Why is there foam under my oil cap?

Foam in the oil pour spout, on the inside of the cap, or on the dipstick or dipstick tube is an indication that moisture has entered into the oil. This could be nothing more then condensation from short driving trips, or as serious as a blowed head gasket or cracked head/block. Drain your oil and take a good look at it. does it appear to be foamy? Brownish in color? If in doubt, have a qualified mechanic look at it. It could be serious.