The simple fix Grandpa thought they could do--adjusting the idling speed of the engine--turns out to be impossible. There's a computer that controls the engine, and all they can do is poke at it till they figure out why it's doing the Wrong Thing. They didn't catch it before they gave it back because their test involved fifteen miles of freeway driving, not the sort of slow, stop-and-go driving you get on residential streets.
So, once again, my car is gone for an unknown, possibly long, period.
*sighs* This is getting annoying. Really annoying.