You can bypass the heater in the resevoir by removing the hoses (one or the other) and shunting the engine coolant from the inflow to the outflow. I had to do this once when the thermo valve sprung a leak.

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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.