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Time and temperature make coke. The heater outlet temperature at 505 C should be hot enough. The drum outlet temperature should be 440 C or higher. If it is less than 440 C, something is removing heat more than just the cracking reactions. Transfer line and coke drum insulation should be confirmed to be intact and minimizing heat loss to the atmosphere. Drum inlet temperatures of less than 480 C would indicate that heat loss is occurring prior to the coke drum or the coil outlet temperatures are not accurate.
A small steam step for no less than 30 minutes should help convert the last oil in the drum and push it up to the top of the coke bed. A big steam step for 30 minutes at the lowest drum pressure possible should strip the unconverted oil off the coke and convey it to the blow down system.
You did not indicate the cycle fill time or the type of feed you were processing. A feed that is difficult to convert will have higher VCM on the coke. A paraffinic feed can have high VCM and a feed that is high in FCC DCO can have high VCM.
Other obvious problems that could cause the coke VCM to be high are leaving the vapor line quench oil or the antifoam carrier oil on during the off-line drum steps.