All You Ever Wanted To Know About Oil And Fuel Filters

On your vehicle, be it gasoline, diesel, or any other combustion fuel type, the oil and fuel filters are of critical importance to long engine life. The most common type of oil filter is the full-flow type while the most common fuel filter type is the cartridge type. Both operate on the same basic principle, though the oil filter is usually more complex (by design) than is the fuel filter.

How An Oil Filter Works


Full-flow oil filters work by taking in engine oil from the sump (oil pan), filtering it, and sending it to the engine for lubrication, where it returns to the sump to repeat the process. As oil runs through the engine's parts, which are mainly metal rubbing against metal, it acts as a lubricant between the contacts, minimizing wear. Some wear does happen, however, and engine particles and contaminants end up in the oil. The filter is there to capture as many of those as possible.

Oil Filter Design

The most common full-flow filter is the spin-on type, which will consist of a metal container shaped like a cylinder. Inside that cylinder is another cylinder made of paper creases in a continual "tube" shape. Oil enters at one side of the filter, passes through the paper in a spinning, centrifugal motion, and out the other side of the filter. When the filter becomes clogged, the centrifugal motion causes the oil to bypass the filtration by hugging the edge of the outer cylinder, to pass through unfiltered. This is to prevent the filter from clogging the oil's circulation and depriving the engine of oil since dirty oil is better than no oil at all.

Another type of oil filter is the bypass filter. These are common on diesel engines but are gaining popularity, usually as an after-market addition, on gasoline engines. These work as a supplemental oil filtration system alongside the full-flow type. A portion of the engine's oil is diverted from the lubrication circuit into the bypass filter, which filters at a much smaller level than does a full-flow filter. This removes more particles than would otherwise be caught. The oil is returned to the sump after filtration. Over time, the bypass will have diverted all of the engine's oil through the extra filter, so over the longer term, the oil is kept cleaner.

NOTE: We've completed some in-depth oil filter comparisons for both the Tundra and the Tacoma - be sure to check them out.

How A Fuel Filter Works

Fuel filters are simpler than are oil filters, but again come in two types. Some fuel filters are also water filters or can be dedicated to water filtration only. This is common on diesel engines and third-world applications where fuel contamination is common. The filters sit in line with the fuel delivery system, stuffed with filter paper similar to that used in oil filters, though generally less compact. These filter large-particle impurities from the fuel before it is burned, making it more efficient and lowering the chances of fuel delivery malfunctions such as clogged injectors and engine deposits.

Water filters work on a centrifuge theory, with a canonical shape, oriented vertically. Since water is heavier than most fuels, it will separate to the bottom of the filter while the fuel passes through the top on the opposite side of the inlet.

Choosing The Right Filters

When it comes to filters of any kind, OEM is the way to go. We've talked extensively in other articles about the aftermarket vs OEM parts. These articles give you more detail on the topic. 

Maintenance Intervals

All vehicles with filters will have maintenance intervals for them. Note that:

  • Full-flow oil filters are changed at every oil change interval while bypass filters will be changed according to their manufacturer recommendations (generally every second or third full-flow change).
  • Fuel filters will have an interval in accordance with their engine manufacturer's specs, usually at every or every other oil change.
  • Filter changes will depend on the application.

For more details on how often to change filters, check out this article on fuel filter replacement and then this article about oil filter replacement.