When you have daily objective of repairing your vehicle's engine and you go to repair your vehicle with the kit, once you start the repair process, it immediately ends giving you the daily objective.
Now this not may seem like a problem, my vehicle was at 260 HP and i started the repair for my objective, as soon as i pressed Y, my vehicle HP was set to 310 and it stopped the animation and gave me the objective.
Perhaps it should still let you repair to 750 without needing to stop you for the daily obj.