'82 Rabbit Diesel, no heat, sweet smell

I was driving one day, the [b]heater was not working[/b], and the [b]engine shut down [/b]about 3 miles from home. It turned right back over and I drove another 10 miles to work with no shut downs. I popped the hood and there was a [b]sweet smell [/b]present.

[i]How do I determine whether this was just an airlocked system, a head gasket or otherwise??[/i]