I have a problem with my 2003 S-Type. If I drive the car for over 2 hours straight with the ac on then all of the coolant will evaporate and the car will over heat. If I drive without the ac on I do not have the problem at all. I was told by a mechanic that my fan needed to be replaced because it was not changing speed. I replaced the fan and that did not solve the problem. Has anyone dealt with this issue before?

 

I don't think it is evaporating, there must be a leak somewhere, can't you see any signs of dried coolant anywhere?

 

I had a pressure test and there are no leaks in the system. There are no visible signs of leaks either.

 

First keep trying to find out where the coolant is going because as was said above it's NOT evaporating. Hopefully no shop or mechanic said that!

Your overheating is just simply lack of coolant. Be sure and check the oil for coolant and I would consider some more pressure testing.

The fan speed may be a problem but you really should be seeing overheating in heavy traffic with the A/C on if so.

Have you changed the coolant tank cap? Just a guess on that one.
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