In between sessions ( or in a session bar the race , but you'll lose most if not all of the time in your session if you do it during a session ) check your laptop and go to the tabs for Engine , Gearbox accordingly . If it's red it's basically on the edge and ready to blow so swap to one of the other engine parts from the 4 of each you have if necessary . If you need to take a 5th item in one of them ( which would incur a grid penalty ) then youre best taking a new one of everything ( including a new gearbox too ) as they cant put you any further back than the back of the grid anyway . Basically all the engine management is in the laptop . Watch some of Aarava's videos on You Tube as he has swapped his engine parts about aplenty and watch how he does it and that'll show you for your own game .
Also , look at the tutorial vids as one of those also shows you how to mix and match engine parts .