I just replaced the heater control valve in my 94 XJ due to it leaking a lot...I put everything together, popped the radiator cap off and started to run it to get any air out of the system and add more coolant (lost a little bit in the process of replacing) -- I noticed that until the vehicle warmed up and the t-stat opened, the coolant at the filler hole had a thin white frothy layer on top of it. It wasn't milky or oily, just frothy. Once the engine got up to temperature and the t-stat opened, this condition disappeared. Only drop in coolant level has been due to heater valve constantly leaking (been watching overflow tank) and my oil is very clean. Is this something to worry about?

 

As long as you dont have "Bubbles/Pressure" pushing the antifreeze out you should be good. I would just keep an eye on it but I suspect its from the air that was trapped in the system...Tj