warming engine oil prior to changing it ("Hot plate warms oil," Issue No. 61).

Using a "modified" hot plate to heat the engine oil may seem like a convenient way of doing things, but it neither accomplishes the task satisfactorily nor is completely safe.

There are sound mechanical reasons for running an engine for a brief period of time prior to changing the oil. To remove all of the impurities that cause engine wear, it's necessary for the engine to be run. (It isn't necessary to run the engine to the point at which the oil is hot and unsafe to handle.) The problem is that the filter which Mr. Black is concerned about only removes 98 to 99% of the dirt and impurities from the oil. Sub-micron particles and particles up to two to three microns will invariably pass through the filter and, over time, contaminate the oil. This is the reason to change the oil in the first place. These particles will settle out into the bottom of the engine oil pan and no amount of external heating will get them back into suspension to be removed when the oil is drained out. Using a hot plate to heat the oil will only get the oil to a point that it will drain well but will do nothing for the removal of the impurities, most of which will have settled out. Eventually, the use of Mr. Black's method will result in excessive engine wear and premature mechanical failures. Hence, there are two reasons for running the engine prior to draining the oil: 1) to get all of the impurities into suspension in the oil for removal and 2) to warm the oil to a point at which it will readily drain from the engine. Experience with your own type and model engine will tell you how long you must run it to get the oil warm. At that point you may be satisfied that all of the smaller particles not trapped in the filter are being removed along with the old oil.

On the safety side, some years ago, the owner of a neighboring boat in the yard told me of an "ingenious" method he used. He then proceeded to set up the hot plate on the bed of his gas engine in his 34-foot ketch. A moment or two later, those of us in the area heard a loud pop and almost immediately a very startled owner retreated to the cockpit. It seems that although the boat had been wet stored for some time, there was still some accumulation of explosive vapors in the bilge. Fortunately, the accumulation was small and did nothing more than convince all around that whenever using a non-intrinsically safe device in the bilges of a gas-powered vessel that the space be well ventilated first.

William S. Full is the master of the Exxon tanker Sea River Long Beach and lives in Cumberland, Maine.

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