coolant

I was parked idiling in my 99 g caravan with a 3.8 ltr motor today for about 10 min after a 20 min drive when I could here something from upfront. I opened the hood & the coolant in the over flow tank was boiling. The temp gauge was reading normal, needle at middle line & the temp was about 70 deg outside. I shut the van off for about 30 min then had a 15 min ride home & every thing was fine.Coolant was changed about 6mths ago. Any suggestions to a possible cause, I'm thinking maybe rad cap.

Most likely from the sound of it. The Expansion tank does not do an awful lot unless the rad pressure exceeds 16lbs pressure which then sends excess to the expansion tank. Check the cap first.