My California 84 doesn't have the black box and neither did my prev 84. Not sure about the '83 300DT CA version. If you are in the South Bay, stop by and look at mine. The line should be plugged but not taken off (just in case someone is snopping around smog check wise, yes I know we don't have smog checks for diesels here in CA).

EGR = Exhaust Gas Recirculation

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)