Clean your throttle body.
The Lancer had a tiny stumble off idle, a small lag from pressing the accelerator and the engine responded, up to maybe 1,200rpm I barely noticed it, but when a learner driver is learning, it's not a good experience. I immediately thought air flow sensor, or spark plugs, after replacing both of them it was still there. "Clean the throttle body JAW" I was told.
After disconnecting the incoming air flow ducting, pulling off an electrical plug, and taking out the four main bolts holding the throttle boy to the plenum chamber I discovered something. This car is drive by wire! There is no throttle cable! Now this should not have come as a surprise, drive by wire has been around for a long time. But this was the first time I'd held one. (Of course now I'm skeptical that the drive by wire throttle actuator isn't working 100%...)
The back of the assembly is coated with carbon, so I attacked it with carby/throttle body cleaner and a rag. Basically spray some cleaner, wipe it out with a clean rag, repeat. I used an old toothbrush to get into the gaps but you don't really need it. The cleaner dissolves the carbon and gum very quickly, but evaporates quickly, so spray, wipe, spray, wipe is your best bet.
I didn't take the body out, it was connected to two water pipes and I wasn't in the mood to deal with coolant, so I did it in situ. Not sure why the throttle body needs the engine temperature...surely the ECU computer already knows the temperature of the engine and would control the throttle actuator using that data as well. I can't believe that there would be mechanical actuation in addition, like old bi-metallic coils in carby chokes, so this is a mystery. Perhaps it is just a redundant temperature reading, but it seems like a lot of effort to pass engine water through the throttle body just for a sensor.
Suffice to say it cleaned up shiny and new; the gakset is one of those re-usuable types so I put it back in and fired her up. Much better. I'm not sure I'd say fixed, but the throttle response for tiny amounts of air flow is much better.
A good lesson. Clean your throttle body regularly, as a minimum it will help the low flow response, but more importantly it improves your fuel economy.