Ive got some LF55s if it is a similar water separator filter to these, I`ve had problems with dirt in the filter head, as some one has said above take the elbow out where the fuel comes in and check there, some have a gauze filter in there, also check the flow / blocked filter sensor, held in the top of the filter head with 2 allen screws. Take the sensor out and the are 2 long metal contacts running parallel which can trap dirt between them.