To deal with karlach's engine problem you'll need to track specific upgrade windows and watch her condition indicators carefully throughout each act. The first critical timing is right after meeting dammon at the grove, where you can get the initial infernal iron upgrade which buys you some breathing room. You'll notice her engine temperature gauge stabilizing temporarily but keep an eye on those heat warning indicators that start flashing more frequently as you progress

The second upgrade window opens once you reach the steel watch foundry in act 2, and you absolutely want to get this done before pushing into act 3, since her engine stability drops sharply after that point. During conversations watch for dialogue options where she mentions feeling overheated - these are good indicators that you need to act quickly on repairs. The foundry itself hints at better solutions with those prototype mentions but don't waste time searching since those upgrades aren't actually accessible

For the endgame sequence you're basically looking at three distinct paths with specific triggers: if you've managed both upgrades she'll still face the final choice at the docks but with slightly different dialogue options. The mind flayer path needs specific setup during the colony sequence, while the avernus return requires either wyll's questline completion or specific tav dialogue choices. Watch her condition indicators closely in act 3 since they'll hint at which endings are still available based on your previous choices and engine stability level

The timing here is super precise - missing either upgrade window basically locks you into the self-immolation path so plan your progression carefully especially through acts 2 and 3