PeachParts Mercedes-Benz Forum - View Single Post - Five sided w123 300D oil filter!!! Why???

JimSmith · #14 05-11-2002, 02:27 PM

Going to try once more to make the point. There has to be a mechanism to develop the force necessary to crush the outer can on the bypass section of the oil filter.

This force can not be generated by static conditions as under static conditions (no flow) the pressure on the outside and inside of the can are equal. This is because the can has holes in it (90 of them on a MANN filter as I just counted them on one I have on my desk waiting for the 240D to come back from Troy, NY for a service) which cannot sustain a static differential pressure (the pressure will equalize through the holes). Since all the flow through the upper portion of the filter goes in the little hole in the return flow tube at the center of the oil filter housing cover, and down to the check valve, a blocked check valve will generate a static flow condition and no chance for deforming the outer can on the bypass filter section.

Consequently, the force must be generated by some other conditions. At very low flow rates the situation is much like a static pressure situation. As the flow through the 90 holes is increased by lowering the pressure on the downstream side, the resistance to this flow increases (like drag on your car - at 5 mph it is hard to measure the air resistance but at 80 mph most of your engine's output is devoted to overcoming air resistance to the car's movement). This increasing resistance can sustain a differential pressure across the perforated surface of the can.

Under normal flow conditions the flow rate in the upper, bypass section of the filter is kept low by the size of the hole all the flow must travel through in the side of the return flow tube, and the resistance of the check valve mechanism. At the normal conditions there is very little resistance at the 90 holes in the outer can, as they have about 90 times the flow area of the single hole in the return flow tube.

So, to make conditions that will develop the necessary forces, there needs to be substantially higher flow through the upper, bypass portion of the filter and its outer can. A plugged return flow tube check valve will not generate this condition unless the pressure across the rubber grommet seal at the bottom of the bypass filter section (about one third of the way in the filter looking from the bottom) that results is enough to pop the grommet off. I doubt this can happen, but it is a possiblility. The grommet seal failure would allow the bypass filter flow to access the low resistance flow path of the main filter element.

So, a scenario where high flow in the upper section of the filter is needed, and any one that happens is good enough for me. I think the inner can is much less likely to deform than the outer can, just by the fact that it is much smaller in diameter (less surface area). Suggesting it might have been deformed could have been a red herring and added to the confusion.

I hope this helps, Jim