Ever wonder what goes on inside your BMW motorcycle fuel filter? This is probably nearly identical to most automotive fuel filters as well.

I've split open my old filter, and have photos of the inlet and outlet. The short answer is: it works really well. My motorcycle had two cups (about 400 cc) of water in the fuel tank. This caused some rust in the bottom of the tank, but no other symptoms until recently. I tipped the bike over, and all the water ended up on the fuel inlet side of the tank. The engine developed an intermittent missfire. After checking a couple of things, and thinking about the evidence, I drained the fuel tank, opened it up, and checked the fuel filter. Of course, I discovered the water.

The filter element has been trapping rust for many miles. Normally the filter element is a lighter shade of beige, tan, or brown. The reddish coloration is mostly rust.

This is the inlet. There are small deposits of rust particles. It's hard to see in this photo, so I split open the inlet pipe. See the next image.

Here I have split the fuel filter inlet so we can see if the inlet is clean or dirty. Note the dusting of rust particles in the inlet.

Here is the filter outlet. It is clean. There are no rust particles here, therefore the filter performed very well. You can see some small scratches caused by the nibbler I used to split the pipe.