Well with the engine running and no leaks, it sounds like the cap is not holding and the overflow is not getting back to the expansion tank. On the top of the expansion tank, where the cap is secured, below the cap there is an overflow hose attached to the neck. Trace that to the other end and see wheree it terminates. You will probably need a new cap. With the engine of and hot, the pressure in the cooling system will rise temporarily and this could be the trouble.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.