My 2005 Daihatsu Copen has a "lifetime" fuel filter, inside the fuel pump housing, located in the fuel tank.
I am wary that, over time, crud will get sucked up by this filter and saturate it. What are the downsides of fitting a secondary fuel filter somewhere in the engine bay?
I imagine loss of fuel pressure, as the petrol / gas has to go through two filters
I would worry about the main fuel filter (only slightly), but wouldn't worry too much about a secondary filter. The reason I say not to worry about a secondary is, as fuel filters become plugged, it will just stop flowing. It doesn't allow contaminants to push through, it just becomes plugged. And remember, it only filters out solids ... things which would cause problems inside of the fuel system. As long as the OEM fuel filter has been designed correctly, it should do the job just fine for you. More than likely when it gets to the point of not allowing fuel through, your pump will probably be on the way out at the same time, but that is probably many thousands of miles (or kilometers) down the road.
I go offroad regularly and ditched factory fuel pump in favour of one made by FACET (the U.S. coast guard use them as do some light aircraft)I also use not one but two cheap plastic inline filters-give them a tap every now and then and you will hear if any particles that may cause fuel blockages are present.May sound like overkill but it's saved my fuel pump more than once
There is nothing wrong with a secondary fuel filter in the engine compartment, and it is very unlikely to cause flow problems, assuming you size it correctly.
However, the place to install a disposable filter is BEFORE the fuel pump, so that the disposable unit can be replaced if it gets loaded up. Given that is not practical, in your application, you might just resign yourself to the mercy of the engineers.
In my experience, fuel pumps seem to fail more frequently now than in the past. If it does, you will get a new filter with a new pump.
