Q & A — October 2001
By Capt. Ken Kreisler
consumption, blowby, an electrical leak, and shore-power cord corrosion.
Are oil consumption and blowby the same thing? B.R., via e-mail
No. Blowby is the process, and oil consumption is the result. But before discussing the difference, remember that all engines--even brand new ones--use some oil, so it's not a matter of whether there is oil consumption but how much.
It is the job of piston rings to separate the high pressures in the combustion chamber from the oil in the crankcase, while allowing a thin film of oil to remain on the cylinder walls for lubrication. Excessive oil consumption occurs when these rings, the cylinder walls, or both wear enough to let oil get past the piston and up into the combustion chamber where it is burned. A related form of blowby occurs when there is sufficient wear in the valve guides, valves stems, or both so that oil slips down the valve stem and into the combustion chamber. As an engine is placed under heavy load, cylinder pressures increase, which typically exacerbates blowby.
Blowby is revealed by higher than normal oil consumption. You can often tell if it is the result of worn rings/liners or valve guides/valve stems by performing a compression test. (Worn rings and guides both usually result in a lower than normal reading.) Remove the compression gauge, and squirt a few teaspoons of oil into the cylinder. Crank the engine a few turns, and retest the compression. If the reading goes up, the problem is worn rings/liners and the oil you introduced sealed the gap between the two. If the reading is unchanged, then you probably need to replace the valve guides/stems.My 1996 four-stroke outboard has about 1,000 hours on it. The oil between changes is usually clean and at the full mark. Upon my latest check I noticed the oil was black and overflowed a considerable amount out the dipstick tube. It also appeared thin, but I didn't see any signs of water present. Any suggestions? W.K., via e-mail
It sounds like the fuel pump diaphragm is shot and you've got fuel in the oil. If you don't want to go right to the fuel pump and tear it down, first have the oil analyzed, which will most likely reveal the presence of fuel.
This article originally appeared in the June 2003 issue of Power & Motoryacht magazine.