Might check the return hose nipples. Had one leak on a 6.2l I have. Drove me nuts for a couple of weeks. I had only removed the leak off host to remove a price of metal that was singing to me. The nipple moved just enough that there was a body-tube leak. Replaced injector, and leak was gone. Dropped my mileage by about 2mpg.

HammerWerf

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1985 300DT 241,000 miles
1979 300SD 202,000 miles