Hi,
I'm a bit confused. The heading indicates the AC doesn't blow cold at highway speeds. In your post, you indicate there is no heat unless you are driving on the highway. Based on everything in your post, I suspect the problem is the heater. If that isn't correct, let me know.
First, there is a good chance the heater core is plugged. The first thing I need you to try is this. Here is a link that explains how to repair a heater not working. Take a look through it and see if it helps.
https://www.2carpros.com/articles/car-heater-not-working
As far as the coolant leak, it needs to be identified. I have a feeling we will find it is the water pump. On the pump is what is known as a weep hole. Check for evidence of a leak at that point. I attached a pic showing it.
Also, here are a couple links you may find helpful when checking for a leak:
https://www.2carpros.com/articles/car-is-leaking-coolant
https://www.2carpros.com/articles/radiator-pressure-test
The second link explains how to pressure test the system. It would be a good idea to do this if you can't locate the leak.
Last, a bad head gasket can cause the things you are experiencing. Here is a link that explains how to check if that is the issue:
https://www.2carpros.com/articles/head-gasket-blown-test
Let me know what you find. Please feel free to ask me questions if you have them.
Take care,
Joe
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Sunday, May 17th, 2020 AT 12:33 AM