Processes for oligomerizing olefins to produce diesel. The oligomerization zone temperature is controlled to counteract catalyst deactivation caused by coking, by contaminants such as cyclo C5 and/or cyclo C6 hydrocarbons, or both. The temperature is increased in increments to ensure that that the oligomerization zone is producing product at a target product yield with a target product quality, which may be measured by a product cetane number. The target product yield is at least 50 wt % and a target product cetane number may be at least 35.