The biggest negative of having to run the engine is fuel consumption. There’s something wrong with the cooling system if it overheats after 10-15 minutes of idling. That being said, running (and reving) the engine isn’t preferred, but necessary in many cases. Mechanics are testing & trying to replicate complaints. Aside from the trick I mentioned, we also must run the engine to perform A/C odor treatments. We encounter the livid customers when we don’t adequately dry their interior after detailing when they return days later complaining of the musty smell.
The thing is Cadillacs are some of the most finicky cars on earth, and I've ran into several situations with mechanics who've wound up doing more harm than good and some of them flat out couldn't even fix the problem that they're supposed to specialize in. [i.e. smog check, water pump, distributor] I've had the worst luck + most mechanics simply cannot fix these cars correctly, let alone treat them right. That's why these days my car goes to the dealer anytime something really goes wrong or needs the job done right.
As far as the mechanic needing to idle your car in order to find out what's wrong? Not necessarily, most of that is done with the diagnostic machine and the rest is common sense, it doesn't take your car sitting there idling for them to figure out what's wrong with it.
And then comes after the work's performed, I believe it's called a road test. A lazy mechanic might let your car idle [I found out Pep Boys did this to a previous Cad of mine for over 30min] Road test doesn't mean let my car idle for 30min...
My car wastes alot of premium gas and I don't mind that at all.. But if a Northstar engine overheats it may never be the same or even worse. And the last thing I need is some idiot to make that mistake for me. Odor treatments? On my car they get done with the car off, it's just not worth it.
I may sound crazy, but I've owned 5 of these cars and I know what sets them off. Idling in park for extended periods happens to be 1 of them. Lol.
Btw I use my Master Blaster Sidekick to help speed up the drying if I need to speed it up. It's not an instant dryer, but with a bit of patience over the area I'm always pleased with how much of a difference it can make.