Maintenance ground runs.
I can't speak for the J model! never worked on it! but, a K ground run post Prop/ engine change could last a good while. One run that lasted "forever" was a Pressurisation check where you needed to go to "safety valve" release. Even in the UK in summer! it would take a bit of extra throttle to get up there. Now that in itself is curious because the whole assembly, once in the flight range was running at 100%, regardless of throttle setting. Maybe a bit of aural naughtiness in the sumpies make up !
Smudge
