This question is for my father. He was driving around town when he noticed the car starting to over heat as he went to pull over the car lost all power come to find out the upper radiator hose came apart. So now after I ordered a new one for him and put it on the car will idle fine up until it's warm then just shoot up to high idle. We checked for leaks and checked all the fuses but still can't find the problem. By the way it's a 2000 V40.

The engine management temperature sensor is at the thermostat housing under the coolant hose while the dashboard temperature sensor is about halfway down one side of the radiator. Two different sensors are used. The one at the thermostat housing is probably cooked and giving a wrong reading. They do take a beating and wear out over time, but steam probably killed it. I'm about to replace mine; had a slow hose leak with hot water dripping on the sensor and wires.

Also when the thermostat opens the cooling system starts building up pressure. Sure you are not blowing coolant into the cylinders? Might be slow enough that it takes a while to notice a drop in coolant level. With a cold engine try removing the coolant cap and then idling the car up to temperature so it does not build up pressure. I drove my VW for 3 weeks like that with duct tape around a split radiator hose until I could find another hose. On the VW you can actually remove the pressure valve from the cap.

It could be the engine coolant temp sensor also got cooked and no longer works. Does the dash temp gauge stay low?

Did the top coolant hose get "burped" after adding coolant? Tell your dad to squeeze the hose a few times with the engine running but not hot. Don't want to burn fingers...

Klaus
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Cynthia Amendt, RIP