This might be normal. Consider this.
The cooling system is under pressure, this keeps the coolant from boiling under normal operations.
When the car gets hot, expanded coolant from the engine will flow 'backwards' into the reserve tank. This is by design.
That 'released' coolant gets trapped in the reserve tank. The pressure cap on the reserve tank should keep the tank and the entire engine under the correct pressure so that you don't see any bubbling/boiling.
When you shut off or otherwise cool the motor, the sequence runs in reverse. Coolant from the reserve tank is pulled back into the engine system.
I'd say that as long as the engine isn't overheating and that you don't actually have to add coolant (when everything is cold) to keep the levels in the reserve tank at the correct level, then the system is working as intended.
Do you see the boiling as soon as you stop the car or after it's been stoped for a while. The 'pressure' in the cooling system will bleed down slowly after engine stop. But, as far as I know, the pressure shouldn't bleed down so fast that the reserve tank starts bubbling/boiling.
The quick and easy test/fix might be to to replace the pressure cap on the reserve tank, it's a $5 part. Those things do not last forever, the gasket and spring does wear out with time. Doesn't hurt to replace the cap, it is a key part of the cooling system. I did mine at about 60k miles or so.
Check the 50/50 mix of coolant, use only Mopar or Zerex G-05 coolant, DO NOT USE Ford or GM coolant.
Check for air in the cooling system. A search of the site will find a procedure for burping air put of your motor and cooling system. If you have to burp a lot of air out, chances are that you've got or had a coolant leak somewhwere and that needs to be chased down ASAP. Be sure to change the pressure cap on the coolant tank in this case.
Last edited by '98-ESer; 08-23-2006 at 12:56 PM.