I have a 92 Isuzu Rodeo w/ 3.1 v6
Last weekend I flushed the radiator. As I was adding more water , we noticed that the water at the radiator lip would form "foam" a bit with very small bubbles (kind of like the head of a Guiness, not large bubbles, like a soda pop). Dad (the mechanic), says that it could be caused by air entering the system via a very small head gasket leak.
Sound familiar ? Vehicle runs and cools fine now.
Jim H