300DT stumbling idle/smoking

[Biotour]

1985 300dt, 318k. had been running fine but after driving in hot weather about 150 miles i noticed the idle had become very rough.

i cracked the #1 injector line and noticed the idle didnt change, so i suspected a bad injector. (all other cylinders were fine) i swapped #1 and #2 injectors and the problem stayed in #1.

then i suspected valves had possibly gone out of adjustment. checked valves and they are fine, not sure how to proceed.

idle stays rough no matter if engine is cold or hot. also there is white or light gray smoke at all times.