If he doesn't have a plugged filter, continue driving it until you get one. You need to know what is plugging the filter. There are tons of cases on here of algae plugging filters but very few, if any, of rust plugging filters. If it's algae, the problem is fixable without removing the tank.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13