short trip after engine warm-up
 

OK, I know it's necessary to warm up the engine to normal temp. before shutting down. my question is, once the operation temp. reaches normal (2 marks left from mid), I shut down the engine, come back 30 mins later, and run the car again. At this point, normally the operation temp. should be still just right, then I run the car for very few minutes' trip(5 min or less). In this case, do I still need to rev to 3000/4000 to warm up the engine again.

I normal don't, and guess it should be fine. tho I know this is a stupid question, just wanna make sure what i did is correct.

What you did is correct. Once the engine is warm, it'll still be warm 30 minutes later. No worries.

Nope. Once it's warm and the temp is up, you're fine.

You didn't have to rev the engine the first time, either. That's just done if you warmed it by idling rather than driving.

Ken