I like to use a "friendly coworker" standard. If a friendly coworker who you didn't know that well was asking you for a particular favor would you do it, or would it be over the line?
In the scenarios you note above, if you're already in the garage at some point and her car is there, I'd be willing to put some oil in it, but I wouldn't jump through hoops to do it on her timeline. If its actually your car, or you share joint ownership, or you're worried about the engine blowing up with your kids in the car, then I'd do it with some urgency. If adding oil doesn't address the problem, I wouldn't do any more, that's up to her to take it to a mechanic.
If I had an ECU reader and a friendly coworker asked me to read a check engine light code for them I'd probably do it. Would I then go home and google what it means and try to diagnose it? No, they are perfectly capable of doing that for themselves, so I'd stop at helping them out with my tool that they may not have.
Once again, if its a safety issue that will impact your assets or your kids, or you're worried she'll physically get hurt, then take a look.
So much of DB is subtle. You want to offer tough love, but you don't want to be overtly punishing or passive aggressive, and that requires your discretion. I probably wouldn't say something as blunt as "you've fired me as your partner". While that may be how you feel, it may also come across as a blaming/shaming statement and she's going to resent you for that.
You want your actions to say "I'm not impacted by you" versus "I'm angry at you" -- that's the key to the whole thing
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015