The thermostat is a definite possibility, so is the fan clutch. Inspect the fan clutch and put the thermostat in a pot of water on the stove to see when it's opening. My MKIII always wared up pretty quick but at idle it would still take a while (my MR-2 takes forever)... Did you drive around or just leave it idling?
__________________
If something breaks or you need to contact a member of the administration please post HERE. Unless it's a private or administration matter please post it on the forum. It benefits no one else if car related questions aren't posted for future users and takes away from the time I'm able to spend helping on the rest of the forum.
If you're so inclined I'm always more than happy to accept tips via PayPal. 
Tip Jar ---> 
|