I have a 2001 Expedition with the smaller V8 (4.6L. I think). When I have the Air conditioner on and I accelerate, the air switches to the defrost mode and is warm. Then when I go back to cruise mode the A/C comes back on cold. I'm baffled by this.
I had a tune up about a week ago, so I'm thinking maybe a vacuum hose was left loose. I returned it to the shop and they said they couldn't find any vacuum leak. They now want to charge me $79 to do a diagnostic and find the reason for this problem.

Any ideas?

Thank you!