I had a significant diesel fuel leak near the front of the injection pump on my '83 300CD. What a mess! First thought it was the injection pump or a return line, but it turned out to a hairline crack at the bottom edge of the main fuel filter, only visible when I had the filter out. The attached picture barely shows it. I was grateful it wasn't the pump! The filter was last changed in 2006, about 45,000 miles ago. The crack was near a black mark, perhaps from something hitting it. I can't imagine what could have done this to crack the metal filter housing. Has anyone seen this before?

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Water collects in the bottom of the filter. It may have rusted through.

Or else a stressed area from the stamping process succumbed to years of vibration.

Since you likely were overdue for a new filter, consider it a "free fix."

I'll suggest changing the filter more often.

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83 SD

84 CD

Unusual secondary filter failure

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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

What you are calling the "main fuel filter" is actually the secondary filter.

And yes, it should be changed a hell of lot more often than every 4 years. They cost about a whole $8, and that's for a German brand.

15k miles max on those.

Change it when it gets plugged.

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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