Modern Oil Filters Do a Better Job

Question: I retired after working for 37 years at McClellan Air Force Base. I believe all full flow oil filters, which are based on military specifications that have not been changed since 1952, are in error. It is impossible to flow oil through a filter when it is cold. The result is that the filter’s bypass valve opens and the oil pump sends through dirty oil that has settled to the bottom of the oil pan. I rate these filters a zero. W.M.

Answer: You are at least partly correct that cold oil will not flow through a filter. Rather, thick, cold oil triggers a bypass valve on the filter and the initial oil flow moves through the engine unfiltered.

A modern oil filter is supposed to filter the full volume of oil sent by the oil pump, ensuring that the filter does not block the flow of lubrication from the pump to the engine.

But if the filter becomes plugged with dirt or otherwise obstructs the flow of oil, a bypass valve allows the oil to move through unfiltered.

Once the oil heats up, it thins out enough to flow through the filter and the bypass valve closes. The potential harm occurs if the bypass valve remains open or if the unfiltered cold oil pumped to the engine is especially dirty.

Engineers at Allied-Signal Corp., which makes Fram oil filters, say that oil begins to flow to through a filter very soon after an engine starts. Not until temperatures become very cold, in most cases below zero, does the oil take more than a few minutes to heat up and the bypass valve close.

Oil experts at Valvoline believe that the oil is not any dirtier when it is cold and that a higher concentration of dirt does not necessarily exist around the oil pickup tube.

Modern motor oil contains a dispersant, which suspends dirt particles in the oil rather than allowing them to settle at the bottom of the oil pan.

Obviously, this is not true for all sizes of dirt, because larger and heavier pieces will certainly sink to the bottom of the oil pan. But it certainly works for dirt up to 25 microns--roughly the size of a white blood cell--which cause the most damage to an engine.

In addition, the cold oil typically moves very slowly because it is so thick. It has very little tendency to stir up dirt at the bottom of an oil pan, if any exists there.

Oil sludge does tend to build up in certain areas of the engine, but not to a great extent at the bottom of the oil pan. And the sludge at the bottom of the pan resembles tar that isn’t like to go anywhere. This sludge is quite different than the buildup in the upper valve area, which tends to look more like caked-on dirt.

Although you have pointed up an obvious problem with oil filter, it is also true that filter designs have advanced substantially and that modern filtering medium is much more capable of filtering the dirt most harmful to an engine.

Vartabedian cannot answer mail personally but will attempt to respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, Times Mirror Square, Los Angeles 90053.