Dodge Van Radiator Overflow tank bubbling but temp guage normal

1992 Dodge B350 van 5.9l v8 engine, 203,500 miles.

We were driving back to Va from Florida yesterday and while checking under the hood during a driving break I noticed radiator overflow tank bubbling, even though temp guage was right where it always is, slightly to the low side of normal range. No other signs of distress so I drove another 400 miles home without incident. What are possible causes of tank bubbling?

Was the engine shut off when you were looking at it? It’s normal. When there is no circulation by the water pump, the coolant in the engine will boil. Once you start it back up, it will cool to normal temp. Fear not.

The only other thing that can easily make this happen is a worn out radiator cap. If it does not hold the pressure it’s supposed to, the coolant will boil when you stop. Pressure raises the boiling point of coolant. The coolant circulating in your van is normal if it is circulating over the unpressurized boiling point, and it won’t boil, but if the cap is leaky it will boil just like you say happens. Radiator caps are cheap; just replace it when the system is COLD.

Hmmm. I have opened the hood on this truck many times while hot and recently shut down, and it has never done this before. The fluid level has elevated to he “full hot” level, but has never boiled like that before. Same with my other three vehicles with overflow tanks.

I replaced the radiator and water pump last year and changed the cap when I did those, but I’ll go ahead and replace the cap again and see if that makes a difference.