| Clue | Source | Date |
|---|---|---|
| Functioning well, as machinery | Newsday | 29 Jun 2021 |
| In fine order | Jonesin | 14 Jul 2020 |
| Purring, as an engine | USA Today | 28 Jul 2015 |
| Purring, as an engine | ||
| Emulating a purring engine | ||
| Purring, as an engine | ||
| Emulating a purring engine | ||
| Purring |